Question
1. (50 points) Consider two serially connected packet switches (ie, routers) as shown in the figure below. The link connecting the two switches (Link I) is full-duplex, with a data rate of I Mbps and propagation delay of 3 ms (in each direction). The packets sent through the network are 1000 bits long The input bufferiqueue of Switch 2 can store at most 100 packets. To control congestion (means t?kaniklk in Turkish) and avoid packet loss, the switches employ the so-called 'back pressure mechanism. According to this mechanism, whenever Switch 2 detects congestion on its outgoing link (i.e, no more packets can be sent over Link 2), Switch 2 sends a signal, which has negligible size, back to Switch 1 instructing Switch I to stop further packet transmission over Link I. Assumptions I. There is an unlimited number of packets a Switch therefore, when active, Switeh I sends packet continuously, back-to-back 2. Once detected, the congestion on Link 2 could be prevented at any point in time. Hence, the back pressure signal should not be sent too early (unless there is a real risk of Switch 2 running out of buffer space), nor too late (no packet should ever be lost/dropped) Switch 1 Switch 2 Link 2 Link 1 a) Determine how big (to prevent packet loss, yet allow its buffer to grow to maximally allowable size) Switch 2 should let its buffer grow (in case of congestion on Link 2), before sending a back pressure signal to Switch I. (Ge. When should Switch 2 send a back pressure signal to Switch 1?) Select one of the choices below and explain your choice with a single sentence. 9) when the input buffer/queue of Switch 2 has exactly 100 packets b) when the input buffer/queue of Switch 2 has less than 100 packets c) when the input buffer/queue of Switch 2 has more than 100 packets d) sending time of the back pressure signal does not depend on the Switch 2 number of packets in the input bufferqueue of
Explanation / Answer
Choice (b) is most appropriate for given situation ; Explanation is as follows
Given the switch 2 buffer size is 100 and in back pressure mechanism when number of packet inflow crosses the buffer limit switch drops some packet and sends a back pressure message to previous node to slow the data transfer rate. So for buffer input more than or exact 100 packet will leads to few packet loss (due to packet dropping at switch 2)
To preotect packet from "Packet Dropping" and congestion back pressure signal can be generated when input/Buffer queue is less that 100 Packet.
