1 6/27/ :02 Chapter 15LAN Performance1 Rivier College CS575: Advanced LANs Chapter 15: LAN Performance

2 6/27/ :02 Chapter 15LAN Performance2 LAN Performance Overview 0 Measures of Performance 0 Effect of Propagation Delay and Transmission Rate 0 Factors That Affects Performance 0 Bounds of Performance 0 Comparative Performance of Token Passing and CSMA/CD 0 Behavior of Contention Protocols

3 6/27/ :02 Chapter 15LAN Performance3 Measures of Performance 0 [D]: The delay that occurs between the time a packet or frame is ready for transmission from a node, and the completion of successful transmission. 0 [S]: The throughput of the LAN; the total rate of data being transmitted between nodes (carried load). 0 [U]: The utilization of the LAN medium; the fraction of total capacity being used.

4 6/27/ :02 Chapter 15LAN Performance4 Ideal Channel Utilization

5 6/27/ :02 Chapter 15LAN Performance5 Measurements of Performance

6 6/27/ :02 Chapter 15LAN Performance6 Representative Values of a and U a = Propagation time/Transmission time; Maximum possible utilization, U = Throughput/Data rate = 1/(1 + a)

7 6/27/ :02 Chapter 15LAN Performance7 The Effect of a on Utilization for Baseband Bus

8 6/27/ :02 Chapter 15LAN Performance8 Utilization U as a Function of a

9 6/27/ :02 Chapter 15LAN Performance9 Effect of a on Throughput S

10 6/27/ :02 Chapter 15LAN Performance10 Overhead 0 Administrative Overhead for controlling protocols includes address and synchronization bits. 0 In contention protocols (ALOHA, S-ALOHA, CSMA, CSMA/CD): Time wasted due to collisions; need for acknowledgement frames. S- ALOHA requires that slot size equals transmission plus maximum propagation time. 0 In Token Ring: Time waiting for token if intervening stations have no data to send. 0 In Token Bus: Time waiting for token if logically intervening stations have no data to send; token transmission; acknowledgement frames. 0 Explicit reservation: Reservation transmission, acknowledgements. 0 Implicit reservation: Overhead of protocol used to establish reservation, acknowledgements.

11 6/27/ :02 Chapter 15LAN Performance11 LAN Performance: Bounds on Performance 0 Regions, based on the magnitude of the offered load: 1. A region of low delay through the network, where the capacity is more than adequate to handle the load offered. 2. A region of high delay, where the network becomes a bottleneck. 3. A region of unbounded delay, where the offered load exceeds the total capacity of the system. 0 Maximum possible throughput: THRU < N active /(T idle + T msg ) 0 The breakpoint between these two bounds occurs at: N active /(T idle + T msg ) = 1/ T msg or N active = (T idle + T msg )/T msg 0 For a lower bound on delay: T delay > T msg or T delay > N active T msg - T idle

12 6/27/ :02 Chapter 15LAN Performance12 Performance of Token Passing and CSMA/CD 0 For Token Ring: Throughput S = 1/(1 + a > 1, a < 1 S = 1/a(1 + a > 1 and S = N >> 1, a > 1 Delay D = N + a – a < 1 D = a >1 0 For CSMA/CD: S = 1/[1 + 2a(1-A)/A] and S = 1/(1 + N >> 1; A = (1 – 1/N)^(N – 1) Delay for CSMA/CD is difficult to express and depends on the exact nature of the protocol (persistence, retry policy).

13 6/27/ :02 Chapter 15LAN Performance13 Performance Analysis Conclusions 0 For the given parameters, the smaller the mean frame length, the greater the difference in maximum mean throughput rate between token passing and CSMA/CD. This reflects the strong dependence of CSMA/CD on a. 0 Token ring is the least sensitive to workload. 0 CSMA/CD offers the shortest delay under light load, while it is most sensitive under heavy load to the workload.