Token Ring and Token Bus


Token bus(802.4):

       Token bus was standardized by IEEE standard 802.4. It is mainly used for industrial applications. Token bus was used by GM (General Motors) for their Manufacturing Automation Protocol (MAP) standardization effort.

       This is an application of the concepts used in token ring networks. The main difference is that the endpoints of the bus do not meet to form a physical ring.

       The IEEE 802.4 Working Group is disbanded. In order to guarantee the packet delay and transmission in Token bus protocol, a modified Token bus was proposed in Manufacturing Automation Systems and flexible manufacturing system



Token ring(802.5):

       A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time.

       The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second.


       Very briefly, here is how it works:

       1. Empty information frames are continuously circulated on the ring.
       2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
       3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
       4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
       5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.



Types of Ethernet:

Fast Ethernet:

       Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the 100 megabit Ethernet standards 100baseTX (T="Twisted" Pair Copper) is by far the most common and is supported by the vast majority of Ethernet hardware currently produced.

       Full duplex fast Ethernet is sometimes referred to as "200 Mbit/s" though this is somewhat misleading as that level of improvement will only be achieved if traffic patterns are symmetrical. Fast Ethernet was introduced in 1995 and remained the fastest version of Ethernet for three years before being superseded by gigabit Ethernet.

Gigabit Ethernet:

       Gigabit Ethernet is the latest version of Ethernet. It offers 1000 Mbps ( 1 Gbps ) raw bandwidth, that is 100 times faster than the original Ethernet, yet is compatible with existing Ethernets, as it uses the same CSMA/CD and MAC protocols. When Gigabit Ethernet enters the market it will compete directly with ATM.




Comparison of 802.3, 802.4, 802.5:

802.3 (Ethernet)
– pros: popular, simple, reliable
– cons: non-deterministic, no priorities, min frame size

802.4 (Token Bus)
– pros: reliable equipment, more deterministic, priorities
– cons: complex protocols, hard to implement in fiber,not popular

802.5 (Token Ring)
– pros: fully digital, cheap to install, priorities
– cons: delay at low load, monitor is critical component
• Usually, all perform roughly the same

   802.3, 802.4, 802.5 not good for MAN
 –cable length limitations
 –thousands of stations degrade performance 


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