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.