1. 1 Layer 2 Technologies Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

2. 2 Overview n Ethernet, Fiber Distributed Data Interface (FDDI), and Token Ring are widely used LAN technologies n IEEE specifications for these three technologies n LAN standards that specify cabling and signaling at the physical and data link layers n Ethernet troubleshooting

3. 3 Token Ring n Developed by IBM in 1970’s n IEEE 802.5 specification is almost identical to, and completely compatible with, IBM's Token Ring n Either 4 or 16 Mb/s

4. 4 Token Frame Format n 3 bytes in length n The access control byte contains the priority and reservation field

5. 5 Data/Command Frame Format n Data frames carry information for upper-layer protocols n Command frames contain control information and have no data for upper-layer protocols

6. 6 Data/Command Frame Format n Like token frames, data and command frames contain start delimiter, access control, and end delimiter fields

7. 7 Access Method for Token Ring n Token is passed around the ring n Possession of token grants the right to transmit data n If node has no information to send, it passes the token to the next station n Each station can hold the token for a maximum period of time, depending on the specific technology

8. 8 Access Method for Token Ring (cont.) n If a station has information to transmit, it seizes the token and alters 1 bit of it. n The token becomes a start-of-frame sequence, appends information to transmit to the token, and sends data to next station n No token is on the network while information frame is circling the ring, unless the ring supports early token release ( with early token release, a new token can be released when the frame transmission has been completed )

9. 9 Access Method for Token Ring (cont.) n Other stations cannot transmit until token becomes available again n Information frame circulates ring until it reaches intended destination station, which copies the information for processing n Information frame circles the ring until it reaches sending station and is then removed

10. 10 Fiber Distributed Data Interface n Began with ANSI standard that was then internationalized by the ISO n Four specifications - MAC, PHY, PMD, and SMT n Spans the OSI physical layer and MAC sublayer of the data link layer n Dual ring provides fault tolerance n 100 Mb/s

11. 11 Media Access Control (MAC) n defines how the medium is accessed, including:  frame format  token handling  addressing  algorithm for calculating a cyclic redundancy check and error recovery mechanisms

12. 12 Physical Layer Protocol (PHY) n defines data encoding/decoding procedures, including:  clocking requirements  framing  other functions

13. 13 Physical Layer Medium (PMD) n defines the characteristics of the transmission medium, including:  fiber optic link  power levels  bit error rates  optical components  connectors

14. 14 Station Management (SMT) n defines the FDDI station configuration, including:  ring configuration  ring control features  station insertion and removal  initialization  fault isolation and recovery  scheduling  collection of statistics

15. 15 FDDI Token Frame Format

16. 16 FDDI Data Frame Format

17. 17 FDDI Nodes n Single-attachment stations (SAS) attach to the primary ring through a concentrator. n A dual-attachment station (DAS) has two ports, each of which is connected to both rings.

18. 18 FDDI Media n Single mode fiber often used between buildings (inter-building) n Multi-mode fiber often used within a building (intra-building)

19. 19 FDDI Signaling FDDI uses an encoding scheme called 4B/5B: every 4 bits of data are sent as a 5 bit code.

20. 20 Ethernet n CSMA/CD developed and used first at the University of Hawaii in the 1960’s using radio transmissions n Xerox developed first experimental Ethernet system in 1970’s n This formed basis for IEEE 802.3 released in 1980 n Ethernet II developed by DEC, Intel, and Xerox shortly after n Provides services corresponding to layers 1 and 2 of the OSI model n Shared media broadcast technology

21. 21 18 Varieties of Ethernet Here are a few.

22. 22 Ethernet Frame Formats The top format is that of Ethernet II developed or DIX (Digital, Intel, Xerox) format. Note that the two headers have the same length.

23. 23 Errors n A destination Ethernet host checks a frame for errors. If the device detects errors, the frame is discarded. The destination device will not notify the source device regardless of whether the packet arrived successfully or not. n Ethernet is a connectionless network architecture and is referred to as a best-effort delivery system.

24. 24 Ethernet Signaling n Signal encoding is a way of combining both clock and data information into a stream of signals over a medium n Manchester encoding is used (low- to-high is a 1 and high-to-low is a 0) n 10BASE-T transceivers are designed to send and receive signals over a segment that consists of 4 wires

25. 25 Troubleshooting n It is best to start troubleshooting at layer 1. n Then proceed upward through the layers. n Very often difficult problems turn out to be layer 1 issues. Usually troubleshooting internetworking devices is limited to layers 1, 2, and 3.

26. 26 Summary n IEEE defines LAN standards n the term token-ring refers both to IBM's token-ring and to IEEEs 802.5 specification n FDDI has 4 specifications: FDDI has four specifications: MAC, PHY, PMD, and SMT n the term Ethernet is often used to refer to all CSMA/CD LAN’s that generally conform to Ethernet specifications, including IEEE 802.3

27. 27 The End