Unit 4
Text Networking Devices（1）
1．Hub
Networks using a star topology require a central point for the devices to connect.Originally this device was called a concentrator since it consolidated the cable from all network devices.The basic form of concentrator is the hub.
The hub is a hardware device that contains multiple，independent ports that match the cable type of the network.1 Most common hubs interconnect Category 3 or 5 twisted pair cable with RJ-45ends，although Coax BNC and Fiber Optic BNC hubs also exist.Hubs offer an inexpensive option for transporting data between devices，but hubs don’t offer any form of intelligence.Hubs can be active or passive.
An active hub strengthens and regenerates the incoming signals before sending the data on to its destination.
Passive hubs do nothing with the signal.
1.1 Ethernet hubs
An Ethernet hub is also called a multiport repeater.A repeater is a device that amplifies a sig-nal as it passes through it，to counteract the effects of attenuation.If，for example，you have a thin Ethernet network with a cable segment longer than the prescribed maximum of 185 meters，you can install a repeater at some point in the segment to strengthen the signals and increase the maximum segment length.2 This type of repeater only has two BNC connectors，and is rarely seen these days.
The hubs used on UTP Ethernet networks are repeaters as well，but they can have many RJ45ports instead of just two BNC connectors.When data enters the hub through any of its ports，the hub amplifies the signal and transmits it out through all of the other ports.This enables a star network to have a shared medium，even though each computer has its own separate cable.The hub relays every packet transmitted by any computer on the network to all of the other computers and it also amplifies the signals.
The maximum segment length for a UTP cable on an Ethernet network is 100 meters.A seg-ment is defined as the distance between two communicating computers.However，because the hub also functions as a repeater，each of the cables connecting a computer to a hub port can be up to 100meters long，allowing a segment length of up to 200 meters when one hub is inserted in the network.
1.2 Multistation Access Unit
A Multistation Access Unit（MAU）is a special type of hub used for token ring networks.The word“hub”is used most often in relation to Ethernet networks，and MAU only refers to token ring networks.On the outside，the MAU looks like a hub.It connects to multiple network devices，each with a separate cable.
Unlike a hub that uses a logical bus topology over a physical star，the MAU uses a logical ring topology over a physical star.
When the MAU detects a problem with a connection，the ring will beacon.Because it uses a physical star topology，the MAU can easily detect which port the problem exists on and close the port，or“wrap”it.The MAU does actively regenerate signals as it transmits data around the ring.3
2．Switches
Switches are a special type of hub that offers an additional layer of intelligence to basic，physi-cal-layer repeater hubs.A switch must be able to read the MAC address of each frame it receives.This information allows switches to repeat incoming data frames only to the computer or computers to which a frame is addressed.This speeds up the network and reduces congestion.
Switches operate at both the physical layer and the data link layer of the OSI Model.
3．Bridges
A bridge is used to join two network segments together，it allows computers on either segment to access resources on the other.They can also be used to divide large networks into smaller segments.Bridges have all the features of repeaters，but can have more nodes，and since the network is divid-ed，there is fewer computers competing for resources on each segment thus improving network per-formance.
Bridges can also connect networks that run at different speeds，different topologies，or different protocols.But they cannot join an Ethernet segment with a Token Ring segment because these use different networking standards.Bridges operate at both the Physical Layer and the MAC sublayer of the Data Link layer.Bridges read the MAC header of each frame to determine on which side of the bridge the destination device is located，the bridge then repeats the transmission to the segment where the device is located.
4．Routers
Routers are networking devices used to extend or segment networks by forwarding packets from one logical network to another.Routers are most often used in large internetworks that use the TCP／IP protocol suite and for connecting TCP／IP hosts and local area networks（LANs）to the Internet using dedicated leased lines.
Routers work at the network layer（layer 3）of the Open Systems Interconnection（OSI）refer-ence model for networking to move packets between networks using their logical addresses（which，in the case of TCP／IP，are the IP addresses of destination hosts on the network）.Because routers operate at a higher OSI level than bridges do，they have better packet-routing and filtering capabili-ties and greater processing power，which results in routers costing more than bridges.
Routers contain internal tables of information called routing tables that keep track of all known network addresses and possible paths throughout the internetwork，along with cost of reaching each network.4 Routers route packets based on the available paths and their costs，thus taking advantage of redundant paths that can exist in a mesh topology network.
Because routers use destination network addresses of packets，they work only if the configured network protocol is a routable protocol such as TCP／IP or IPX／SPX.This is different from bridges，which are protocol independent.The routing tables are the heart of a router；without them，there’s no way for the router to know where to send the packets it receives.
Unlike bridges and switches，routers cannot compile routing tables from the information in the data packets they process.This is because the routing table contains more detailed information than that is found in a data packet，and also because the router needs the information in the table to process the first packets it receives after being activated.A router can’t forward a packet to all pos-sible destinations in the way that a bridge can.
Static routers：These must have their routing tables configured manually with all network ad-dresses and paths in the Internet work.
Dynamic routers：These automatically create their routing tables by listening to network traffic.
Routing tables are the means by which a router selects the fastest or nearest path to the next“hop”on the way to a data packet’s final destination.5 This process is done through the use of rou-ting metrics.
Routing metrics are the means of determining how much distance or time a packet will require to reach the final destination.Routing metrics are provided in different forms.
You can use routers to segment a large network and to connect local area segments to a single network backbone that uses a different physical layer and data link layer standard.They can also be used to connect LAN’s to a WAN’s.
5．Brouters
Brouters are a combination of router and bridge.This is a special type of equipment used for networks that can be either bridged or routed，based on the protocols being forwarded.Brouters are complex，fairly expensive pieces of equipment and as such are rarely used.
A brouter transmits two types of traffic at the exact same time：bridged traffic and routed traffic.For bridged traffic，the brouter handles the traffic the same way a bridge or switch would，forwarding data based on the physical address of the packet.This makes the bridged traffic fairly fast，but slo-wer than if it were sent directly through a bridge because the brouter has to determine whether the data packet should be bridged or routed.
6．Gateways
A gateway is a device used to connect networks using different protocols.Gateways operate at the network layer of the OSI model.In order to communicate with a host on another network，an IP host must be configured with a route to the destination network.If a configuration route is not found，the host uses the gateway（default IP router）to transmit the traffic to the destination host.The de-fault gateway is where the IP sends packets that are destined for remote networks.If no default gate-way is specified，communication is limited to the local network.Gateways receive data from a net-work using one type of protocol stack，remove that protocol stack and repackage it with the protocol stack that the other network can use.
Examples：
● Email gateways，a gateway that receives Simple Mail Transfer Protocol（SMTP）Email，translates it into a standard X.400 format，and forwards it to its destination
● Gateway Service for NetWare（GSNW），which enables a machine running Microsoft Win-dows NT Server or Windows Server to be a gateway for Windows clients so that they can ac-cess file and print resources on a NetWare server
● Gateways between a Systems Network Architecture（SNA）host and computers on a TCP／IPnetwork，such as the one provided by Microsoft SNA Server
● A packet assembler／disassembler（PAD）that provides connectivity between a local area net-work（LAN）and an X.25 packet-switching network
New Words
Phrases
twisted pair 双绞线
fiber optic 光缆，光纤
active hub 主动式集线器
thin Ethernet network 细线以太网，细缆以太网
token ring network 令牌环网
logical bus topology 逻辑总线拓扑
logical ring topology 逻辑环形拓扑
physical star 物理星形
divide…into… 把……分为……
network layer 网络层
Open Systems Interconnection reference model 开放式系统互联参考模型
routing table 路由表
mesh topology 网状拓扑
routable protocol 可路由协议
static router 静态路由器
dynamic router 动态路由器
packet-switching network 分组交换网络
Abbreviations
RJ（registered jack） 注册的插座
BNC（Bayonet Nut Connector） 刺刀螺母连接器，同轴电缆接插件
COAX（Coaxial Cable） 同轴电缆
UTP（Unshielded Twisted Paired） 非屏蔽双绞线
OSI（Open System Interconnect） 开放式系统互联
LAN（Local Area Network） 局域网
IPX（Internetwork Packet Exchange protocol） 因特网分组交换协议
SPX（Sequences Packet Exchange） 顺序分组交换
SMTP（Simple Mail Transfer Protocol） 简单邮件传输协议
GSNW（Gateway Service for NetWare） NetWare网关服务
PAD（Packet Assembler／Disassembler） 分组组合／分解器
Notes
［1］The hub is a hardware device that contains multiple，independent ports that match the ca-ble type of the network.
本句中，that contains multiple，independent ports是一个定语从句，修饰和限定a hardware device。that match the cable type of the network也是一个定语从句，修饰和限定ports。
［2］If，for example，you have a thin Ethernet network with a cable segment longer than the prescribed maximum of 185 meters，you can install a repeater at some point in the segment to strengthen the signals and increase the maximum segment length.
本句中，If引导了一个条件状语从句。with a cable segment longer than the prescribed maxi-mum of 185 meters做定语，修饰和限定a thin Ethernet network。to strengthen the signals and in-crease the maximum segment length是动词不定式短语，做目的状语。
［3］The MAU does actively regenerate signals as it transmits data around the ring.
本句中，does起强调作用，意思是“的确，确实”。
［4］Routers contain internal tables of information called routing tables that keep track of all known network addresses and possible paths throughout the internetwork，along with cost of reaching each network.
本句中，called routing tables是过去分词短语，做定语，修饰和限定internal tables of infor-mation。that keep track of all known network addresses and possible paths throughout the internet-work，along with cost of reaching each network是一个定语从句，修饰和限定routing tables。keep track of的意思是“跟踪”。
［5］Routing tables are the means by which a router selects the fastest or nearest path to the next“hop”on the way to a data packet’s final destination.
本句中，by which a router selects the fastest or nearest path to the next“hop”on the way to adata packet’s final destination是一个介词前置的定语从句，修饰和限定the means。on the way to的意思是“在……的途中”。