Chapter 13Internetworking Technologies

Part III: Wide Area Networks and Internetworking Technologies

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Topics Addressed in Chapter 13 Internetworking technologies and the OSI model Business rationale for internetworking technologies Using repeaters to connect LAN segments Using bridges to connect two LANs Routers and network layer connections Using gateways to connect networks above the network

layer Internetworking via switches Remote access technologies Wireless access to corporate networks Intranets and extranets: Web-based internetworking

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Internetworking and the OSI Model Internetworking technologies are used to

interconnect networks The OSI reference model provides an appropriate

context for understanding internetworking technologies (see Figure 13-3)

Although some internetworking technologies span two or more layers of the OSI model, most can be classified as physical layer, data link layer, network layer, or higher layer technologies

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Figure 13-3

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Physical Layer Technologies One of the main responsibilities of physical layer interconnection

technologies is to overcome signal attenuation (see Figure 13-1) Repeaters are used in digital communication systems Amplifiers do this in analog systems

Repeaters are also used to overcome distance limitations; in this role they function as signal relay stations (see Figure 13-2)

Repeaters can be standalone devices and be used for media conversion. Repeating capabilities are typically included in LAN shared

media hubs, patch panels, and punchdown blocks Optical repeaters are available for fiber optic networks

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Figure 13-1

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Data Link Layer Connections Three key functions of data link layer protocols are data

delineation, error detection, and address formatting Bridges are used to interconnect two LANs at the data

link layer (see Figure 13-4) Bridges have more intelligence than physical layer

technologies; they have to examine (filter) data link layer frames transmitted in one network to determine if they should be forwarded to the other network (see Figure 13-5)

Layer 2 switches are also used to connect two networks at the data link layer

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Figure 13-5

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Network Layer Connections The network layer of the OSI reference model is

responsible for packet routing in networks with multiple alternative paths from sender to receiver (see Figure 13-6)

Routers are widely used network layer internetworking technologies After determining the destination address of the

recipient, a router chooses the best route for a packet based on routing tables and routing algorithms

Layer 3 switches have routing capabilities

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Figure 13-6

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Higher Layer Connections Network connections that operate above

the network layer are generically called gateways

Gateways often support protocol conversion because the networks they interconnect use different network layer protocols (see Figure 13-7)

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Figure 13-7

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Business Rationale for Internetworking Technologies Internetworking technologies enable LANs to be

interconnected. LANs can also be connected to LANs. In addition, WANs can be interconnected. Hence, internetworking technologies are used by businesses to create enterprise-wide networks

Internetworking technologies can also be used to form interorganizational systems that connect an organization and one or more business partners

The ability to forge larger networks from smaller ones facilitates resource sharing and communication enterprise-wide

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Repeaters: Connecting LAN Segments Repeaters are used in LANs to overcome signal

attenuation and distance limitations They are also used to connect LAN segments (see

Figures 13-8 and 13-9) Some LAN standards specify the maximum number

of LAN segments that can be created Repeater capabilities are specified in Table 13-1. Repeater limitations include insensitivity to data

errors and the recreation of collisions that originate in one segment to all other segments

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Figure 13-8

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Table 13-1

Figure 13-9

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Bridges: Connecting Two LANs Bridges are used to connect two LANs at the data link

layer of the OSI model (see Figure 13-11) Bridges possess more intelligence than repeaters and are

typically more costly Unlike repeaters, bridges handle complete frames

This means that they can isolate problems to a LAN and reduce the likelihood of transferring noise or collisions from one network to the other

Bridges listen to traffic on each network; they are often called promiscuous listening technologies

Bridges are typically implemented as standalone devices

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Figure 13-11

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Bridge Functionality When a bridge receives a data link layer frame from one network (or segment),

it verifies that it is correctly formatted and if necessary, forwards it to the other network.Two LANs connected by a bridge behave like a single LAN

Frame filtering is one of the most important functions performed by a bridge; this is the process of reading the destination address in the frame’s header and determining if it should be forwarded to the other network;Filtering rates are measured in frames or packets per second

Forwarding is the process used by a bridge to send a frame from one network to the other Format conversion is necessary if the bridge connects LANs with

dissimilar data link protocols Bridges that connect dissimilar LANs are called translating bridges (see

Figure 13-12) Forwarding rates are measured in frames per second

Additional bridge functions are summarized in Table 13-2

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Figure 13-12

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Table 13-2

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Types of Bridges Various kinds of bridges exist including:

Transparent: connect two similar LANs Translating: connect two different LANs Learning (adaptive): builds routing tables from network traffic

The spanning tree algorithm enables bridges to exchange routing information with each other

Source routing bridges: used in token ring networks Remote bridges: used to interconnect LANs via WAN services

(see Figure 13-14 and Table 13-4) Wireless: can be used to bridge remote LANs located within a

few miles of each other

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Figure 13-14

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Routers: Network Layer Connections Key network layer functions include:

Routing: forwarding data to its destination along its “best” route

Network control: exchanging node status information among routing nodes to facilitate the best routing for messages

Congestion control: attempting to reduce transmission delays by sharing information about network traffic and message queue length among routers or network switches

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Message Routing Processes Message routing processes can be centralized or distributed

In networks that centrally determine packet routing, one router is designated as the network routing manager to which all other routers periodically forward network status information

Distributed routing determination requires each router to periodically send network status updates to the other routers in the network

Routing can be categorized as static, weighted or dynamic In static routing, the same path between two nodes is always used In weighted routing, each alternative path is given a weight based on

perceived use; random numbers are generated for incoming packets to the same destination to determine which path to use (see Figure 13-16)

Dynamic (adaptive) routing attempts to select the best current route based on network conditions; it considers path failures and congestion (see Figure 13-17)

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Figure 13-16

Figure 13-17

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IP Routing When an Internet node sends a message to

another Internet node, it must know the destination node’s IP address This may be resolved from a URL using the

Internet’s domain name system

The IP routing process is summarized in Figure 13-20

IP routing may also be used in networks that are not attached to the Internet (see Figure 13-21)

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Figure 13-20

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Types of Routers Three types of routers can be identified for organizations whose networks are

attached to the Internet: Internal: used to route packets between the subnets or the networks

included in a particular subnet Border: used to route messages between an organization’s network and the

Internet External: route messages between border routers across the Internet

backbone (these are also called Internet backbone routers) These are illustrated in Figure 13-19

Dial-up routers enable geographically dispersed LANs to be connected over dial-up digital WAN services such as ISDN (see Figure 13-22)

High-speed routers, such as edge routers, enable network traffic to be routed over high-speed ATM networks or SONET services Terabit routers are capable of forwarding hundreds of millions of packets

per second and have throughput rates of more than one trillion bits per second

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Figure 13-19

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Figure 13-22

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Router Functionality Like bridges, filtering and forwarding rates are often used as router

performance measures Unlike bridges, routers only process packets that are addressed to

them Also unlike bridges, forwarding decisions are based on destination

addresses in network layer packet headers Routers can also be used to limit access to a network; many have

firewall capabilities Multiprotocol routers are capable of forwarding messages using more

than one network layer protocols Encapsulation may be used to enable non-routable data link

protocols, such as SDLC, to be routed over TCP/IP networks

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Routing Protocols Routing protocols enable routers to adapt to changes in network conditions and

topologies; they enable routers to exchange network status updates in order to keep the information in routing tables current

There are three major categories of routing protocols: Distance vector protocols base routing decisions on the distance (number

of hops) to every other router in the network Examples include RIP (Routing Information Protocol) and EIGRP

(Enhanced Interior Gateway Routing Protocol) Link state protocols compute best routes by consulting a complete copy of

the network topology and traffic conditions Examples include OSPF (Open Shortest Path First), NLSP (NetWare

Link Services Protocol) , and IS-IS (Intermediate System-to-Intermediate System)

Path vector protocols maintain comprehensive lists of known routes and networks between senders and receivers. BGP (Border Gateway Protocol) is an example

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Gateways: Connecting Networks Above the Network Layer Gateways connect dissimilar networks; networks that do

not share a common physical, data link, or network layer protocol A gateway can connect two or more networks above the

network layer of the OSI model A gateway reconciles differences between the networks it

connects and serves as a protocol converter In some instances, a complete network or WAN service

may serve as a gateway between two networks (see Figure 13-23); this is possible if gateways exist to connect each of the two networks to the WAN service

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Figure 13-23

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Switches Switches are widely used to interconnect networks. Like other internetworking

technologies, these correspond to OSI model layers (see Figure 13-24) These include:

Layer 2 switches: function like bridges by sending frames to destinations based on MAC addresses (see Figure 13-25)

Layer 3 switches: are capable of layer 2 switching and layer three routing; both layer 2 and layer 3 switches may be used to create virtual LANs (VLANs)

Layer 4 switches: can route TCP/IP messages based on well known port addresses in TCP headers in addition to layer 2 or layer 3 addresses

Backbone attached LAN switches: enable switched connections between devices attached to the same LAN as well as switched access to a high-speed backbone network or router (see Figure 13-26)

Backbone switches: enable switched interconnections among various types of LANs as well as switched access between LANs and a backbone network or WAN services (see Figure 13-27)

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Figure 13-24

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Figure 13-25

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Figure 13-26

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Figure 13-27

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Remote Access Technologies Remote access technologies provide network access to teleworkers Two major types of remote access exist:

Remote client (node) computing occurs when client applications on remote nodes communicate with server applications via dial-up or other WAN links (see Figure 13-28a)

Remote control applications are run on the server rather than the client; remote nodes function as terminals or thin clients (see Figure 13-28b)

Three major approaches exist for remote users to access LAN resources (see Figure 13-29): Dial-in connection to a LAN-attached microcomputer Dial-in connection to a LAN modem

A LAN modem is essentially a modem with a NIC Dial-in connection to a communication server (see Figure 13-30)

A communication server provides dial-in and dial-out services for LAN users; these are also called remote access servers, remote node servers, and telecommuting servers

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Figure 13-29

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Figure 13-30

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Wireless Access to Corporate Networks Explosive growth in wireless communication

technologies is fueling interest in wireless internetworking technologies

Two important wireless internetworking technologies are wireless bridges and mobile IP Wireless bridges enable organizations to link LANs that are

located within a few miles of each other These enable organizations to avoid carrier service charges

Mobile IP enables users to “roam” among wireless LANs Mobile IP clients must be installed on each mobile wireless device

to enable it to communicate with mobile IP servers or routers in corporate network offices

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Interconnections via Web Technologies Organizations are increasingly leveraging TCP/IP

applications to create intranets and extranets Firewalls enable remote users to access corporate

intranets from virtually anywhere via Web browsers

Clientless network operating systems, such as NetWare 6, also enable remote users to access corporate network resources via Web browsers

Chapter 13Internetworking Technologies

Part III: Wide Area Networks and Internetworking Technologies