LAN as a Productivity tool

Productivity

Productivity depends on ensuring that people have timely access to the equipment and information required to perform their job. LAN increases productivity because key individuals in the organization will be able to get access to and share database, documents and expensive peripherals.


As a productivity tool, a LAN can:

• Enable wider distribution of information and the technologies needed to deal with it.


• Improve information retrieval, processing, storage and dissemination through a distributed database.


• Minimize or even if possible, eliminate redundant and repetitive tasks.


• Improve efficiency by facilitating the unification of systems and procedures.

• Provide graphic capabilities and other specialized application that are not cost-effective on stand-alone micros.

More characteristic of LAN

• LAN as a Resource Sharing tool.
• LAN as a Communication resource.
• LAN as a Management tool.

LAN as a Resource Sharing Tool

Resource Sharing

LAN eliminates the possibility of overspending by allowing workstations to share peripherals like printers, plotters, digitizers, tape drives and hard disks. The lowers the overall cost of data processing. Provides for efficient and flexible communication. By providing a facility through which a wide variety of computer equipment can shared by many people, the local area network presents a cost-effective solution. In a LAN the shared resources need not be just hardware, software and information also may be shared.

As a resource sharing tool, a LAN can:

• Permit sharing of expensive hardware.
• Facilitate sharing of complex programs and the information that they generate and manage.

Aid in the integration of all aspects of information processing, particularly transforming a group of individual, not very powerful microcomputers into a powerful distributed processing system.

More characteristics of LAN:

• LAN as a Productivity tool.
• LAN as a Communication Resource.
• LAN as a Management tool.

Characteristics of LAN

A common communication medium over which all user devices can share information, programs and equipment without regard to the physical location of the user or the resource. A high transmission rate intended to accommodate the needs of both people and equipment. The system normally is able to support transmission between workstations at the maximum speed at which they can communicate, a limited geographic range: generally defined as less than 10 miles or 16 kilometers. To minimize the error rate, a built-in method of detecting and compensating for system errors is implied.

A LAN is characterized by the following:

1. Resource Sharing
2. Productivity
3. Communication
4. Management

LAN Cable

LAN Cable as a comp0nent of LAN

LAN uses coaxial cable RG-62. This is a relatively superior cable that allows for base band transmission. The cable is capable of transferring up to 10 Mbps. Special end connectors are used to interface with network interface card or hubs.

The advantage of the coaxial cable are:

• Wider band width
• Interference resistance
• High conductivity without distortion
• Longer distance covered.

Active Hub and Passive Hub

Active and Passive Hub as a component of LAN

Active Hub
An active hub is a powered distribution point with active devices which drive distant nodes up to 1 kilometer away. Active hubs can be cascaded to connect 8 connections to which passive hubs, file servers or another active hubs can be connected Maximum distance covered by an active hub is about 2000 ft.

Passive Hub
It is passive distribution point which dies not use power or active devices in a network to connect up to 4 nodes within a very short distance. Maximum distance covered by a passive hub is about 300 ft.

Network Interface Unit (NIU)

Network Interface Unit (NIU) as a component of LAN

The network interface unit is a microprocessor-based device containing hardware and software which supply the intelligence to control access and communication across the network and to perform all communication processing. It is the means by which the workstations are connected functionally and physically to the network.

On most microcomputer, the network interface is a printed circuit board installed in the microcomputer. On some networks, the network interface may be implemented as a stand-alone box, termed a wiring center, or hub, attached between the main network cable and the workstation.

Network interface functions are realized through chips on the interface unit: Network bus drivers, communication controller chips, specialized microprocessors, RAM buffers and ROM code are executed by the workstation itself. For most LANs, the network interface unit for all user workstations is identical. Server interface units may include additional ROM code to implement additional functions. Physical connection to the network is provided through a standard communication or input/output interface.

Through the network interface, data on the medium is available to all attached workstations and peripherals. System users never need to know what it takes to get from one point to another; they simply indicate the desired destination. The network interface unit provides transmission and data control, formats the data into manageable units, translates the data rate and protocols of the attached workstation to the network communication medium and vice versa, and supplies address recognition capabilities. Details of network operation are hidden from users of the attached workstations.

Network Interface can be classified in to the communication interface, containing network oriented functions and the host interface, containing computer specific functions.

The communication interface is the unit which logically interfaces to the network. It performs all transmission related functions. It accepts data from the attached workstations, buffers the data until the communication channel is available and then transmits the data. The communications interface also monitors the channel for messages addressed to its workstation, stores the data and transfers the data to the device.

The host interface supplies the connection between a specific workstation's internal circuitry and the communication interface unit. It fits into the input/output structure of a particular computer and governs all data exchange between the workstation and the communication-oriented portion of the network interface.

Gateway

Gateway as a component of LAN

This component is used to connect two different LANs which are having dissimilar components. The gateway assists in transferring bits from one LAN to the other. A workstation is dedicated to act as the gateway. Network adapter cards for both types of LANs are inserted in the machine, and a special set of Novell program transfers the bits from one LAN to the other. Similarly a LAN can also be connected to another mainframe computer by a gateway.

File Server

File Server as a powerful component of LAN

The file server is a powerful computer which runs a special software to act as a file server. As the name suggests, it serves to networked computers which share and use these files. The files can be programs, text or data. The file server is a completely enclosed logical structure, which is secure against accidental or malicious abuse as it can be accessed only through Network Operating System (NOS).

The activity of each file server can be monitored from the file server's screen. The system supervisor monitors and control operation of each individual network through the file server and uses it to control the print spooling, send/broadcast messages and perform many other system functions. The file server has a large volume of memory which is used for caching directories, files and directory hashing. File servers also support backup facilities, print serving and so on.

Novell Net Ware, for example, requires on a workstation to log on to a file server to enter the LAN. Under Novell Net Ware, specifically designed machines are converted to file servers. Under the PC network Program, any workstation can perform the function of file serving on any files that have been designated as public.

A file server is a resource that should be available to all workstations. If the file server is forced to operate as a workstation at the same time, it then must divide its processing time and memory between two tasks, with the performance of both suffering.

Workstation

Workstation as a component of a LAN

As the most common component, a workstation is an individual single-user microcomputer with communications capabilities added. The term includes in the microcomputer itself as well as all its attached bits and pieces-memory cards, CRT, floppy disk drives, hard disks and printers. A workstation is distinguished from a personal computer by the network operating system software that controls what the workstations can and cannot do and by a network interface unit that supplies the communications capabilities.

Every workstation will run memory resident software, called Workstation Shell, which is the software interface between file server and workstation. This will filter local and network requests/commands.

A workstation can send or receive messages to or from other workstation or file server. For some LANs, the connection to a workstation can be made with a serial port. In that case, the LAN interface-unit is not a plug-in board internal to the computer but an external component.

Workstations may have one to several floppy-disk drives and hard-disk drives. Workstations may be divided into two classes: users and servers, User Work stations are microcomputers on the network which have a primary responsibility to an individual user. Server workstations perform a service to other workstation on the network. All workstations on the network communicate and cooperate with one another. The primary difference between server and user workstations is directly attached resources and programs which they run.

User workstations normally do not and cannot fulfill request from other workstations.
Resources attached to a user workstation, such as floppy disk drives, can only be accessed by the user of that workstation.

More than one server may be attached to a network, with each server providing a different function or one server fulfilling several roles.

Server workstations are of two kinds:

• Dedicated: The microcomputer is restricted to network function and often incorporates more powerful capabilities than user workstations do. It can support more features, such as electronic mail service or multiple hard disks and provides faster system response. Larger networks usually require dedicated servers.
• Non-dedicated: The microcomputer can act as an individual workstation even while it controls the network. Additional memory is required for all but the simplest tasks. Under light load, performance of a non-dedicated server may be slightly less than that of a workstation; under heavy processing demand, the individual user of the server may find work impossible.

Some network servers are capable of operating in both dedicated and non-dedicated mode, depending on the user's selection.

Components of a LAN

Local Area Network (LAN) has several distinct components like work-station, file-server, gateway, Network interface card, active hub, passive hub, LAN cables etc.

1. Workstation
   
   
   • User Workstations
   
   
   • Server Workstations
     • Dedicated Server Workstations
     • Non-dedicated Server Workstations
   
   
   
   
   2. File Server
   
   
   3. Gateway
   
   
   4. Network Interface Unit
   
   
   5. Active and Passive Hub
   
   
   6. LAN Cable.

LAN (Local Area Network)

LAN is computer network that spans a relatively small area. Most LAN's are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a Wide Area Network (WAN).

Most LANs connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it also is able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.

Related Topics:

1. Components of a LAN
   
   
   
   • Workstation
   
   
   • File Server
   
   
   • Gateway
   
   
   • Network Interface Unit
   
   
   • Active Hub
   
   
   • Passive Hub
   
   
   • LAN Cable


2. Characteristics of LAN
   
   
   
   • Resource Sharing
   
   
   • Productivity
   
   
   • Communication
   
   
   • Management


3. LAN Topologies
   
   
   
   • Bus Topology
   
   
   • Ring Topology
   
   
   • Star Topology
   
   
   • Tree Topology


4. LAN Media-Access Methods
   
   
   
   • Carrier Sense Multiple Access Collision Detect (CSMA/CD)
   
   
   • Token Passing


5. LAN Transmission Methods
   
   
   
   • Unicast
   
   
   • Multicast
   
   
   • Broadcast


6. Wireless LANs




• Line-of-sight


• Scattered infrared

HDLC Protocol Operation

The two basic functions in the protocol

• Link Management
• Data transfer (which includes error and flow control).

1. Link Management - Prior to any kind of transmission (either between two stations connected by a point to point link or between a primary and secondary station a multidrop link) a logical connection between the two communication parties must be established.
2. Data Transfer - In NRM all data (information frames) is transferred under the control of the primary station. The unnumbered poll frame with the P bit set to 1 is normally used by the primary to poll a secondary. If the secondary has no data to transmit, it returns an RNR frame with the F bit set. If data is waiting, it transmits the data, typically as a sequence of information frames. The two most important aspects associated with the data transfer phase are error control and flow control. Essentially, error control uses a continues RQ procedure with either a selective repeat or a go back N transmission strategy, while flow controls bases on a window mechanism.

Some standard subsets are listed in the following table.

HDLC Subset		Used for
802.2	Logical Link Control	FDDI, Token Ring, and some Ethernet LAN's
LAP	Link Access Procedure	Early X.25 implementations
LAPB	Link Access Procedure Balanced	Current X.25 implementations
LAPD	Link Access Procedure for the ISDN D Channel	ISDN D channel and frame relay
LAPM	Link Access Procedure for Modems	Error-correcting modems (specified as part of V.42

HDLC Frame Types

The following are the Supervisory Frame Types in HDLC

RR	Information frame acknowledgement and indication to receive more.
REJ	Request for retransmission of all frames after a given sequence number
RNR	Indicates a state of temporary occupation of station (e.g. window full)
SREJ	Request for retransmission of one given frame sequence number.

The following are the Unnumbered Frame Types in HDLC

DISC	Request disconnection
UA	Acknowledgement frame
DM	Response to DISC indicating disconnected mode.
FRMR	Frame reject.
SABM	Initiator for asynchronous balanced mode. No master/slave relationship
SABME	SABM in extended mode.
SARM	Initiator for asynchronous response mode. Semi master/slave relationship
SARME	SAMR in extended mode.
REST	Reset sequence numbers.
CMDR	Command reject.
SNRM	Initiator for normal response mode. Full master/slave relationship
SNRME	SNRM in extended mode.
RD	Request disconnect
RIM	Secondary station request for initialization after disconnection.
SIM	Set initialization mode.
UP	Unnumbered poll.
UI	Unnumbered information. Sends state information/data.
XID	Identification exchange command.

There is one information Frame Type in HDLC

Info	Information frame.

HDLC Frame Classes

In the HDLC protocol, three classes of frames are used:

1. Unnumbered frames are used for link management, for example they are used to set up the logical between the primary station and a secondary station and to inform the secondary station about the mode of operation which is used.
2. Information frames are those who carry the actual data. The information frames can be used to piggyback acknowledgment information relating to the flow of Information frames in the reverse direction when the link is being operated in ABM or ARM.
3. Asynchronous Balanced Mode (ABM) is used mainly on full duplex point-to-point links for computer to computer communications and for connections between a computer and a packed switched data network, in this case each station has an equal status and performs the role of both primary and secondary functions. This mode is used in the protocol set known as X.25
4. Asynchronous Response Mode (ARM) is used in unbalanced configurations. It allows a secondary station to initiate a transmission without receiving permission from the primary station. This mode is normally used with point-to-point configurations and full duplex links and allows the secondary station to send frames asynchronously with respect to the primary station.
5. Supervisory frames are use for error and flow control. They contain, send and receive sequence numbers.

HDLC Frame Formats

The standard frame of the HDLC protocol handles both data and control messages. It has the following format:

The HDLC frame structure consists of:

1. Flag - The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.

2. Address field - The first byte of the frame after the header flag is known as the Address Field. HDLC is used on multipoint lines and it can support as many as 256 terminal control units or secondary stations per line. The address field defines the address of the secondary station which is sending the frame or the destination of the frame sent by the primary station.

3. Control Field - The field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type. Every frame holds a one bit field called the Poll/Final bit. In the NRM (Normal Response Mode) mode of HDLC this bit signals which side is 'talking' and provides control over who will speak next and when. When a primary station has finished transmitting a series of frames, it sets the Poll bit, thus giving control to the secondary station. At this time the secondary station may reply to the primary station. When the secondary station finishes transmitting its frames, its sets the Final bit and control returns to the primary station.

4. Modes of operation - HDLC has 3 modes of operation according to the strength of the master/slave relationship. This is determined by a unique frame type specifier. The three modes of operation are:

• Normal Response Mode (NRM) - The primary station initiates the session and full polling is used for all frame transmissions.
• Asynchronous Response Mode (ARM) - This mode is similar to NRM and is signified by the SARM(E) frame. The difference, however, is that secondary stations can transmit freely without waiting for a poll.
• Asynchronous Balanced Mode (ABM) - This mode is totally balanced (i.e. no master/slave relationship) and is signified by the SABM(E) frame. Each station can initialize, supervise, recover from errors and send frames at any time.

5. FCS (Frame Check Sequence) - The Frame Check Sequence (FCS) enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.

HDLC (High Level Data Link Control)

HDLC (High Level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In HDLC, data is organized into a into (called a frame) and send across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent.

A bit-oriented, synchronous, link layer, data-framing, flow control and error detection and correction protocol. Uses a header with control information and a trailing cyclic redundancy check character (which is usually 16 or 32 bits in length). Implementations are both standard subsets or vendor-specific (such as that used for the 56,000-bits/s interfaces on a vendor's remote bridge or router). IBM calls HDLC as SDLC.

HDLC uses zero insertion/deletion process (commonly known as bit stuffing) to ensure that the bit pattern of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and therefore relies on the physical layer to provide method of clocking and synchronizing the transmission and reception of frames.

The HDLC protocols is defined by ISO for use on both point-to-point and multipoint (multidrop) data links. It supports full duplex transparent-mode operation and is now extensively used in both multipoint and computer networks.

Selective Repeat Sliding Window Protocol

Another strategy is to re-send only ones that are actually lost or damaged. The receiver buffers all the frames after that lost one. When the sender finally noticed the problem (e.g. no ack for the lost frame is received within time-out limit), the sender retransmits the frame in question.

Main Window
The main window can be subdivided roughly into four areas:

1. Sender - The representation assumes that the implementation of the Sliding-Window protocol takes place on Stack n. Layer n+1 represents the "data-supplier" for the protocol. It is represented through the text-input-field. One can hand over individual data-packages with "Step" button. So that the animation speed can be adjusted by clicking on it with the right mouse button one can choose the protocol to be simulated.

2. Receiver - It is similar to the sender. The only difference is that the step button here controls the data transfer from the layer n+1 to the layer n. The received Packets is presented in the input text field. By clicking on it with the right mouse button one can choose the protocol to be simulated.

3. Channel - Represent the network channel. By clicking on it with the right mouse button. One can simulate errors.

4. History-Chart - One can adjust the number of messages to be displayed in this area.

Go-Back-n Sliding Window Protocol

In case of satellite communication, it takes 270 msec to transmit a frame and 520 msec is needed before the acknowledgement arrives. The solution here is the sender is allowed to send up to 'w' frames and the acknowledgment will arrive after the roundtrip time gets equal. The technique of allowing the sender to send the data and receiver acknowledging them parallel is called Pipelining.

If the channel capacity is b bits/sec and frame size 1 bits and the roundtrip propagation time R sec, the time required to transmit a single frame is 1/b sec. There is a delay of R/2 before the last bit arrives and another R/2 before acknowledgement arrives. The line utilization for the stop-and-wait is 1(1+bR). Pipelining has a serious drawback when one of the frames gets damaged in the middle. This could be overcome by the Go Back n protocol.

This approach is mainly used for dealing with errors when the frames are pipelined. The receiver simply discards all subsequent frames, sending no acknowledgments. Here the receiver window is of size 1. In other words, the data link layer refuses to accept any frame except the next one it must give to the network layer. If the sender's window fills up before the timer runs out, the pipeline will begin to empty. Eventually, the sender will time out and retransmit all unacknowledged frames in order, starting with damaged or lost one. This approach can waste a lot of bandwidth if the error rate is high.

One Bit Sliding Window Protcol

One bit sliding window protocol is also called Stop-And-Wait protocols. In this protocol, the sender sends out one frame, waits for acknowledgment before sending next frame, thus the name Stop-And-Wait.

Problem with Stop-And-Wait protocol is that it is very inefficient. At any one moment, only in frame is in transition. The sender will have to wait at least one round trip time before sending next. The waiting can be long for a slow network such as satellite link.

This protocol uses Stop-And-Wait, since the sender transmits a frame and waits for its acknowledgment before sending the next one. One of the two data link layers goes first. The starting machine fetches the first packet from its network layer, builds a frame from it and sends it. When this (or any ) frame arrives, the receiving data link layer checks to see if it is a duplicate. If the frame is the one expected, it is passed to the network layer and the receiver's window is slid up. The acknowledgement field contains the number of the last frame received without error. If this number agrees with the sequence number of the frame the sender is trying to send, the sender knows it is done with the frame stored in buffer and can fetch the next packet from its network layer. If the sequence number disagrees, it must continue trying to send the same frame. Whenever a frame is received, a frame is also sent back.

Introduction to Sliding Window Protocols

Sliding window is used by most connection oriented network protocols. In fact, TCP also uses sliding window. It assumes two -way communication (full duplex). It uses two types of frames:

• Data
• Ack (sequence number of last correctly received frame).

The basic data idea of sliding window protocol is that both sender and receiver keep a "window" of acknowledgment. The sender keeps the value of expected acknowledgment; while the receiver keeps the value of expected receiving frame. When it receives an acknowledgment from the receiver, the sender advances the window. When it receives the expected frame, the receiver advances the window.

The characteristics of sliding windows used at the sender and receiver usually involve

• error correction (by retransmission)
• flow control and
• message ordering by sender (FIFO).

The latter property can easily be incorporated in a sliding window protocol, but sometimes, it is preferred to be implemented as a separate protocol for easier maintenance.

Data Link Layer Protocols

The Data Link Layer defines how data is formatted for transmission and how access to the network is controlled. This layer has been divided by the IEEE 802 standards committee into two sublayers:

• Media Access Control (MAC)
• Logical Link Control (LLC)

The following data link layer protocols are described:

• Ethernet
• Token Ring
• FDDI
• LLC
• CIF

FDDI, Token ring and Ethernet may be physical interference or may act as logical protocols encapsulated over a WAN protocol or ATM.

Some of the Data Link protocols have discussed in the following:

1. Ethernet - Ethernet is a widely used data communications network standard developed by DEC, Intel, and Xerox. It uses a bus topology and CMSA/CD access method. The terms Ethernet and IEEE 802.3 standard are often used interchangeably.
2. Token Ring - Token ring is a LAN protocol where all stations are connected in a ring and each station can directly hear transmissions only from its immediate neighbor. Permission to transmit is granted by a message (token) that circulates around the ring.
3. FDDI - Fiber distributed Data Interference (FDDI) is a 100 Mega-bit technology using a timed token over a dual ring of trees. FDDI is standardized by the American National Standards Institute (ANSI)
4. LLC - The IEEE 802.2 Logical Link Control (LLC) protocol provides a link mechanism for upper layer protocols. LLC type I service provides a data link connectionless mode service, while LLC type II provides a connection-oriented service at the datalink layer.
5. CIF - CIF (Cells in frames) describes the mechanism by which ATM traffic is carried across a media segment and a network interface and conforming to the specification for Ethernet Version 2, IEEE 802.5 Token Ring or IEEE 802.3. ATM cells be carried over many different physical media, from optical fiber to spread spectrum radio. ATM is not coupled to any particular physical layer.

CIF defines a new pseudo-physical layer over which ATM traffic can be carried. It is not simply a mechanism for translation between frames and cells; neither is it simple encapsulation. CIF carries ATM cells in legacy LAN frames. This defines a protocol between CIF end system software and CIF attachment devices (CIF-AD) which makes it possible to support ATM services, including multiple classes of service, ovr an existing LAN NIC just as if an ATM NIC were in use. CIF specifies how the ATM layer protocols can be made to work over the existing LAN framing protocols in such a way that the operation is transparent to an application written to an ATM compliant API. Over Ethernet, CIF frames have an Ethernet header and trailer. CIF frames are encapsulated in Token Ring and LLC by use of SNAP header.

Optical Transmission Modes

There are three primary types of transmission modes using optical fiber. They are

1. Step Index
2. Graded Index
3. Single Mode.

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This cause some rays of take longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and fourth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. A typical Core is 62.5 microns.

Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core is 62.5 microns.

Both step index and graded index allow more than one light source to be used (different colors simultaneously), so multiple channels of data can be run at the same time.

Single index mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small of 9 microns.

Frequency is not used to talk about speed any more, instead of that wavelengths are used. The wavelength of light sources is measured in nanometers or 1 billionth of a meter.

Indoor cable specifications:

• LED (Light Emitting Diode) light source
• 3.5 dB/Km attenuation (loses 3.5 dB of signal per kilometer)
• 850 nM - wavelength of light source
• Typically 62.5/125 (core dia/cladding dia)
• Multimode - can run many light sources.

Outdoor cable specifications:

• Laser Light Source
• 1 dM/Km Attenuation (loses 1 dB of signal per kilometer)
• 1170 nM - wavelength of light source
• Monomode (single mode).

1. Advantage of Fiber Optic
   • Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI - Electromagnetic Interference)
   • Security:cannot tap into cable.
   • Large Capacity due to BW (bandwidth)
   • No corrosion
   • Longer distances than copper wire
   • Smaller and lighter than copper wire
   • Faster transmission rate.
2. Disadvantages of Fiber Optic
   • Physical vibration will show up a signal noise.
   • Limited physical are of cable. Bend it too much and it will break.
   • Difficult to splice.

The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

Fiber Optic

Optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron=10^-6 meters). Typically Cladding has a diameter of microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.

An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other.

If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.
Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it-it bounds the medium.

1. Advantages of Fiber Optic Line (Glass Fibers):
   • Smaller
   • Lighter
   • Faster (speed of light)
   • No interference.
2. Disadvantages of Fiber Optic Line (Glass Fibers)
• Expensive
• Harder to install and modify

Coaxial Cable

A single insulated inner wire is surrounded by cylindrical conductor which is covered with a shield; it transmits electromagnetic signals. Coaxial cable is classified into two categories:

• Base Band (uses digital signals)
• Broad Band (uses analog signals) coaxial cable.

1. Advantages of coaxial cable (Round Insulated Wire):
   • Not susceptible to interference
   • Transmits faster.
2. Disadvantages of coaxial cable (Round Insulated Wire):
   • Heavy and bulky
   • Needs booster over distance

1. Base band Coaxial Cable

It has been used for many years in the telephone network in applications with requirements similar to those of a LAN. Both base band and broadband coaxial cable are available. Although their structure is same, their installation and applications differ.

In base band coaxial cable, a central carrier wire is surrounded by a fine woven mesh of copper which forms an outer shell. The space between the wire and the outer shell is insulated to separate the two conductors and to maintain the electrical properties. The entire cable is covered by protective insulation to minimize electrical emissions. The cable is usually approximately 3/8 inch in diameter.

2. Broadband Coaxial Cable

This comes in several different diameters with varying amounts of insulation. The cable may have the same construction as base band coaxial. The central carrier may be surrounded by an aluminium sleeve. The space between the core and the shell is filled with insulation and the whole is enclosed in a protective coat of insulation. Broadband coaxial cable can carry 50 to 100 television channels or thousands of voice and low speed data channels at the rates of 9.2 to 50 Kpbs.

Twisted Pair

The wires in twisted pair cabling are twisted together in pairs. Each pair consists of a wire used for the +ve data signal and wire used for the negative data signal. Any noise that appears on 1 wire of the pair will also occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase (180 degrees - phasor definition of opposite polarity). When the noise appears on both wires, it cancels or nulls itself out at the receiving end. Twisted pair cables are most effectively used in systems that use a balanced line method of transmission:polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTL logic).

1. Advantages of Twisted Pair (Phone Line):
   • Easy to string
   • Cheap
2. Disadvantages of Twisted Pair (Phone Line):
   • Subject to interference =static and garble.

1. Unshielded Twisted Pair

The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference. To further improve noise rejection, a foil or wire braid "shield" is woven around the twisted pairs. This shield can be woven around individual pairs or around a multi-pair conductor (several pairs).

2. Shielded Twisted Pair

Cables with a shield are called shielded twisted pair and are commonly abbreviated STP. Cables without a shield are called unshielded twisted pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.

UTP or unshielded twisted pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc.)

STP or shielded twisted pair is used with traditional Token Ring cabling or ICS-IBM Cabling System. It requires a custom connector. IBM STP (shielded twisted pair) has a characteristic impedance of 150 ohms.

Open Wire

Open wire is traditionally used to describe the electrical wire strung along power poles. There is a single wire strung between poles. No shielding or protection from noise interference is used. The traditional definition of open wire is extended to include any data signal path without shielding or protection from noise interference. This can include multi conductor cables or single wires. This medium is susceptible to a large degree of noise and interference and consequently is not acceptable for data transmission except for short distances under 20 ft.

Introduction to Network Transmission Media

Transmissions lines are the backbone to a network. It is available in two basic varieties:Base band and broadband. Twisted pair wires are Base band communication links whereas coaxial and fiber optic cables are broadband links. There are two basic categories of transmission media:

• Guided
• Unguided.

Guided transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the cabling system. Guided media is also known as bound media. "Cabling" is meant in a generic sense and is not meant to be interpreted as copper wire cabling only.

Unguided transmission media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and are therefore often called unbound media.

There are four types of guided media:

• Open Wire
• Twisted Wire
  • Unshielded Twisted Pair
  • Shielded Twisted Pair
• Coaxial Cable
  • Base Band Coaxial Cable
  • Broad Band Coaxial Cable.
• Optical Fiber.

SNA (System Network Architecture)

It is IBM's network architecture. It was developed mainly to interconnect dissimilar user machines. An SNA network consists of machines called nodes of which there are four types:

• Type 1 - Terminals.
• Type 2 - Controllers, machines that supervise the behaviors of the terminals and other peripherals.
• Type 3 - Front End Processor (FEP), relieve the Host of the work and interrupt handling associated with data communication.
• Type 4 - Hosts.

Each nodes contains one or more NAUs (Network Addressable Unit) which is a piece of software that allows a process to use the network. There are three kinds of NAUs. An LU (Logical Unit) is the usual variety to which user processes can be attached. A PU (Physical Unit) is used to being a node on-line, take it off-line, test it and perform similar network management functions. The third kind of NAU is SSCP (System Services Control Point) has complete knowledge of and control over all the front ends, controllers and terminals attached to the host.

Protocol Hierarchy in SNA

End User

NAU services

Data Flow Control

Transmission Control

Path Link Control

Data Link Control

Physical Link Control

Layer1 (Physical Link Layer) take care of physically transporting bits from one machine to another. The Layer2 (data link layer) constructs frames from the raw bit stream, detecting and recovering from transmissions errors.

Layer3 in SNA, called Path Control, is concerned with establishing a logical path from source NAU to destination NAU. Path control consists of three sublayers. The highest sublayer doest the global routing, deciding which sequence of subareas should be used to get from the source subarea to the destination subarea. The sequence is called virtual route. Two subareas may be connected by several kinds of communication lines so that the next sublayer chooses the specific lines to use, giving and explicit route. The lowest sublayer splits traffic among several parallel communication links of the same type to achieve greater bandwidth and reliability.

It is the job of Layer4 (Transmission Control Layer) to create, manage and delete transport connections (sessions). The Data Link Layer keeping track of which end of session is supposed to talk next. This layer is also doing error recovery functions. The NAU services layer provides two classes of services to the user processes. First there are presentation services such as text compression. Second there are session services for setting up connections. In addition there are network services which maintain the operation of the network.

ARPANET

The ARPANET (American Research Project agency NET work), now called DARPA, owned by US Defense, doest not follow the OSI model at all. The IMP-IMP protocol really corresponds to mixture of the layer 2 and layer 3 protocols. Layer 3 also contains an elaborate routing mechanism. In addition, there is a mechanism that explicitly verifies the correct reception at the destination IMP of each and every of protocol sent by the source IMP.

The ARPANET does have protocols that roughly cover the same territory as the OSI network and transport protocols. The network protocols, called IP (Internet Protocol) is connectionless and was designed to handle the interconnection of the vast number of WAN and LAN. The ARPANET transport protocol is a connection-oriented protocol called TCP (Transmission Control Protocol). There is no session or presentation layer protocol in ARPANET.
The ARPANET services include files transfer, email and remote login. These services are supported by well-known protocols FTP, SMTP (Simple Mail Transfer protocols) and TELNET (remote login)

Public Networks

The networks which is owned neither by Government or by private organization but by an individual network operator, provides communication services for the customers' hosts and terminals. Such a system is called public networks.

All of them use OSI model and the standard CCITT (Consultative Committee International for Telephony and Telegraphy, a French standard organization) or OSI protocols for all the layers. For the lowest three layers, CCITT has issued recommendations that have been universally adopted by public networks worldwide. These layers are always known collectively as X.25 standards.

They physical layer protocol, called X.25, specifies the physical, electrical and procedural interface between the host and the network. The data link layer standard has a number of variations. They all are designed to deal with transmission errors on the telephone line between the user's equipment and the network. The network layer protocol deals with addressing, flow control, delivery confirmation, interrupts and related issues.

ISO has developed standards for a connection-oriented transport layer service definition and a connection-oriented transport layer protocol. Also it has adopted standards for the connection-oriented session service, protocol and presentation service and protocol.

The Application layer contains following protocols:

1. FTAM (File transfer, access and management) protocol, which provides a way to transfer, access and generally manipulate remote files in a uniform way.
2. MOTIS (Message-Oriented Text Interchange Systems) protocol is used for electronic mail.
3. VTP (Virtual Terminal Protocol) provides a terminal-independent way for programs to access remote terminals.
4. JTM (Job Transfer and Manipulation) protocol is used for submitting jobs to remote mainframe computers for batch processing.

Example Networks

Numerous networks are currently operating around the world. Some of them owned by governments, some for research work, some are owned by private organizations and so on.

Physical Layer

They physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as 1 bit, not as 0 bit. Typical questions here are how many volts should be used to represent a 1 and how many for a 0, how many microseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are are finished and how many pins the network connector has and what each pin is used for. The design issues here deal largely with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below the physical layer. Physical layer design can properly be considered to be within the domain of the electrical engineer.

Data Link Layer

The main task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames (typically a few hundred bytes), transmit the frames sequentially and process the acknowledgment frames sent back by the receiver. Since the physical layer merely accepts and transmits a stream of bits without any regard to meaning of structure, it is up to the data link layer to create and recognize frame boundaries. This can be accomplished by attaching special bit patterns to the beginning and end of the frame. If there is a chance that these bit patterns might occurs in the data, special care must be taken to avoid confusion.

The data link layer should provide error control between adjacent nodes.

Data Link Layer: Error Control

A noise on the line can destroy a frame completely. In this case, the data link layer software on the source machine must retransmit the frame. However, multiple transmissions of the same frame introduce the possibility of duplicate frames. A duplicate frame could be sent, for example, if the acknowledgement frame from the receiver back to the sender was destroyed. It is up to this layer to solve the problems caused by damaged, list and duplicate frames. The data link layer may offer several different service classes to the network layer, each of a different quality and with a different price.

Network Layer

The network layer is concerned with controlling the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes could be based on static tables that are "wired into" the network and rarely changed. They could also be determined at the start of each conversation, for example a terminal session. Finally, they could be highly dynamic, being determined a new for each packet, to reflect the current network load.

If too many packets are present in the subnet at the same time, they will get in each other's way, forming bottlenecks. The control of such congestion also belongs to the network layer.

Since the operators of the subnet may well expect remuneration for their efforts, there is often some accounting function built into the network layer. At the very least, the software must count how many packets or characters or bits are sent by each customer, to produce billing information. When a packet crosses a national border, with different rates on each side, the accounting can become complicated.

When a packet has to travel from one network to another to get to its destination, many problem can arise. The addressing used by the second network may be different from the first one. The second one may not accept the packet at all because it is too large. The protocols may differ and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.

In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.

Transport Layer

The basic function of the transport layer, is to accept data from the session layer, split it up into smaller units if need be, pass these to the network layer and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done efficiently, and in a way that isolates the session layer from the inevitable changes in the hardware technology.

Under normal conditions, the transport layer creates a distinct network connection for each transport connection required by the session layer. If the transport connection requires a high throughput, however, the transport layer might create multiple network connections, dividing the data among the network connections to improve throughput. On the other hand, if creating or maintaining a network connection is expensive, the transport layer might multiplex several transport connections onto the same network connection to reduce the cost. In all cases, the transport layer is required to make the multiplexing transparent to the session layer.
The transport layer also determines what type of service to provide to the session layer, and ultimately, the users of the network. The most popular type of transport connection is an error free point-to-point channel that delivers messages in the order in which they were sent. However, other possible kinds of transport, service and transport isolated messages with no guarantee about the order of delivery and broadcasting of messages to multiple destinations. The type of service is determined when the connection is established.

The transport layer is a true source-to-destination or end-to-end layer. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control message.

Many hosts are multiple-programmed, which implies that multiple connections will be entering and leaving each host. There needs to be some way to tell which message belongs to which connection. The transport header is one place this information could be put.

In addition to multiplexing several message streams onto one channel, transport layer must take care of establishing and deleting connections across the network. This requires some kind of naming mechanism, so that process on one machine has a way to describing with whom it wishes to converse. There must also be a mechanism to regulate the flow of information, so that a fast host cannot overrun a slow one. Flow control between hosts is distinct from flow control between switches, although similar principles apply to both.

Session Layer

The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides some enhanced services useful in a some applications. A session might be used to allow a user to log into a remote time-sharing system or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic can only go one way at a time, the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, the session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical operation.

Another session service is synchronization. Consider the problems that might occur when trying to do a two-hour file transfer between two machines on a network with a 1 hour mean time between crashes. After each transfer was aborted, the whole transfer would have to start over again , and would probably fail again with the next network crash. To eliminate this problem, the session layer provides a way to insert checkpoints into the data stream, so, after a crash, only the data after the last checkpoint has to be repeated.

Presentation Layer

The presentation layer performs certain functions that are requested sufficiently often to warrant finding a general solution for them, rather than letting each user solve the problems. In particular, unlike all the lower layers, which are just interested in moving bits reliably from here to there, the presentation layer is concerned with the syntax and semantics of the information transmitted.

A typical example of a presentation service is encoding data in a standard, agreed upon way. Most user programs do not exchange random binary bit strings. They exchange things such as people's names, dates, amounts of money and invoices. These items are represented as character strings, integers, floating point numbers and data structures composed of several simpler items. Different computers have different codes for representing character strings, integers and so on. In order to make it possible for computers with different representation to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used "on the wire". The job of managing these abstract data structures and converting from the representation layer.

The presentation layer is also concerned with other aspects of information representation. For example, data compression can be used here to reduce the number of bits that have to be transmitted and cryptography is frequently required for privacy and authentication.

Application Layer

The application layer contains a variety of protocol that are commonly needed. For example, there are hundreds of incompatible terminal types in the world. Consider the plight of a full screen editor that is supposed to work over a network with many different terminal types, each with different screen layouts, escape sequences for inserting and deleting text, moving the cursor, etc.

One way to solve this problem is to define an abstract network virtual terminal for which editors and other program can be written to deal with. To handle each terminal type, a piece of software must be written to map the functions of the network virtual terminal onto the real terminal. For example, when the editor moves the virtual terminal's cursor to the upper left-hand corner of the screen, this software must issue the proper command sequence to the real terminal to get its cursor there too. All the virtual terminal software is in the application layer.

Another application layer function is file transfer. Different file systems have different file naming conventions, different ways of representing text lines, and so on. Transferring a file between two different systems requires handling these and other incompatibilities. This work, too, belongs to the application layer, as do electronic mail, remote job entry, directory lookup, and various other general-purpose and special-purpose facilities.

Characteristics of the OSI Layers

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.

The Upper Layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another type in the OSI model.

The Lower Layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for e.g.) and is respoinsible for actually placing information on the medium.

Two sets of Layers make up the OSI Layers
Application	Application
	Presentation
	Session
Data Transport	Transport
	Network
	Data Link
	Physical

OSI Reference Model

Modern computer networks are designed in a highly structured way. To reduce their design complexity, most networks are organized as a series of layers, each one built upon its predecessor.
Open System Interconnection (OSI) Reference Model was developed by the International Standard Organization (ISO) to provide a framework for understanding how information is sent from one computer to another. The model is so called because it deals with connecting open systems - that is, the system is open for communication with other systems. The OSI model describe seven layers, with each layer intended to provide a well defined service in order to ensure data has been successfully transmitted between devices.

These layers are:

• Application - Provides different services to the applications.
• Presentation - Converts the information
• Session - Handles problems which are not communication issues.
• Transport - Provides end to end communication control.
• Network - Routes the information in the network.
• Data Link - Provides error control between adjacent nodes.
• Physical - Connects the entity to the transmission media.

Topologies of Common Networks

As mentioned earlier, a network can have a logical topology different from its physical topology. The following are some common types of networks:

• Ethernet
• Token Ring.

1. Ethernet

An older, common wiring system for Ethernet (10Base2) and (10Base5) uses coaxial cable in a linear bus topology. In the most type of Ethernet, each node connects to the coax through a T-connector that taps into the signals on the coaxial cable. The nodes both transmit and receive through connector. Therefore, 10Base2 Ethernet is a logical as well as a physical bus.

A newer variation of Ethernet, 10Base-T and 100Base-TX, are cabled using wiring hubs (concentrators). Each station is connected to the hub via an individual UTP twisted pair cable. Within the hub, however, the individual signals are combined into a bus. Therefore 10Base-T and 100Base-TX are physical stars,but logical buses.

2. Token Ring

In the wiring of a Token Ring, it meets all the specifications of a star. Token Ring uses central wiring hubs and each node is wired to the hub with an individual run of cable.

Starting at the hub, the signal travels through a pair of wires to the receive circuit on the node's network interface. The receive circuit passes the signal to the transmit circuit, which repeats the signal on a separate pair of wires and sends the signal back to the hub.

If there is signal around the entire network, it completes a circuitous path, proving that Token Ring has a ring logical topology.

Token ring is wired in a physical star to obtain the advantages of a central wiring hub. All stations can be connected and disconnected at a central point and the wiring hub can be equipped with hub management and diagnostic systems. Sometimes Token Ring referred as a star-wired-ring.

Logical Topologies

Logical topologies have the same names as physical topologies. But a physical topology describe the network, whereas the logical topology describes the network from the viewpoint of the data traveling on the network. Networks can have different physical and logical topologies.

The following two logical topologies are discussed in the following sections:

• Ring Logical topology
• Bus logical topology

1. Ring Logical Topology

Ring topology functions by passing data transmission from one node to the next. This operation is clearest when the physical topology is also a ring. Any time data are passed from node-to-node, the network has a ring logical topology.

Another way to identify a ring is to determine whether each node has separate receive and transmit circuits. If that is the case, the node is functioning as a repeater and is probably connected in a logical ring network.

2. Bus Logical Topology

In a bus topology, each data transmission passes by each node on the network. Essentially, each transmission is broadcast throughout the network and the nodes use addresses to determine whether they should pay attention. Any time all transmissions are available to all nodes on the network, the network has a bus logical topology.

If the nodes on a network use the same circuits to transmit and receive, the logical network is a bus.

Considerations when choosing a topology:

1. Money: A linear bus network may be the least expensive way to install a network; Need not to purchase concentrators.
2. Length of cable needed: The linear bus network uses shorter lengths of cable.
3. Future growth: With a star topology, expanding a network is easily done by adding another concentrators.
4. Cable type: The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.

Hybrid Topology

The hybrid topology scheme combines multiple topologies into one large topology. The hybrid network is common in large wide-area networks. Because each topology has its own strengths weaknesses, several different types can be combined for maximum effectiveness.

1. Advantages of Hybrid topology
   • One company can combine the benefits of several different types of topologies.
   • Workgroup efficiency and traffic can be customized.
2. Disadvantages of Hybrid topology
   • Devices on one topology cannot be placed into another topology without some hardware changes.
3. Examples of Hybrid topology
   • A Company can place its accounting database users on a ring and its secretarial staff on a bus for ease of cabling

Tree Topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network and enable schools to configure a network to meet their needs.

1. Advantages of a Tree Topology
• Point-to-point wiring for individual segments.
• Supported by several hardware and software vendors.
2. Disadvantages of a Tree Topology
   • Overall length of each segment is limited by the type of cabling used.
   • If the backbone line breaks, the entire segment goes down.
   • More difficult to configure and wire than other topologies.
3. Rule (Ethernet)

A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A populated segment is one which has one or more nodes attached to it. The furthest two nodes on the network have 4 segments and 3 repeaters/concentrators between them.

This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling is used.

Ring Topology

The ring topology is a physical, closed loop consisting of point-to-point links. All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. Each node on the ring acts as a repeater. It receives a transmission from the previous node and amplifies it before passing it on.

1. Advantages of Ring topology
   • Each repeater duplicates the data signals so that very little signal degradation occurs.
2. Disadvantages of Ring topology
   • A break in the ring can disable the entire network. Many ring designs incorporate extra cabling that can be switched in if a primary cable fails.
   • Because each node must have the capability of functioning as a repeater, the networking devices to be more expensive.
3. Examples of Ring topology
   • IBM Token Ring (although wired as a Star)
   • Fiber Distributed Data Interface (FDDI).

Star Topology

A star topology is designed with each node (file server, workstations and peripherals connected directly to a central network hub or concentrator.

Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

• All peripheral nodes may thus communicate with all others by transmitting to and receiving from, the central node only.
• The failure of a transmission line, that is, channel linking any peripheral node to the central node will result in the isolation of that peripheral node from all others.
• If the star central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two way transmission time, that is, to and from the central node, plus any delay generated in the central node.
• An active star network has an active central node that usually has the means to prevent echo-related problems.

The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology called the star-wired ring.

1. Advantages of a Star Topology
   • Easy to install and wire.
   • No disruptions to the network when connecting or removing devices.
   • Easy to detect faults and to remove parts.
2. Disadvantages of a Star Topology
   • Requires more cable length than a linear topology.
   • If the hub or concentrator fails, nodes attached are disabled.
   • More expensive than linear bus topologies because of the cost of the concentrators.
3. Examples of Star Topology
   • ARCnet
   • 10Base-T, 100Base-TX
   • StarLAN.

Bus Topology

A linear bus topology consists of a main run of cable with a terminator at each end. All nodes (file server, workstations and peripherals) are connected to the linear cable. Ethernet and LocalTalk networks can use a linear bus topology. All devices are connected to a central cable called the bus or backbone.

1. Advantages of a Bus Topology
   
   • Easy to connect a computer or peripheral to a linear bus.
   • Requires less cable length than a star topology.

2. Disadvantages of a Bus Topology
   • Entire network shuts down if there is a break in the main cable.
   • Terminators are required at both ends of the backbone cable.
   • Different to identify the problem if the entire network shuts down.
   • Not meant to be used as a stand-alone solution in a large building.

3. Example of Bus Topology
   • ARCnet (Token Bus)
   • Ethernet (10Base2).

Physical Topologies

All physical topologies are variations of two fundamental methods of connecting devices. They are:

• Point-to-point
• Multipoint

1. Point-to-point Topology -

Point-to-point (PTP) topology connects two nodes directly together.

The following examples are pure point-to-point links:

• Two computers communicating via modems
• A mainframe terminal communicating with a front-end processor
• A workstation communicating along a parallel cable to a printer.

In a point-to-point link, two devices monopolize a communication medium. Because the medium is not shared, a mechanism is not needed to identify the computers. Therefore, a simple, two device point-to-point network has no need for addressing.

Point-to-point links can be simplex, half-duplex or full-duplex. When devices must engage in bi-directional communication on a half-duplex link, some turnaround mechanisms must be in place to switch the roles of the sending and receiving devices.

2. Multipoint Topology -

Multipoint topologies link three or more devices together through a single communication medium. Multipoint topologies work much like a party-line telephone service where several subscribers are connected to the same telephone line.

Because multipoint topologies share a common channel, each device needs a way to identify itself and the device to which it wants to send information. The method used to identify senders and receivers is called addressing.

The following are different types of physical topologies are frequently used in computer networking:

• Bus
• Star
• Ring
• Tree
• Hybrid.

Network Topologies

Network Topology refers to the shape of network or the network's layout. How different nodes in a network are connected to each other and how they communicate is determined by the network's topology. Two network have the same topology if the connection configuration is the same, although the network may differ in physical interconnections, distances between nodes, transmissions rates and /or signal types.

There are two types of topology: physical and logical.

• The physical topology of a network refers to the configuration of cables, computers and other peripherals.
• Logical topology is the method used to pass the information between workstations.

Relationship of Services to Protocols

Service is a set of primitives (operations) that a layer provides to the layer prepared to perform on above it. The service defines what operations the layer is prepared behalf of its users, but it says nothing at all about how these operations are implemented. A service relates to an interface between two layers, with the lower layer being the service provider and the upper layer being the service user.

A protocol is a set of rules governing the format and meaning of frames, packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols in order to implement their service definitions. They are free to change their protocols at will, provided they do not change the service visible to their users. In this way, the service and the protocol are completely decoupled.

Service Primitives

A service is formally specified by a set of primitives (operations) available to a user or other entity to access the service. These primitives tell the service to perform some action or report on an action taken by a peer entity. Service primitives are divided into four classes. They are:

Operations	Meaning
Request	An entity wants the service to do some work
Indication	An entity is to be informed about an event
Response	An entity wants to respond to an event
Confirm	The response to an earlier request has come back

Connection-Oriented and Connectionless Services

Layers can offer different types of services to the layers above them:

• Connection-oriented
• Connectionless.

In the Connection-oriented service, the service user first establishes a connection, uses the connection and then releases the connection. The essential aspect of a connection is that it acts like a tube: the sender pushes objects (bits) in at one end, and the receiver takes them out in the same order at the other end.

In the Connectionless service, each message carries the full destination address and each one is routed through the system independent of all the others. Normally, when two messages are sent to the same destination, the first one sent will be the first one to arrive. However, it is possible that the first one sent. It can be delayed so that the second one arrives first. With a connection-oriented service this is impossible.

Interfaces and Services

The function of each layer is to provide services to the layer above it. The active elements in each layer are often called entities. An entity can be a software entity ( such as a process ) or a hardware entity ( such as an intelligent 1/0 chip ). Entities in the same layer on different machines are called peer entities. The entities in layer n implement a service used by layer n+1. In this case layer n is called the service provider and layer n+1 is called the service user. Layer n may use the services of layer n-1 in order to provide its service. It may offer several classes of service, for example, fast, expensive communication and slow, cheap communication.

Services are available at SAPs (Service Access Points), the layer n SAP is the places where layer n+1 can access the services offered. Each SAP has an address that uniquely identifies itself. To make this point clearer, the SAPs in the telephone system are the sockets into which modular telephones can be plugged and the SAP addresses are the telephone numbers of these sockets. To call someone, caller's SAP address should be known. Similarly, in the postal system, the SAP addresses are street addresses and post office box numbers. To send a letter, the addressee's SAP address should be known.

In order for two layers to exchange information, there has to be an agreed upon set of rules about the interface. At a typical interface, the layer n+1 entity passes an IDU (Interface Data Unit) to the layer n entity through the SAP. The IDU consists of an SDU (Service Data Unit) and some control information. The SDU is the information passed across the network to the peer entity and then up to layer n+1. The control information is needed to help the lower layer do its job (e.g. the number of bytes in the SDU) but is not part of the data itself.

In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet. The PDU headers are used by the peer entities to carry out their peer protocol. They identify which PDUs contain data and which contain control information, provide sequence numbers and counts, and so on.

Design - Issues for the Layers

Every layers needs a mechanism for identifying senders and receivers. Since a network normally has many computers, some of which have multiple processes, a means is needed for a process on one machine to specify with whom it wants to talk. As a consequence of having multiple destinations, some form of addressing is needed in order to specify a specific destination.

Another set of design decisions concerns the rules for data transfer. In some systems, data only travel in one direction (simplex communication). In others they can travel -in either direction, but not simultaneously (half-duplex communication). In still others, they travel in both directions at once (full-duplex communication). The protocol must also determine how many logical channels the connection corresponds to and what their priorities are. Many networks provide at least two logical channels per connection, one for normal data and one for urgent data.

Protocol Hierarchies

Networks are organized as a series of layers or levels, to reduce their design complexity. The number of layers, the name of each layer, the contents of each layer and the function of each layer differ from network to network.

Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n protocol. Basically, a protocol is an agreement between the communicating parties on how communication is to proceed. The entities comprising the corresponding layers on different machines are called peers. In other words, it is the peers that communicate using the protocol.

In reality, no data directly transferred from Layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower layer offers to he upper one.

A set of layers and protocols is called network architecture. The specification of architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol.

Networking Software

In the beginning computer networks were designed with the hardware as the main concern and the software as a secondary thing. This strategy no longer works. Network software is now highly structured. The following sections describe the software structuring technique.

• Protocol Hierarchies
• Design - Issues for the Layers
• Interfaces and Services
• Connection - Oriented and Connectionless Services
• Service Primitives
• Relationship of Services to Protocols

Routers

A router translates information from one network to another; it is similar to a super intelligent bridge. Routers select the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions and is smart enough to know when to direct traffic along back roads and shortcuts.

While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges and other routers on the network. Routers can even "listen" to the entire network to determine which sections are busiest -- they can then redirect data around those sections until they clear up.

In a school LAN, one needs to purchase a router to connect to the Internet. In this case, the router serves as the translator between the information on the LAN and the Internet. It also determines the best route to send the data over the Internet. Routers can:

1. Direct signal traffic efficiently
2. Route messages between any two protocols
3. Route messages between linear bus, star, and star-wired ring topologies
4. Route messages across fiber optic, coaxial, and twisted-pair cabling.

Bridges

A bridge is a device that allows segmenting a large network into two smaller, more efficient networks.

A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can "listen" to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and if necessary, broadcast it on the other side of the network.

The bridge manages the traffic to maintain optimum performance on both sides of the network. Bridge is like a traffic cop at a busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges can be used to connect different types of cabling, or physical topologies. They must, however, be used between networks with the same protocol.

Repeaters

Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used when the local length of the network cable exceeds the standards set for the type of cable being used.

A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted-pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a multi-port active concentrator. The concentrators amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100 meter limit.

Concentrators / Hubs

A concentrator is a device that provides a central connection point for cables from workstations, servers and peripherals. In star topology, twisted-pair wire is run from each workstation to a central concentrator. Hubs are multi slot concentrators into which can be plugged a number of multi-port cards to provide additional access as the network grows in size. Some concentrators are passive, that is, they allow the signal to pass from one computer to another without any change. Most concentrators are active, that is, they electrically amplify the signal as it moves from one device to another. Active concentrators are used like repeaters to extend the length of a network.
Concentrators are:

1. Usually configured with 8, 12, or 24 RJ-45 ports
2. Often used in a star or star-wired ring topology
3. Sold with specialized software for port management
4. Also called hubs
5. Usually installed in a standardized metal rack that also may store netmodems, bridges, or routers.

Network Interface Cards

The network Interface Card (NIC) provides the physical connection between the network and the computer workstation. Most NIC's are internal, with the card fitting into an expansion slot inside the computer. Some computers, such as Mac Classics, use external boxes which are attached to a serial port. Laptop computers can now be purchased with a network interface card built-in or with network cards that slip into a PCMCIA slot.

Network inteface cards are a manor factor in detrmining the speed and performance of a network.

The three most common network interface connections are Ethernet cards. LocalTalk connectors and Token Ring cards. According to a International Data Corporation study, Ethernet is the most popular, followed by Token Ring and LocalTalk.

1. Ethernet Cards - Ethernet cards are usually purchased separately from a computer, although many computers (such as th Macintosh) now include an option for a pre-installed Ethernet card. Ethernet cards contain connections for either coaxial or twisted pair cables (or both). If it is designed for coaxial cable, the connection will be a BNC. If it is designed for twisted pair, it will have RJ-45 conenction. Some Ethernet cards also contain an AUI connectors. This can be used to attach coaxial, twisted pair or fiber optics cable to an Ethernet card. When this method is used there is always an external transceiver attached to the workstation.
2. LocalTalk Connectors - LocalTalk is Apple's built-in solution for networking Macintosh computers. It utilizes a special adapter box and a cable that plugs into the printer port of a Macintosh. A major disadvantage of LocalTalk is that it is slow in comparison to Ethernet. Most Ethernet connections operate at 10 Mbps (Megabits per second). In contrast, LocalTalk operates at only 230 Kbps (or .23 Mbps).

Ethernet Cards vs. LocalTalk Connections
Ethernet	LocalTalk
Fast data transfer (10 to 100 Mbps)	Slow data transfer (.23 Mbps)
Expensive - purchased separately	Built into Macintosh computers
Requires computer slot	No computer slot necessary
Available for most computers	Works only on Macintosh computers

3. Token Ring Cards - Token Ring network cards look similar to Ethernet cards. One visible difference is the type of connector on the back end of the card. Token Ring cards generally have a nine pin DIN type connector to attach the card to the network cable.