區域網路設備基礎介紹

Data Communications and Computer Networks: A Business User’s Approach

Fifth Edition (Chap 7)

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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-to-point 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