Token Passing Protocol

Token Passing Protocol

The Token-Passing Protocol relies on a control signal called the token. A token is a 24-bit packet that circulates throughout the network from NIC to NIC in an orderly fashion. If a workstation wants to transmit a message, first it must seize the token. At that point, the workstation has complete control over the communications channel. The existence of only one token eliminates the possibility of signal collisions. This means that only one station can speak at a time.

 Logical Ring Physical Star topology for Token-Passing Standard.

It is sure that any break in the ring at any point will interrupt communications for all machines. To solve this problem, IBM developed a modified ring topology, which they called the logical ring physical star. The central point of the physical star configuration is Token Ring hub called the multi-station access unit (MSAU, pronounced as masow).

Workstations and servers attached to the MSAU through special STP adapter cables. IBM converted stars into a logical ring by connecting all MSAU hubs together through special ring-in (RI) and ring-out (RO) ports.

Advantages of Token Ring.

Here are Token ring’s most useful advantages:

a. It offers excellent throughput under high-load conditions.

b. Token Ring facilitates LAN-to-LAN mainframe connections especially for interfacing with IBM’s broader connectivity strategies.

c. It has built-in troubleshooting mechanisms such as beaconing and auto-reconfiguration and may now be used with UTP cabling.

d. It has the most reliable protocol (token-passing), the most trouble-free configuration (physical star) and the fastest connectivity scheme (r or 16 mb/s).

Disadvantages of Token Ring. 

Few of the disadvantages of Token Ring are:

a. Token Ring is very expensive. All topology components cost much more than other more popular standards.

b. It is relatively proprietary. Token Ring’s complexity is built into the hardware components. This means hat you need to choose a manufacturer and stick with it.

c. Engineers must have considerable expertise to manage and troubleshoot token ring components.

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