Homework #5

Solutions HW #5
3] With pure ALOHA, transmission can begin as soon as data is available. 2 overlapping frames will collide,
resulting in damage to both frames. Even if the last bit of a frame overlaps with the 1st bit of the new frame, both
frames will be damaged. At low loads, fewer collisions will occur & ALOHA will not incur many retransmission
In slotted ALOHA, time is divided into discrete slots equivalent to the transmission time of a single frame.
Transmission occurs only on slot boundaries, so that frames either completely overlap or do not overlap at all. So
with slotted ALOHA, when a frame is ready for transmission, it has to wait until the start of the next slot. This
introduces half a slot time of average delay. Hence, at low load pure ALOHA has lesser delay than slotted ALOHA.
19] The Transmitter sends the 256 bit frame. The receiver needs to send a 32 bit acknowledgement after receiving
the frame.
1 way propagation delay is 1km/ (200m/microsecond) = 5microseconds.
Transmission delay for 256 bits is 256 bits/10Mbps = 25.6 microseconds.
Transmission delay for 32 bits is 32 bits/10Mbps = 3.2 microseconds.
Total time to transmit a frame is (5+25.6) + (5+3.2) microseconds = 38.8 microseconds.
During this time the useful data transmitted is 256 bits – 32 bits = 224 bits (header and ack do not count as useful
Effective data rate is 224 bits / 38.8 microseconds = 5.77 Mbps.
23] Fast Ethernet reduces the maximum cable length by a factor of 10 to compensate for the increased speed.
43] Store-and-forward switches receive the entire frame before switching. They can process the CRC & discard
damaged frames. Cut-through switches forward the frames as soon as the destination MAC address is received. So
they cannot analyze the CRC & discard damaged frames.
44] Switches or bridges in VLANs require a configuration table to identify which VLANs are accessible via each
port. E.g. a frame coming in on a VLAN1 port needs to be forwarded on all VLAN2 ports for unicasts, multicasts &
Hubs have all ports within a single collision domain. Frames arriving on any port are sent out on all the hub ports.
So a VLAN configuration table is not required for hubs.
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