1.6 计算机网络结构与互联
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Transcript of 1.6 计算机网络结构与互联
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§1.6 1.6 计算机网络结构与互联计算机网络结构与互联
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1 网络拓扑结构
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Mesh Topology(1) 网状结构
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Bus Topology(2) 总线结构
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LAN Topologies: BusLAN Topologies: Bus
Multipoint mediumStations attach to linear medium (bus) using
tapFull-duplex between station and tapTransmission from any stations travels
entire medium (both directions)Termination required at ends of bus
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Bus LAN DiagramBus LAN Diagram
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Tree LAN DiagramTree LAN Diagram
(3) 树形结构
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LAN Topologies: TreeLAN Topologies: Tree
Generalization of bus topologyBranching cable with no closed loopsCable(s) begin at headend, travel to
branches which may have branches of their own
Each transmission propagates through network, can be received by any station
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Tree Topology
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Ring LAN DiagramRing LAN Diagram
(4) 环形结构
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LAN Topologies: RingLAN Topologies: Ring
Repeaters are joined by unidirectional point-to-point links in a ring
As a frame circulates past a receiver, the receiver checks its address, and copies those intended for it into a local buffer
Frame circulates until it returns to source, which removes it from network
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Ring Topology
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Star Topology(5) 星形结构
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LAN Topologies: StarLAN Topologies: Star
Each station connected directly to central node, usually with two unidirectional links
Central node can broadcast info, or can switch frames among stations
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Star LAN DiagramStar LAN Diagram
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四种网络拓扑结构:四种网络拓扑结构:
星形星形
环形环形
总线型总线型 网状型网状型
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Hybrid Topology
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Choosing a TopologyChoosing a Topology
Factors to consider include reliability, flexibility/expandability, and performance
Bus/tree is most flexibleTree topology easy to lay outRing provides high throughput, but
reliability problemsStar can be high speed for short distances,
but has limited expandability
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Hub Physical All transferredBridge Data link Filtered using
data link layer add. Switch Data link Switched using
data link layer add.Router Network Routed using
network layer add.Gateway Network Routed using
network layer add.
Device Operates at Messages
2 网络互联设备
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(1)(1) HubHubThe active central element of the star
layout.When a single station transmits, the hub
repeats the signal on the outgoing line to each station.
Physically a star; logically a bus.Hubs can be cascaded in a hierarchical
configuration.
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HubHub
Operating at the physical layer, hubs are very simple devices that pass all traffic in both directions between the LAN sections they link.
They may connect different types of cable, but use the same data link and network protocol.
Strictly speaking, hubs are not considered part of a backbone network, but are usually repeaters or amplifiers.
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Two-Level Star TopologyTwo-Level Star Topology
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(( 22 )) BridgesBridges
Allow connections between LANs and to WANs Operates at Layer 2 (Data Link Layer) of OSI Used between networks using identical
physical and link layer protocols Provide a number of advantages
– Reliability: Creates self-contained units– Performance: Less contention– Security: Not all data broadcast to all users– Geography: Allows long-distance links
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BridgesBridges
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Bridge FunctionsBridge Functions
Read all frames from each network Accept frames from sender on one network that are
addressed to a receiver on the other network Retransmit frames from sender using MAC protocol
for receiver Must have some routing information stored in order to
know which frames to pass
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BridgesBridges
If a bridge receives a packet with a destination address that is not in the address table, it forwards the packet to all networks or network segments except the one on which it was received.
Bridges are a combination of both hardware and software, typically a “black box” that sits between the two networks, but can also be a computer with two NICs and special software.
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Bridge OperationBridge Operation
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(3) Switches(3) Switches
Like bridges, switches operate at the data link layer. Switches connect two or more computers or network segments that use the same data link and network protocol. They may connect the same or different types of cable.
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Ethernet Hubs and SwitchesEthernet Hubs and Switches
Shared medium hubs
Switched LAN hubs
x
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Switches vs. HubsSwitches vs. HubsEthernet 10
One device sending at
a time
Hub
All nodes share 10 Mbps
Layer 1 DomainLayer 1 Domain
Ethernet Switch
Each node has 10 Mbps
Backbone Switched Ethernet 10
Multiple devices
sending at the same time
Layer 2 DomainLayer 2 Domain
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SwitchesSwitches
Switches operate at the same layers as bridges but differ from them in two ways:
– First, most switches enable all ports to be in use simultaneously, making them faster than bridges.
– Second, unlike bridges, switches don’t learn addresses, and need to have addresses defined.
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Switched EthernetSwitched Ethernet
A simple concept behind switched Ethernet - replace the LAN hub with a switch. Each computer now has its own dedicated point-to-point circuit.
By increasing the number of connections from the server to the switch, the throughput of the server will be improved because of more circuits.
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A C
B
2
4
1
10 Mbps
10 Mbps
Forwards packets based on a forwarding table
– Forwards based on the MAC (Layer 2) address
Operates at OSI Layer 2Learns a station’s location by e
xamining source address– Sends out all ports when des
tination address is broadcast, multicast, or unknown address
– Forwards when destination is located on different interface
Interface
Sta
tio
ns
1 2 3 4
A X
B X
3
LAN Switch OperationLAN Switch Operation
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Types of SwitchesTypes of Switches
Store and forward switch– Accepts a frame on input line– Buffers it briefly– Routes it to appropriate output line
Cut-through switch– Begins repeating the frame as soon as it
recognizes the destination MAC address– Higher throughput, increased chance of error
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(4) Routers(4) Routers
Routers operate at the network layer. Routers connect two or more LANs that use the same or different data link protocols, but the same network protocol.
Routers may be “black boxes,” computers with several NICs, or special network modules in computers.
In general they perform more processing on each message than bridges and therefore operate more slowly.
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RoutersRouters
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Routers vs BridgesRouters vs Bridges
Routers can choose the best route. Routers also only process messages specifically
addressed to it. Routers can connect networks using different data
link layer protocols. Therefore, routers are able to change data link layer packets.
Routers may split a message into several smaller messages for transmission.
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Layer 3 SwitchesLayer 3 Switches Problems With Layer 2 Switches
– Broadcast overload because of the single MAC broadcast address (e.g. using ARP for Data Link Layer address resolution)
– Lack of multiple links - only one path Normally, the above problems can be solved with several
subnets connected by routers. However, – A MAC broadcast frame is then limited to only the
devices and switches contained in a single subnet.– A router does all IP-level processing, some of which
could be not necessary.– It is implemented in software and slow.
Layer 3 switches implement the packet-forwarding logic of the router in hardware.
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(5) Gateways(5) Gateways
Gateways operate at the network layer and use network layer addresses in processing messages.
Gateways connect two or more LANs that use the same or different (usually different) data link and network protocols. The may connect the same or different kinds of cable.
Gateways process only those messages explicitly addressed to them.
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GatewaysGateways
One of the most common uses of gateways is to enable LANs that use TCP/IP and ethernet to communicate with IBM mainframes that use SNA.
The gateway provides both the basic system interconnection and the necessary translation between the protocols in both directions.
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GatewaysGateways
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3 3 传输介质传输介质 ( ( 11 )同轴电缆)同轴电缆 粗缆粗缆
Thick Ethernet
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10BASE510BASE5
RG-8 CableTransceiver
– Each station is attached by an AUI cable to an intermediary device called a medium attachment unit (MAU) or a transceiver
– Check for voltages and collisions on the line
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10BASE510BASE5
AUI Cables– Each station is linked to its corresponding
transceiver by an attachment unit interface (AUI), also called a transceiver cable.
– 15-wire cable– DB-15 (15-pin) connector. – A maximum length of 50 meters
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Ethernet Segments
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10BASE5
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Thin Ethernet
细缆
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10BASE210BASE2
NIC– Provides the station with an address and checks for
voltages on the link. Thin Coaxial Cable
– RG-58– Are relatively easy to install and move around
BNC-T– The BNC-T connector is a T-shaped device with three
ports: one for the NIC and one each for the input and output ends of the cable
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10BASE2
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(2) 双绞线
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10BASET10BASET
A star-topology LAN using UTP cablePlaces all of its networking operations in an
intelligent hub with a port for each station.Stations are linked into the hub by four-pair
RJ-45 cable
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10BASET
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PL: Twisted pairPL: Twisted pair
5 categories– Category 1
Voice only (telephone wire)
– Category 2 Data to 4Mbs (LocalTalk)
– Category 3 Data to 10Mbs (Ethernet)
– Category 4 Data to 20Mbs (16Mbs Token Ring)
– Category 5 (100 MHz) Data to 100Mbs (Fast Ethernet)
– Category 5e (350 MHz) Data to 1000Mbs (Gigabit Ethernet)
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(3) 光纤电缆
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PL: FiberPL: Fiber
Center core made of glass or plastic fiberTransmit light versus electronic signals
– Protects from electronic interference, moisture
Plastic coating to cushion coreKevlar fiber for strengthTeflon or PVC outer insulating jacket
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PL: FiberPL: FiberSingle-mode fiber
– Smaller diameter (12.5 microns)– One mode only– Preserves signal better over longer distances– Typically used for SONET or SDH– Lasers used to signal– More expensive
Multi-mode fiber– Larger diameter (62.5 microns)– Multiple modes– LEDs used to signal
Photodiodes at receivers
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