1 Layer 2: Concepts Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

2 Overview n Access to the networking media n Physical addressing n Error notification, network topology, and flow control n How the data link layer provides reliable transit of data across a physical link by using the Media Access Control n Technology-independent IEEE LLC standard n Grouping of bits - framing

3 IEEE n Institute of Electrical and Electronic Engineers n Professional organization that defines network standards n Standards include IEEE 802.2, IEEE 802.3, and IEEE n Best known LAN standards in the world today n Only involves the two lowest layers n Divides the OSI layer 2 into the MAC sublayer (transitions down to media) and the LLC sublayer (transitions up to network layer)

4 Logical Link Control n Allows part of the data link layer to function independently from existing technologies n Participates in the encapsulation process - there is an LLC PDU n Adds two addressing components of the specification - the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). n Repackaged IP packet passed to MAC sublayer for handling by the required technology.

5 Media Access Control n Coordinates access to specific physical media. n Controls access to media; example access methods are CSMA/CD and token passing. n Used to determine which host has access to the media at any given point in time.

6 How NIC’s use MAC’s n When a source device sends data out on an Ethernet network, the data carries the MAC address of its intended destination. As this data propagates along the network media, the NIC in each device on the network checks to see if its MAC address matches the physical destination address carried by the data packet. If there is no match, the NIC discards the data packet. n As data travels along the wire, the NIC in each station checks it. The NIC verifies the destination address in the frame header to determine if the packet is properly addressed. When the data passes its destination station, the NIC for that station makes a copy, takes the data out of the envelope and gives it to the computer.

7 Framing n A frame is layer 2 PDU. n Framing allows for: determining which computers are communicating with one another, when communication between individual computers begins, and when it terminates n Allows for dealing with errors that occurred during the communication

8 Frame Formats n The portions of a frame are called fields. n Each field is composed of bytes. "Here comes a frame!" Source and Dest.

9 Frame Formats (cont.) n In some technologies, a length field specifies the exact length of a frame. Some have a type field, which specifies the Layer 3 protocol making the sending request. There is also a set of technologies where no such fields are used.

10 Frame Formats (cont.) n There are three primary ways to calculate the Frame Check Sequence number:  cyclic redundancy check (CRC) - performs polynomial calculations on the data  two-dimensional parity - adds an 8 th bit that makes an 8 bit sequence have an odd or even number of binary 1's  Internet checksum - adds the values of all of the data bits to arrive at a sum

11 Summary n IEEE defines LAN standards. n IEEE divides OSI Layer 2 into LLC and MAC sublayers. n MAC controls access to media - deterministic or otherwise. n Framing organizes bit stream - frame format has fields containing bytes of various types of information.

12 The End