區域網路設備基礎介紹
Data Communications and Computer
Networks: A Business User’s Approach
Fifth Edition (Chap 7)
Introduction
• A local area network is a communication
network that interconnects a variety of data
communicating devices within a small
geographic area and broadcasts data at high
data transfer rates with very low error rates
• Since the local area network first appeared in
the 1970s, its use has become widespread in
commercial and academic environments
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Primary Function of Local Area Networks
(continued)
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Hubs
• Hub interconnects two or more workstations into
a local area network
• When a workstation transmits to a hub, hub
immediately resends the data frame out to all
connecting links
• Can be managed or unmanaged
– A managed hub possesses enough processing
power that it can be managed from a remote
location
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Bridges
• A bridge (or bridge-like device) can be used to
connect two similar LANs, such as two
CSMA/CD LANs
• Can also be used to connect two closely similar
LANs, such as a CSMA/CD LAN and a token
ring LAN
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Bridges (continued)
• Examines destination address in a frame and
either forwards this frame onto next LAN or does
not
• Examines source address in a frame and places
this address in a routing table, to be used for
future routing decisions
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Bridges (continued)
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Transparent Bridge
• A bridge observes each frame that arrives at a
port, extracts the source address from the frame,
and places that address in the port’s routing
table
• A transparent bridge is found with CSMA/CD
LANs
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Transparent Bridge (continued)
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Transparent Bridge (continued)
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Switches
• A combination of hub and bridge
• Can interconnect two or more workstations, but
like a bridge, it observes traffic flow and learns
• When a frame arrives at a switch, switch
examines destination address and forwards
frame out the one necessary connection
• Workstations that connect to a hub are on a
shared segment
• Workstations that connect to a switch are on a
switched segment
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Switches (continued)
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Switches (continued)
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Virtual LANs
• Virtual LAN (VLAN) – logical subgroup within a
LAN that is created via switches and software
rather than by manually moving wiring from one
network device to another
• Even though employees and their actual
computer workstations may be scattered
throughout the building, LAN switches and VLAN
software can be used to create a “network within
a network”
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Wired Ethernet
• Most common form of LAN today
• Star-wired bus is most common topology but bus
topology still not totally dead yet
• Comes in many forms depending upon medium
used and transmission speed and technology
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Wired Ethernet (continued)
• Originally, CSMA/CD was 10 Mbps
• Then 100 Mbps was introduced
– Most NICs sold today are 10/100 Mbps
• Then 1000 Mbps (1 Gbps) was introduced
• 10 Gbps is now being installed in high-end
applications
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Wired Ethernet (continued)
• 1000 Mbps introduces a few interesting wrinkles:
– Transmission is full-duplex (separate transmit and
receive), thus no collisions
– Prioritization is possible using 802.1p protocol
• Topology can be star or mesh (for trunks)
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Wired Ethernet (continued)
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Wireless Ethernet
• Not really a specific topology
– Workstation in a wireless LAN can be anywhere
as long as it is within transmitting distance to an
access point
• Several versions of IEEE 802.11 standard define
various forms of wireless LAN connections
• Workstations reside within Basic Service Set,
while multiple basic service sets create an
Extended Service Set
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Wireless Ethernet (continued)
• Two basic components necessary:
– Client radio
• Usually a PC card with an integrated antenna installed in a
laptop or workstation
– Access point (AP)
• An Ethernet port plus a transceiver
• AP acts as a bridge between the wired and wireless
networks and can perform basic routing functions
• Workstations with client radio cards reside within Basic
Service Set, while multiple basic service sets create an
Extended Service Set
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Wireless Ethernet (continued)
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Wireless Ethernet (continued)
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Wireless Ethernet (continued)
• IEEE 802.11
– Original wireless standard, capable of
transmitting data at 2 Mbps
• IEEE 802.11b
– Second wireless standard, capable of transmitting
data at 11 Mbps
– In actual tests, 11 Mbps 802.11b devices
managed 5.5 Mbps (from a July 2000 test by
Network Computing)
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Wireless Ethernet (continued)
• With directional antennae designed for point-topoint transmission (rare), 802.11b can transmit
for more than 10 miles
• With an omni-directional antenna on typical AP,
range may drop to as little as 100 feet
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Wireless Ethernet (continued)
• IEEE 802.11a
– One of the more recent standards
– Capable of transmitting data at 54 Mbps
(theoretical) using the 5-GHz frequency range
• IEEE 802.11g
– The other recent standard
– Also capable of transmitting data at 54 Mbps
(theoretical) but using the same frequencies as
802.11b (2.4-GHz)
– Is backwards compatible with 802.11b
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Wireless Ethernet (continued)
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Wireless Ethernet (continued)
• As we have already seen, IEEE has created the
802.11b, 802.11a, and 802.11g wireless
standards
• IEEE 802.11n (100 Mbps) will be ratified soon
and should start appearing in product form in
2006 (maybe?)
• Latest wireless Ethernet is using MIMO
technology (multiple input multiple output)
– Sender and receiver have multiple antennas for
optimum reception
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