• Collection of LAN standards
• Subdivides data link layer into
• LLC : Logic Link Control (upper) sublayer; architecture independent
• MAC: Media Access Control (lower) sublayer; several recognized technologies
• Major sections are:
• 802.1 : internetworking standards (network layer)
• 802.2 : LLC specifications
• 802.3 : Ethernet specifications
• 802.4 : Token Bus specifications
• 802.5 : Token Ring specifications

• High-level Data Link Control (see chapter 11)
• Reference model for bit-oriented frame structures
• Supports half and full duplex
• Supports point-to-point or multipoint
• Frame contains up to six fields

• Independent of LAN technology used
• Frame structure based on HDLC
• Frame structure called PDU (protocol data unit)

802.3 : Ethernet details

• Not as popular as ethernet and token ring.
• A token is like a potty pass. The network only has one, and no station can transmit until it possesses the token. If token passes by and the station is ready to transmit, it grabs the token, transmits, then releases the token.
• Developed at GM for factory automation (real-time). Emphasis on:
• Token: guaranteed limit on delay, thus token with priorities. can't risk ethernet.
• Bus: minimize damage of cut cable, thus bus rather than ring.
• Coax cabling, with analog signalling using amplitude + phase modulation.
• Basically, broadcast token on the bus, and devise elaborate protocols to assure integrity of token and network, and prioritized delivery.
• Logical topology is ring! Stations are sequenced, and each station knows sequence number of its two neighbors. Protocols are very complex.

802.5 : Token Ring Details

• Fiber Data Distributed Interface
  
• FDDI is not part of IEEE 802.
  
• Token ring based on fiber optic cabling. Transmisson rate is 100Mbps.
  
• Fiber is unidirectional; stations connected by pair of fibers transmitting in opposite directions. "inner ring" normally not used. In case of cut or station failure, stations on either side close a switch connecting the pair: loops back to re-form the ring!
  
• Transmission protocol similar to token ring. Uses "early token release"; token is released as soon as last bit transmitted -- doesn't wait until last bit drained. Why? To achieve higher fiber utilization -- if error occurs (highly-unlikely on fiber), re-transmit on next go-around.
  
• frame format nearly identical to token ring
  
• uses NRZ-I, 4-out-of-5 digital signal. Each 4 bit combination has 5 bit encoding having maximum 2 consecutive 0 bits (synch aid). Leaves some 5-bit codes available for control use.

• Asynchronous Transfer Mode.
  
• Intended for LAN/WAN use
  
• Switch-centered technology with fiber optic cabling. Transmission rate 155Mbps. Each switch acts as the hub of a star.
  
• Two fibers connect station to switch, or switch to another switch (one for each direction).
  
• Key is high-speed routing (switching) of small frames called cells. Allows multiplexing of different kinds of data to support real-time multimedia.
  
• Frame contains 5 byte destination address and 48 byte data
  
• Also see Chapter 18. We’ll cover if time allows.

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Peter Sanderson ( pete@csc.smsu.edu )