Download Part III: Wide Area Networks and Internetworking Technologies

Chapter 13
Internetworking Technologies
Part III: Wide Area Networks and Internetworking
Technologies
Topics Addressed in Chapter 13
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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
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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
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One of the main responsibilities of physical layer interconnection
technologies is to overcome signal attenuation (see Figure 13-1)
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Repeaters are used in digital communication systems
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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.
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Repeating capabilities are typically included in LAN shared
media hubs, patch panels, and punchdown blocks
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Optical repeaters are available for fiber optic networks
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Figure 13-1
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Data Link Layer Connections
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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 135)
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
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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
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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
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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
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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 enterprisewide
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Repeaters: Connecting LAN
Segments
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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)
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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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Figure 13-9
Table 13-1
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Bridges: Connecting Two LANs
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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
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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
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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
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Format conversion is necessary if the bridge connects LANs with
dissimilar data link protocols
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Bridges that connect dissimilar LANs are called translating bridges
(see Figure 13-12)
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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
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Various kinds of bridges exist including:
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Transparent: connect two similar LANs
Translating: connect two different LANs
Learning (adaptive): builds routing tables from network traffic
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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
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Key network layer functions include:
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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
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Message routing processes can be centralized or distributed
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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
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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
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When an Internet node sends a message to
another Internet node, it must know the
destination node’s IP address
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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
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Three types of routers can be identified for organizations whose networks are
attached to the Internet:
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Internal: used to route packets between the subnets or the networks
included in a particular subnet
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Border: used to route messages between an organization’s network and
the Internet
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External: route messages between border routers across the Internet
backbone (these are also called Internet backbone routers)
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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
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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
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Like bridges, filtering and forwarding rates are often used as router
performance measures
Unlike bridges, routers only process packets that are addressed to
them
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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
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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:
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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)
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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 Systemto-Intermediate System)
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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
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Gateways connect dissimilar networks; networks that do
not share a common physical, data link, or network layer
protocol
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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:
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Layer 2 switches: function like bridges by sending frames to
destinations based on MAC addresses (see Figure 13-25)
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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)
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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
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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)
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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:
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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)
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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):
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Dial-in connection to a LAN-attached microcomputer
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Dial-in connection to a LAN modem
 A LAN modem is essentially a modem with a NIC
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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
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Explosive growth in wireless communication technologies
is fueling interest in wireless internetworking
technologies
Two important wireless internetworking technologies are
wireless bridges and mobile IP
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Wireless bridges enable organizations to link LANs that are
located within a few miles of each other
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These enable organizations to avoid carrier service charges
Mobile IP enables users to “roam” among wireless LANs
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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
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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
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Chapter 13
Internetworking Technologies
Part III: Wide Area Networks and Internetworking
Technologies