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Solutions to Assignment 3
1
[15 marks] A character-interleaved time-division multiplexer is used to combine the data streams of a
number of 110-bps asynchronous terminals for data transmission over a 2500-bps synchronous
digital line. Each terminal sends asynchronous characters consisting of 7data bits, 1 parity bit, 1 start
bit, and 2 stop bits. Assume that one synchronization character is sent over every 29 data characters
and, in addition, at least 3% of the line capacity is reserved for pulse stuffing to accommodate speed
variations from various terminals.
a) Determine the number of bits per character.
7+1+1+2=11 bits
b) Determine the number of terminals that can be accommodated by the multiplexer.
In synchronous transmission, start bit and stop bits are not needed. So 8
bits per character is needed. The available capacity is 2500*97%=2425 bps.
Each terminal generates 110 bits per second, but only 110*8/11=80 bits per
second are needed to be transmitted. So roughly the number of terminals
that can be accommodated is 2425/80=30. Considering the synchronization
character, the multiplexer can accommodate 29 terminals.
c) Sketch a possible framing pattern for the multiplexer, i.e. show how you would allocate slots in
the synchronous frame to the supported terminals.
The frame can carry one character from each terminal and there will be 10
frames per second. The frame can consist of one SYN, 29 8-bit characters,
and some bits for stuffing.
2
[15 marks] A synchronous non-statistical TDM is to be used to combine four 4.8-kbps and one 9.6kbps signals for transmission over a single leased line. For framing, a block of 7 bits (pattern
1011101) is inserted for each 48 data bits. The reframing algorithm (at the receiving demultiplex) is as
follows:
(1) Arbitrarily select a bit position.
(2) Consider the block of 7 contiguous bits starting with that position.
(3) Observe that block of 7 bits each frame for 12 consecutive frames.
(4)
If 10 of the 12 blocks match the framing pattern the system is “in-frame”; if not
advance one bit position and return to step (2).
a) Draw the multiplexed bit stream (note that the 9.6-kbps input may be treated as two 4.8-kbps
inputs).
b) What is the % overhead in the multiplexed bit stream?
c) What is the multiplexed output bit rate?
d) What is the minimum reframe time? What is the maximum reframe time?
a) see below:
b) 7/(48+7)=12.73%
c) 4.8*6*(48+7)/48=33 kbps
d) Minimum: Suppose the first arbitrarily chosen bit is the first bit of the framing pattern, then it will
only take 10 frames to be “in-frame”, so it is 55/33*10=16.7 ms.
Maximum: This is the worst situation. It just missed the first bit of the framing pattern. It will check
55 bits in order to find the first bit of the next framing pattern. Suppose it will have to check 12
frames. So it is 55/33*12*55=1100 ms.
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3
[20 marks] The following three traces were captured by using a network protocol analyzer, Ethereal.
These are Ethernet II frames. The printout includes the complete Ethernet II frame less the pre-amble
(first 8 bytes) and the CRC (last 4 bytes).
a) What is the higher layer protocol data carried by these three frames?
Frame1: ARP (0806H). Frame2: ARP (0806H). Frame3: IP (0800H).
b) Identify the MAC address of the two computers in the third frame.
DA=788DF7B06D72, SA=001D0941CC59.
c) Identify the source IP address and destination IP address in the third frame.
Source IP: C0A80011, Destination IP: 3D813B61.
d) Identify the source port number and destination port number in the third frame.
Source port#: 042A, Destination port#: 1388.
e) Is the third-layer header correct in the third frame (checksum)?
The following 20 bytes constitute the IP header:
45 00 00 30 2f f2 40 00 80 06 91 3a c0 a8 00 11 3d 81 3b 61
IP checksum is 16 bits (2 bytes). Adding all the 20 bytes together in unit
of 16 bits results 2FFFD in hexadecimal, which is 0. So the IP header is
correct.
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4
[20 marks] Explain why a 10Mbps Ethernet frame has a minimum data size of 46 bytes? What are
the minimum frame size for Fast Ethernet and Gigabit Ethernet?
In addition there being a maximum frame length, there is also a minimum
frame length. While a data field of 0 bytes is sometimes useful, it causes
a problem. When a transceiver detects a collision, it truncates the
current frame, which means that stray bits and pieces of frames appear on
the cable all the time. To make it easier to distinguish valid frames from
garbage, Ethernet requires that valid frames must be at least 64 bytes
long, from destination address to checksum, including both. If the data
portion of a frame is less than 46 bytes, the Pad field is used to fill
out the frame to the minimum size.
Another (and more important) reason for having a minimum length frame is
to prevent a station from completing the transmission of a short frame
before the first bit has even reached the far end of the cable, where it
may collide with another frame. This problem is illustrated in Fig. 4-18.
At time 0, station A, at one end of the network, sends off a frame. Let us
call the propagation time for this frame to reach the other end τ. Just
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before the frame gets to the other end (i.e., at time τ-ε), the most
distant station, B, starts transmitting. When B detects that it is
receiving more power than it is putting out, it knows that a collision has
occurred, so it aborts its transmission and generates a 48-bit noise burst
to warn all other stations. In other words, it jams the ether to make sure
the sender does not miss the collision. At about time 2τ, the sender sees
the noise burst and aborts its transmission, too. It then waits a random
time before trying again.
If a station tries to transmit a very short frame, it is conceivable that
a collision occurs, but the transmission completes before the noise burst
gets back at 2τ. The sender will then incorrectly conclude that the frame
was successfully sent. To prevent this situation from occurring, all
frames must take more than 2τ to send so that the transmission is still
taking place when the noise burst gets back to the sender. For a 10-Mbps
LAN with a maximum length of 2500 meters and four repeaters (from the
802.3 specification), the round-trip time (including time to propagate
through the four repeaters) has been determined to be nearly 50 μsec in the
worst case, including the time to pass through the repeaters, which is
most certainly not zero. Therefore, the minimum frame must take at least
this long to transmit. At 10 Mbps, a bit takes 100 nsec, so 500 bits is
the smallest frame that is guaranteed to work. To add some margin of
safety, this number was rounded to 512 bits or 64 bytes. Frame with fewer
than 64 bytes are padded out to 64 bytes with the Pad field.
To increase the data rate without changing the minimum size of a frame, we
decrease the round-trip time. With the speed of 100 Mbps, the round trip
time reduced 10 times. This means that the collision domain must be
decreased 10 times, from 2500 meters to 250 meters.
For Gigabit Ethernet (1000Base-T), a minimum frame size of 520 bytes is
used to solve the problem.
5
[10 marks] This question is about sliding window. For a k-bit sequence number field, which provides
a sequence number range from 0 to 2k-1, the maximum window size is limited to 2k-1. Use k=3 as
an example to explain why the window size cannot be 8.
Let’s consider the following situation. If k=3 and window size=8, the
sequence number of frames can range from 0 to 7. Suppose there are frames
0 through 7 in the sliding window of the receiver:
01234567012345670123...
The sender sent out frames 0 through 7 and is waiting for the ACK for
frame 0. The receiver received these 8 frames successfully and sent back
an ACK 0. Then the sliding window of the receiver became:
0123456701234567012345670123...
The ACK is lost due to some reasons. The sender didn’t receive the
expected ACK before the time-out timer expires. The sender re-sends the
frames starting from 0. The receiver receives the re-sent frames. It
cannot distinguish whether these are expected frames or repeated frames,
so frame duplication happens.
If we limit the sliding window size to 7 the problem will not happen (see
below). The receiver is expecting frame 7. If it receives frame 0, it
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knows for sure that the frame 0 is a repeated frame which needs to be
ignored.
01234567012345670123...
0123456701234567012345670123...
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[20 marks] Finish the question on following pages on sliding window.
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