Chapter 7

 

1.      Define the following parameters for a switching network:

· N: number of hops between two given stations

·L: message length, in bits

·B: data rate, in bps, on all links

·P: packet size, in bits

·H: overhead (header), in bits per packet

·S: call setup time (circuit-switched or virtual circuit), in seconds

·D: propagation delay per hop, in seconds

 

  1. For N = 4, L = 3200, B = 9600, P =1024, H = 16, S = 0.2, D =0.001,

Compute the end-to-end delay for circuit switching, virtual circuit packet switching, and datagram packet switching. Assume that there is no node delay.

 

2.      Consider a packet-switched network of N nodes, connected by the following topologies:

 

a. Star: one central node with no attached station; all other nodes attach to the central    

          node.

      

  1. Full connected: each node is directly connected to all other nodes.

For each case, give the average number of hops between stations.

 

Answer:

  1. Each connection takes 2 hops - thus average path length is 2.

c.   One hop

 

3.      Flow control mechanisms are used at both levels 2 and 3 of X.25. Are both necessary,

Or is this redundant? Explain.

Layer 2 flow control regulates total flow of data between DTE and DCE.

Layer 3 flow control regulates flow over a single virtual circuit.

 

4.      In X.25, the virtual-circuit number used by one station of two communicating stations

Is different from the virtual-circuit number used by the other station. Why would this

Be so? After all, it is the same full-duplex virtual circuit.

      The virtual circuit number only had local significance (no global management of

       Numbers).