§1.6 1.6 计算机网络结构与互联计算机网络结构与互联

1 网络拓扑结构

Mesh Topology(1) 网状结构

Bus Topology(2) 总线结构

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

Bus LAN DiagramBus LAN Diagram

Tree LAN DiagramTree LAN Diagram

(3) 树形结构

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

Tree Topology

Ring LAN DiagramRing LAN Diagram

(4) 环形结构

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

Ring Topology

Star Topology(5) 星形结构

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

Star LAN DiagramStar LAN Diagram

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

Hybrid Topology

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

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

(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.

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.

Two-Level Star TopologyTwo-Level Star Topology

（（ 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

BridgesBridges

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

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.

Bridge OperationBridge Operation

(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.

Ethernet Hubs and SwitchesEthernet Hubs and Switches

Shared medium hubs

Switched LAN hubs

x

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

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.

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.

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

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

(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.

RoutersRouters

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.

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.

(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.

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.

GatewaysGateways

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

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

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

Ethernet Segments

10BASE5

Thin Ethernet

细缆

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

10BASE2

(2) 双绞线

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

10BASET

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)

(3) 光纤电缆

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

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