Token Ring Technical Overview

 <  Free Open Study  >  

The IEEE officially adopted Token Ring as IEEE 802.5. The specifications are almost identical, with a few minor differences. IBM Token Ring calls for the stations to be attached by twisted-pair cable to a multistation access unit (MSAU) in a physical star topology. The IBM 8228 is a common example of a MSAU. The IEEE committee chose not to specify a medium or topology, allowing Token Ring networks to be more flexible.

Token Ring/IEEE 802.5 operates strictly at Layer 2, like most LAN protocols. Like Ethernet, the IEEE committee divides the data link layer into two sublayers : 802.5 as the MAC layer and 802.2 as the LLC layer. Functionally, IEEE 802.5 Token Ring parallels IEEE 802.3. The LLC layer ”802.2, in this case ”is a standardized interface between a hardware-specific MAC and a Layer 3 protocol.

Token Ring Operation

Token Ring networks have a physical topology of a star, but the network is treated as a logical ring. Figure 2-17 shows the logical and physical Token Ring topologies.

Figure 2-17. Token Ring Topologies

graphics/02fig17.gif

Token Ring networks utilize a token-passing technology on the ring. Token-passing networks move a small frame, called a token, around the ring. Token Ring uses two types of frames, a token and an information/data frame. Token frames contain a start delimiter, an access control field, and an end delimiter . The data/command frames contain the same fields, plus a few more for user data, and also contain a source and destination address. When a station wants to transmit information, it must posses the token. Possession of token grants the right of the station to transmit data. If the station has no data to transmit, it simply passes the token along to the next station on the ring.

When a station posses the token and wants to transmit, it changes a single bit in the access control field, the T bit. The station then appends its information to the frame and sends it on to the next station on the ring. The frame circulates the ring until it reaches the destination station, where the frame is then copied by the station and tagged as having been copied . The frame continues around the ring until it reaches the station that originated the frame. At this point, it is removed from the ring by that station. The concept of early token release allows a station that seizes a token to transmit a new token onto the ring after first sending its information frame.

The following list describes some other important features that Token Ring employs to help data circulate throughout the ring:

  • Access control field ” The access control field is an 8-bit field found in both types of Token Ring frames, the token and information/data frame. It contains the following information:


    graphics/02fig17a.gif


    - P ” 3-bit priority field. Only stations with a priority equal to or higher than the priority of a token can claim that token. After the token is claimed and changed to an information frame, only stations with a priority higher than that of the transmitting station can reserve the token for the next pass around the network.

    - R ” 3-bit reservation field used to reserve the token for the next pass around the ring.

    - T ” Simply called the T bit. If the bit is set to 0, the frame is a token; if it is set to 1, the frame is an information/data frame.

    - M ” Monitor bit, used by the active monitor to remove endlessly circulating frames from the ring.

  • Active monitor ” One station on the network serves as the active monitor. This station acts as a centralized source of timing information for other stations and performs a variety of ring-maintenance functions. One such function is the removal of endlessly circulating frames from the ring. For example, when an originating station fails, it cannot remove its frame from the ring. This, in turn , will not allow any other stations to transmit on the ring. The active monitor can detect this type of failure through the use of the M bit mentioned previously and can properly remove the frame from the ring.

  • Reliable delivery ” Token Ring uses a 2-bit frame status field to ensure reliable delivery of frames. These bits commonly are referred to as the A and C bits. An originating station generates a frame with the A and C bits set to 0. When the frame transverses the ring and arrives at the originating station, the bits are examined to verify delivery. Receiving stations modify the bits as documented in Table 2-15.

Table 2-15. A and C Bit Modification by Receiving Station
A Bit C Bit Meaning
Destination not found. The receiving station did not modify the bits.
1 Invalid.
1 The frame was accepted, but the station was incapable of copying data from the frame.
1 1 Station found. The frame was accepted and copied.
  • Token Ring frame size ” Token Ring frames have a significantly larger frame size than the 1518-byte Ethernet frame. Token Ring frames have a minimum size of 21 bytes at 4-Mbps and 16-Mbps ring operation. The largest frame size is 4511 bytes on 4-Mbps rings and 17,839 bytes on 16-Mbps rings.

NOTE

Canonical Versus Noncanonical Address Format

Ethernet networks transmit data in what is called a canonical address format. This means is that if you have the bit stream 0110 1010, the most significant bit (MSB) is on the left and the least significant bit (LSB) is on the right. Ethernet networks transmit data in a canonical form, with the least significant bits get transmitted first. If an Ethernet network is sending the previous data steam , it would send 0 1 0 1 0 1 1 0. Token Ring and FDDI networks transmit data in a noncanonical format. Noncanonical format transmits the most significant bit first. If a Token Ring network were transmitting the original bit stream mentioned, it would transmit 0 1 1 0 1 0 1 0. Source-route translation bridges and DLSw will perform address translation when needed.


 <  Free Open Study  >  


CCIE Practical Studies, Volume I
CCIE Practical Studies, Volume I
ISBN: 1587200023
EAN: 2147483647
Year: 2001
Pages: 283
Authors: Karl Solie

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net