Overview of OSI - Layer Model

Layer 1 of the layer model is the Physical Layer and defines the physical and electrical characteristics of the network. The NIC cards in the PC and the interfaces on the routers all run at this level and eventually have to pass strings of ones and zeros down the wire.

Layer 2 is known as the Data Link Layer. It defines the access strategy for sharing the physical medium, including data link and media access issues. Protocols such as PPP, SLIP and HDLC exist here.

On an Ethernet, of course, access is governed by a device's MAC address, the six byte number that is unique to each NIC. Devices which depend on the level include bridges and switches, which learn which segment's devices are on by learning the MAC addresses of devices attached to various ports.

This is how bridges are eventually able to segment off a large network, only forwarding packets between ports of two devices on separate segments need to communicate. Switches quickly learn a topology map of the network and can thus switch packets between communicating devices very quickly. It is for this reason that migrating a device between different switch ports can cause the device to lose network connectivity for a while, until the switch, or bridge, re-ARP's.

Layer 3 is the Network Layer, providing a means for communicating open systems to establish, maintain and terminate network connections. The IP protocol exists at this layer and so, do some routing protocols. All the routers in the network are operating at this layer.

Layer 4 is the Transport Layer, and is where TCP exists. The standard says that "The Transport Layer relieves the Session Layer of the burden of ensuring data reliability and integrity". It is for this reason that people are becoming very excited about the new Layer 4 switching technology.

Before these devices became available, only software operated at this layer. Hopefully, you will now also understand why TCP/IP is uttered in one breath. TCP over IP, since Layer 4 is above (over) Layer 3. It is at this layer that, should a packet fail to arrive (perhaps due to misrouting, or because it was dropped by a busy router), it will be retransmitted, when the sending party fails to receive an acknowledgement from the device with which it is communicating.

The more powerful routing protocols also operate here, OSPF and BGP, for example, are implemented as protocols directly over IP.

Layer 5 is the Session Layer. It provides for two communicating presentation entities to exchange data with each other.

The Session Layer is very important in the E-commerce field since, once a user starts buying items and filling their "shopping basket" on a Web server, it is very important that they are not load-balanced across different servers in a server pool. This is why, clever as Layer 4 switching is , these devices still operate software to look further up the layer model. They are required to understand when a session is taking place, and not to interfere with it.

Layer 6 is the Presentation Layer. This is where application data is either packed or unpacked, ready for use by the running application. Protocol conversions, encryption/decryption and graphics expansion all takes place here.

Finally, Layer 7 is the Application Layer. This is where you find your end-user and end-application protocols, such as telnet, ftp and mail (pop3 and smtp).

The Stack Our imaginary listener, eavesdropping on the conversations of network engineers, would hear them refer to IP stacks quite frequently. They are called stacks because, in order to get a packet from an application running on device A to an application running on device B, the packets have to descend and then re-ascent the layers (the stack).

Consider the following example:

An application forms a packet of data to be sent; this takes place at Layer 7. As the packet descents the stack, it is wrapped in headers and trailers, as required by the various protocols, until, having reached Layer 1, it is transmitted as a frame across the medium in use.

Upon reaching device B, it reascends the stack, as the device strips off the appropriate headers and trailers, delivering just the application data to the application. The OSI tried to keep to as few layers as possible for the sake of simplicity. The fact that the 7-Layer model is universally used to describe where a device or protocol sits in the scheme of things shows that the designers did an excellent job of achieving their aims.

Bridges, switches and most network devices keep a table mapping IP addresses to Media Access addresses. Moving a device between ports invalidates these tables and hence the device's view of the world.

Fortunately, the devices age their table entries, typically clearing them out five minutes after the last time a packet was seen from a particular entity. This is sometimes called re-ARPing. Most bridges and switches provide management functions to allow you to clear the ARP entry manually, should you have needed to move a device due to a failed port.

Overview of OSI

The ISO (International Standards Organization) has created a layered model, called the OSI (Open Systems Interconnect) model, to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions that can improve Internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.

The OSI Reference model defines seven layers that describe how applications running upon network-aware devices may communicate with each other. The model is generic and applies to all network types, not just TCP/IP and all media types, not just Ethernet. It is for this reason that any network technician will glibly throw around the term "Layer 4" and expect to be understood.

It should be noted however, that most protocols in day-to-day use work on a slightly modified layer system. TCP/IP, for example, uses a 6-rather that a 7-layer model. Nevertheless, in order to ease the exchange of ideas, even those who only ever use TCP/IP will refer to the 7-layer model when discussing networking principles with peers from a different networking background.

Confusingly, the OSI was a working group within the ISO (International Standards Organisation) and therefore,many people refer to the OSI Reference model as the ISO Reference model. They are referring to the same thing.

Traditionally, layer diagrams are drawn with Layer 1 at the bottom and Layer 7 at the top. The remainder of this article describes each layer, starting from the bottom and explains some of the devices and protocols that may be found in the data centre operating at this layer.

Hubs

Provide full bandwidth to each client, unlike BUS networks where the bandwidth is shared. Often include buffering of packets and filtering, so that unwanted packets (or packets which contain errors) are discarded. In standard ethernet, all stations are connected to the same network segment in bus configuration. Traffic on the bus is controlled using the CSMA protocol and all stations share the available bandwidth.

Hubs dedicate the entire bandwidth to each port (workstation). The workstations attach to the hub using UTP. The hub provides a number of ports, which are logically combined using a single backplane, which often runs at a much higher data rate than that of the ports. Ports can also be buffered, to allow packets to be held in case the hub or port is busy. As each workstation has their own port, they do not contend with other workstations for access, having the entire bandwidth available for their exclusive use.

The ports on a hub all appear as one single ethernet segment. In addition, hubs can be stacked or cascaded (using master /slave configurations) together, to add more ports per segment. As hubs do not count as repeaters, this is a better option for adding more workstations than the use of a repeater.

Hub options also include SNMP (Simple Network Management Protocol) agent. This allows the use of network management software to remotely administer and configure the hub. Detailed statics related to port usage and bandwidth is often available, allowing informed decisions to be made concerning the state of the network.

Routers

Packets are only passed to the network segment they are destined for. They work similar to bridges and switches in that they filter out unnecessary network traffic and remove it from network segments. Routers generally work at the protocol level. Routers were devised in order to separate networks logically. For instance, a TCP/IP router can segment the network based on IP subnets. Filtering at this level ( on IP addresses) will take longer than that of a bridge or switch which only looks at the MAC layer.

Most routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving the network.

Typically, an organization which connects to the Internet will install a router as the main gateway link between their network and the outside world. By configuring the router with access lists (which define what protocols and what hosts have access ) this enforces security by restricted (or allowing) access to either internal or external hosts.

For example, an internal WWW server can be allowed IP access from external networks, but other company servers which contain sensitive data can be protected, so that external hosts outside the company are prevented access.