The propagation delay on each of the three links is $1$ ms$,$ and each is a $100$ Mbps link.

Computer A transmits $100$ byte network layer PDUs to B$.$ The DLC in use at all nodes

for each interface adds a $25$ byte overhead to each SDU. The communication between

DLC and network layers is via a shared memory which can be read from and written to at

$1$ Gbps$.$ The DLC software must completely read a packet from shared memory before

transmitting it on the bitpipe, similarly the network layer software must completely read a

packet from shared memory before forwarding it$,$ and each layer must wait until a packet

is completely available in shared memory before starting to read it$.$

Draw a timing diagram to show the progress of a single PDU originating at the network

layer of A and destined for the network layer of B$,$ under the following cases $($a$)$ and $($b$).$

In each case, the timing diagram will have the general nature of the one shown in the

lecture slide $$ your solutions should be in the same form $($the two cases for Parts $($a$)$ and

$($b$)$ below will, of course, be different$)$ During the period under

consideration, the system can be assumed to be is idle other than the single PDU we

consider and other PDUs as specifically stated below, and the transmission of no other

PDU need be shown.

$($a$)$ N$1$ and N$2$ are routers, and forward packets at Layer $3.$ The store$-$and$-$forward

process at the network layer software takes $5$ ms at each of N$1$ and N$2.$ Other

characteristics of N$1$ and N$2$ are all same as A and B$.$

$($b$)$ N$1$ and N$2$ are simply electrical junction wires.

$($c$)$ N$1$ is a router as in Part $($a$),$ but N$2$ is a bridge, with two different DLC protocols on

the incoming and outgoing interfaces, such as a WiFi$-$Ethernet bridge. At N$2,$ all

characteristics are the same as A$,$ B and N$1,$ but it buffers frames as they are received

from the physical layer prior to retransmitting them, and at the time the PDU of interest

arrives at the shared memory buffer of N$2$s Layer $2,$ there are five other similar frames

already buffered in that buffer.