§1.6 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: Bus Multipoint medium Stations attach to linear medium (bus) using tap Full-duplex between station and tap Transmission from any stations travels entire medium (both directions) Termination required at ends of bus

Bus LAN Diagram

Tree LAN Diagram (3) 树形结构

LAN Topologies: Tree Generalization of bus topology Branching cable with no closed loops Cable(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 Diagram (4) 环形结构

LAN 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: 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 Diagram

四种网络拓扑结构： 星形 环形 总线型 网状型

Hybrid Topology

Choosing a Topology Factors to consider include reliability, flexibility/expandability, and performance Bus/tree is most flexible Tree topology easy to lay out Ring provides high throughput, but reliability problems Star can be high speed for short distances, but has limited expandability

HubPhysicalAll transferred BridgeData linkFiltered using data link layer add. SwitchData linkSwitched using data link layer add. RouterNetworkRouted using network layer add. GatewayNetworkRouted using network layer add. DeviceOperates atMessages 2 网络互联设备

(1)Hub The 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.

Hub 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 Topology

（ 2 ） Bridges 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

Bridges

Bridge 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

Bridges 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 Operation

(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 Switches Shared medium hubs Switched LAN hubs x

Switches vs. Hubs Ethernet 10 One device sending at a time Hub All nodes share 10 Mbps Layer 1 Domain Ethernet Switch Each node has 10 Mbps Backbone Switched Ethernet 10 Multiple devices sending at the same time Layer 2 Domain

Switches 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 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 Mbps Forwards packets based on a forwarding table – Forwards based on the MAC (Layer 2) address Operates at OSI Layer 2 Learns a station ’ s location by examining source address – Sends out all ports when destination address is broadcast, multicast, or unknown address – Forwards when destination is located on different interface Interface Stations AX BX 3 LAN Switch Operation

Types 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 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.

Routers

Routers 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 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 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.

Gateways 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.

Gateways

3 传输介质 （ 1 ）同轴电缆 粗缆 Thick Ethernet

10BASE5 RG-8 Cable Transceiver – 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

10BASE5 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

10BASE2 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

10BASET A star-topology LAN using UTP cable Places 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 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: Fiber Center core made of glass or plastic fiber Transmit light versus electronic signals – Protects from electronic interference, moisture Plastic coating to cushion core Kevlar fiber for strength Teflon or PVC outer insulating jacket

PL: Fiber Single-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