Transcript Ans

Answers of Exercise 5
1. Explain why a hub Ethernet is logically bus but physically star.
Answer: A Ethernet hub acts as one segment, and use same Ethernet frame format
as well as CSMA/CD access control protocol. Thus it is logically bus topology.
Because all computers connect the central device, it is physically star topology.
2. Answer: 10Base5 uses thick coax cable, 10Base2 uses thin coax cable,
10Base-T uses twisted pair, 100Base-T/T4 uses twisted pair,
1000Base LX/SX uses optical fiber.
3. when we connect two LANs in two buildings, one bridge is used. However, when two LANs
are connected by a least line, microwave or satellite, two bridges are used. Why?
Answer: Usually, the bit rate of optical fiber is higher than that (<=100Mbps) of LAN.
There is almost no delay in data transfers between two LANs in two building
across a fiber link. Therefore, one bridge is enough to connect two LANs using
optical fiber and fiber modem. However, the bit rate of a least link, microwave,
or satellite is in the range of 30Kbps~3Mbps, which is lower than LAN speed,
e.g., 10Mbps for 10Base-T and 100Mbps for fast Ethernet). High speed data
from LAN can’t be directly sent to the low speed links, and low speed signals
are not suited for transmission in LAN. Thus, each LAN need a bridge to overcome
speed differences between LAN and the above links.
Answers of Exercise 5
4. Explain similarities and differences between a repeater and a bridge.
Answer:
Similarity: Both connects two LAN segments and can forward packets in two directions.
Difference: (1) A repeater reconstruct signal to overcome the signal attenuation across a
link. It does not understand frame format and thus will forward errors and collisions. It
doubles segment length. Because CSMA/CD can’t work well in a long distance link,
number of repeaters between any two computers in LAN is limited to 4.
(2) A bridge will not only reconstruct signal but also understand frame header
and trailer. It checks packet destination address and forward it if necessary. It does not
forward errors and collisions. It allows concurrent use of different segments if traffic is
local. Actually, no strict limits on numbers used in LAN.
5. Explain similarities and differences between an Ethernet switch and an Ethernet hub.
Answer:
Similarity: Both connect computers and LAN segments. They physically act as a central
device of the star topology but logically bus since both follow the CSMA/CD protocol.
Difference: (1) Hub can be seen as one segment whose length is extremely short. Only
one computer can send data in a time. It is often used to connect computers to build a
subnet of a LAN.
(2) Switch can be seen as a bridge array where one segment has only one
computer attached. Therefore, it allows two or more computers send data at the same
time. Thus its performance is much better than a hub, but it is more expensive than a
hub per port. It is often used to connect LAN segments or LANs because of its cost.