Layer 2 Technologies

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

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

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

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

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

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

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

8. Access Method for Token Ring (cont.) • If a station has information to transmit, it seizes the token and alters 1 bit of it. • The token becomes a start-of-frame sequence, appends information to transmit to the token, and sends data to next station • 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. Access Method for Token Ring (cont.) • Other stations cannot transmit until token becomes available again • Information frame circulates ring until it reaches intended destination station, which copies the information for processing • Information frame circles the ring until it reaches sending station and is then removed

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

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

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

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

14. Station Management (SMT) • 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. FDDI Token Frame Format

16. FDDI Data Frame Format

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

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

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

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

21. 18 Varieties of Ethernet Here are a few.

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. Errors • 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. • Ethernet is a connectionless network architecture and is referred to as a best-effort delivery system.

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

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

26. Summary • IEEE defines LAN standards • the term token-ring refers both to IBM's token-ring and to IEEEs 802.5 specification • FDDI has 4 specifications: FDDI has four specifications: MAC, PHY, PMD, and SMT • 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. The End