14:332:423            Telecommunication Networks

Homework #1
(to be discussed in the recitation on September 23, 2005)

1. [15 points]   Suppose two nodes, sender A and receiver B, communicate using stop-and-wait protocol. Show that the receiver B will never be confused about the packet duplicates if the following is true:
   Subsequent packets are alternately numbered with 0 or 1 and there is only a single path between the sender and the receiver.

   In case there are multiple paths between the sender and the receiver and subsequent packets are numbered with 0 or 1, show step-by-step an example scenario where receiver B cannot distinguish between an original packet and its duplicates.




2. [25 points]   Assume the following system characteristics:
   The link transmission speed is 1 Mbps; the physical distance between the hosts is 300 m; the link is a copper wire with signal propagation speed of 2 × 10⁸ m/s. The data packets to be sent by the hosts are 2 Kbytes each, and the acknowledgement packets (to be sent separately from the data packets) are 10 bytes long. Each host has 100 packets to send to the other one. Assume that the transmitters are somehow synchronized, so they never attempt to transmit simultaneously from both endpoints of the link.

   Each sender has a window of 5 packets. If at any time a sender reaches the limit of 5 packets outstanding, it stops sending and waits for an acknowledgement. Since there is no packet loss (as stated below), the timeout timer value is irrelevant. This is similar to a Go-Back-N protocol, with the following difference.

   The hosts do not send the acknowledgements immediately upon a successful packet reception. Rather, the acknowledgements are sent periodically, as follows. At the end of an 81 msec period, the host examines whether any packets were successfully received during that period. If one or more packets were received, a single acknowledgement packet is sent to acknowledge all of those received packets. Otherwise, no acknowledgement packet is sent.
   
   

   Consider the two scenarios depicted in the figures (a) and (b) below. The router in (b) is 150 m away from either host, i.e., it is located in the middle. If the hosts in each configuration start sending packets at the same time, which configuration will complete the exchange sooner? Show the process.

   Assume no loss or errors on the communication links. The router buffer size is unlimited for all practical purposes and the processing time at the router is approximately equal to zero seconds. Notice that the router can simultaneously send and receive packets on different links.



3. [10 points]   Assume that the network configuration described in Example 4.1 in the class notes (available here) runs the Stop-and-Wait protocol. The signal propagation speed for both links is 2 × 10⁸ m/s. The length of the link from the sender to the router is 100 m and from the router to the receiver is 10 km. Determine the sender utilization.

Note 2: Please bring your own printed copy of the homework assignment, so the TA doesn't need to write the problem statement on the board.