Overview
Token rings were used for some of the earliest local area networks andthey are still widely used today, although not nearly as commonly as Ethernet type networks. They are typically very reliable and have very good performance characteristics.
Objectives
- Understand the fundamentals of the token-ring architecture.
- Know the basic properties of the IEEE 802.5 standard.
- Know the basic properties of the FDDI standard.
- Understand the simple access algorithm for token rings.
- Understand the performance characteristics of token rings.
Preparation
Study sections 2.7 in the book and the notes.
Knowledge, Comprehension and Problem Solving
Terms
- Access Control Byte
- Differential Manchester Encoding
- Fiber Distributed Data Interface (FDDI)
- Frame Control Byte
- IEEE 802.5
- Token bit
- An FDDI frame has 21 bytes of overhead. If the average payload is 100 bytes, what is the utilization of a network with 10 stations that has 1 station ready to send at any one time. Ignore bit and progagation delays and assume a data rate of 100 Mbps.
- How long in meters is a bit on a 10 Mbps token ring network?
- If the token ring is 1000 m in length, will it hold a message of 1000 bits?
- What happens when the sending station starts receiving the bits that it previously sent.
- A major issue with token ring networks is "ring maintenance" - tasks that have to be accomplished either manually or automatically to insure proper operation. List 5 things that have to be considered that are not a problem on a CSMA/CD network.
Analysis, Synthesis & Evaluation
- What is meant by the statement that "a token-ring has to be at least as long as its token"?
- Consider an IEEE 802.5 token ring network with a sending probability for each station of 0.10, and an average message length of 1024 bits. Calculate the utilization for N = 5, 10, 15, 20, 25 and 30. What does this indicate? Compare this to the results of the same problem in the unit on CSMA and analyze. Assume a data rate of 10 Mbps and ignore bit and propagation delays.
- What happens on a token ring if the node holding the token stop sending for some reason? Be specific about how the ring would recover from this problem?
- What happens when it starts sending again, thinking that it has the token?
- What are the impacts of using a long THT time in a token-ring network versus a shorter THT time. How would you decide which to use and why?