Solutions to Homework #2

Chapter 3: 21, 22, 23

21. (10 pts) An Ethernet frame has a minimum frame size (5 pts) of 64 bytes; it pads smaller packets. Protocols above the Ethernet must not confuse padding with actual data, so it must remember where the data ends (5 pts).

22. (10 pts) The calculated delay is 1500/2*10^8 = 7.5us (2 pts). The 51.2us value is conservative, and reflects several factors.

Original Ethernet limited to 1500m, now up to 2500m. (1 pt)
51.2us reflects round trip time, not one-way latency. (5 pts)
Takes latency introduced by repeaters into account. (2 pt)

23. (10 pts) A remote terminal is a latency-dominated application (4 pts); a small amount of data is sent in each packet. The Ethernet is better because, except under heavy load, the time it takes to acquire the network is small (ie no waiting for token). FDDI is better suited for file transfer because such an application is higher bandwidth and 100MBps is better than 10Mbps (4 pts). Whether the same holds on a 100Mbps ethernet depends on the network load. Under high load, FDDI is more efficient for file transfer (2 pts).

Chapter 4: 1, 2

1. (10 pts) Datagram-happy applications are ones that can handle packet loss and retransmission, like FTP, SMTP, HTTP, etc. (5 pts) Connection-requiring applications are ones like real-time applications that the data MUST get there on time (real time voice, video, etc.) (5 pts)

2. (10 pts)

Subpart Switch Input
Port Input
VCI Output
Port Output
VCI

a

1 2 0 1 0

2 3 0 0 0

3 0 0 3 0

b

1 3 0 1 1

2 3 1 1 1

4 1 1 1 0

c

2 2 0 0 1

3 0 1 2 0

d

1 0 0 1 2

2 3 2 0 2

3 0 2 3 1

e

4 2 0 3 0

2 1 0 0 3

3 0 3 1 0

f

4 0 0 3 1

2 1 1 3 0

1 1 0 2 0

Subpart	Switch	Input Port	Input VCI	Output Port	Output VCI
a
	1	2	0	1	0
	2	3	0	0	0
	3	0	0	3	0
b
	1	3	0	1	1
	2	3	1	1	1
	4	1	1	1	0
c
	2	2	0	0	1
	3	0	1	2	0
d
	1	0	0	1	2
	2	3	2	0	2
	3	0	2	3	1
e
	4	2	0	3	0
	2	1	0	0	3
	3	0	3	1	0
f
	4	0	0	3	1
	2	1	1	3	0
	1	1	0	2	0

Chapter 5: 15

15. (10 pts) Since the lower level network is a connection oriented one, there would be a connection set-up and tear-down phase. This would imply that IP would need to set-up and tear-down a connection for each packet it forwards. This would be very inefficient (8 pts). To solve this problem, IP might cache (and reuse) the recently used connections, and close down the least recently used connection whenever it needs to open a new connection (2 pts).

In addition:

1. (5 pts) Baseline drift if there are long idle periods (3 pts), but NRZI encoding is used (2 pts) so no problem.

2. (10 pts) A CRC's error correcting capability is based on its ability to detect more than one error; the reason frames are simply discarded is because recovery is very computationally intensive and burst errors are hard to differentiate from correctable single-bit errors.

3. (5 pts) Easier to implement in hardware.

Bonus Question

(5 pts) If two stations transmit claim frames with TTRT equal to the lowest TTRT on the ring, they'll both get them back and assume that they hold the token.

Subpart	Switch	Input Port	Input VCI	Output Port	Output VCI
a
	1	2	0	1	0
	2	3	0	0	0
	3	0	0	3	0
b
	1	3	0	1	1
	2	3	1	1	1
	4	1	1	1	0
c
	2	2	0	0	1
	3	0	1	2	0
d
	1	0	0	1	2
	2	3	2	0	2
	3	0	2	3	1
e
	4	2	0	3	0
	2	1	0	0	3
	3	0	3	1	0
f
	4	0	0	3	1
	2	1	1	3	0
	1	1	0	2	0

Subpart	Switch	Input Port	Input VCI	Output Port	Output VCI
a
	1	2	0	1	0
	2	3	0	0	0
	3	0	0	3	0
b
	1	3	0	1	1
	2	3	1	1	1
	4	1	1	1	0
c
	2	2	0	0	1
	3	0	1	2	0
d
	1	0	0	1	2
	2	3	2	0	2
	3	0	2	3	1
e
	4	2	0	3	0
	2	1	0	0	3
	3	0	3	1	0
f
	4	0	0	3	1
	2	1	1	3	0
	1	1	0	2	0

Subpart	Switch	Input Port	Input VCI	Output Port	Output VCI
a
	1	2	0	1	0
	2	3	0	0	0
	3	0	0	3	0
b
	1	3	0	1	1
	2	3	1	1	1
	4	1	1	1	0
c
	2	2	0	0	1
	3	0	1	2	0
d
	1	0	0	1	2
	2	3	2	0	2
	3	0	2	3	1
e
	4	2	0	3	0
	2	1	0	0	3
	3	0	3	1	0
f
	4	0	0	3	1
	2	1	1	3	0
	1	1	0	2	0