Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics
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Token Ring MAC
Token Ring is the second most popular MAC layer protocol (Exhibit 3-3-6). It is based on a deterministic token passing algorithm and conforms to the IEEE 802.5 specification. The Token Ring protocol is more complex than its 802.3 counterpart. Due to the nature of normal operation, three different frame formats are required.
Exhibit 3-3-6. IEEE 802.5 Frame Structures
The starting and ending delimiters are used for error detection and definition of the frame boundaries. The access control field is used to set up a proprietary access and reservation system. A workstation on a LAN segment uses the priority and reservation mechanisms to capture the token as it circulates around the ring. Once the ring is captured, a variable length frame type containing user data, LLC information, and source route information is generated by the workstation. Token Ring is a deterministic protocol that uses a rules-based access mechanism as the core structure of the protocol. The same consideration applies to FDDI.
The first two fields are taken directly from the token and are used for access control as the frame circulates around the ring. The frame control field identified whether a LLC frame transporting user data or a MAC frame carrying ring management data is sent. The source and destination field are similar to the 802.3 frames and contain the respective MAC layer addresses.
The variable length route information field (RIF) composed of route control information and routing data handles the transport of data between LAN segments (through a process called source route bridging). This particular field is discussed in greater detail in the source route bridging section. The information field contains the LLC protocol data unit (the 802.2 sublayer in user frames) and additional ring management information for MAC frames. As with the 802.3 frame, the 802.5 frame makes use of the LLC link sublayer to define source and destination service access pints (SSAP and DSAP) to the network layer protocol.
The 802.2 and subnetwork access protocol (SNAP), illustrated in Exhibit 3-3-7, has been defined as an extension to the 802 MAC and 802.2 LLC layers. The reason for the additional link level frame extension is to support the encapsulation of IP and address resolution protocol (ARP) datagrams in the 802 frames. The SNAP subheader adds a protocol ID similar in function to the original Ethernet type field and an Ethertype field specifying further framing information. Many of the higher level protocols (e.g., AppleTalk, NetWare, and TCP/IP) either require or can invoke framing options to support the SNAP frame extensions. The SNAP protocol is also compatible with the ANSI FDDI frame format. The 802.2 and SNAP extensions permit the movement of data between different MAC layer protocols (e.g., Ethernet, Token Ring, and FDDI).
Exhibit 3-3-7. IEEE 802.2 and SNAP Frame Structures
FDDI MAC
The FDDI standard American National Standards Institute (ANSI) X3T.9 is a layered protocol that uses a framing format similar to Token Ring. The operation of the FDDI ring differs from Token Ring in that it is based on a dual counterrotating ring that can have a circumference of 200 km while transmitting data at 100M bps. Fault tolerance is a result of the dual-ring architecture. Limits are more typically 100 km and 500 devices, to ensure proper operation in failure mode. Internode distance limits of 2 km for multimode fiber and 10 km for single-mode fiber are also in the specification.
The physical (PHY) layer concerns itself with the actual encoding schemes for data. The physical medium dependent (PDM) layer controls the 1,300-nm light-emitting idodes (LEDs) used to generate light pulses over the optical fiber.
The media access control (MAC) and link level control (LLC) layers handle token passing as well as frame formation and addressing. The LLC protocol used on FDDI networks is based on the 802.2 specification, easing integration with other MAC-level LAN protocols.
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