© 2001 by Prentice Hall13-1 Local Area Networks, 3rd Edition David A. Stamper Part 5: Connecting to Other Systems and Networks Chapter 13 Wide Area Networks

13-1© 2001 by Prentice Hall

Local Area Networks, 3rd EditionDavid A. Stamper

Part 5: Connecting to Other Systems

and Networks

Chapter 13

Wide Area Networks


Chapter Preview

• WAN terminology and topology

• Functions of the data link layer

• WAN data link protocols

• Functions of the network layer

• Network routing

• Differences between LANs and WANs

In this chapter you will study:


WAN Terminology

• Link– The direct connection of two nodes in a network. A link, therefore, connect tow

computers.

• End-to-End Routing– Sometimes, when one node wants to send a message to another node, the two

are not directly connected. The message must then pass through one or more intermediate nodes before arriving at the final destination. Determining how this is done is called end-to-end routing.

• Path– Th links that the message traverses.

• Hop– The number of hops a message takes in going from its source to its destination is

the number of links it traverses.


WAN Terminology (cont.)

• Store-and-Forward– A technique used by some networks to send data along a path. Each intermediary node

along the path stores the message, sends an acknowledgment of message receipt to the sender, and then forwards the message to the next node on the path. When the sender receives an acknowledgement that the message bas been received by the next node, it is no longer responsible for retransmitting the message if an error occurs.

• Session– A communication between two users of a network. A user can be a terminal operator,

an application, or any other originator of messages. In some systems, sessions are quite formal, with well-defined conventions for establishing, continuing, and terminating the dialogue.

• Packet Switching– The technology of transmitting a message in one or more fixed-length data packets.


Hierarchical Network

• The hierarchical network topology is also called a tree structure.

• This type of network closely resembles corporate organization charts, and corporate computer centers are one place in which this topology can be found.

• Information flowing from a district in one division to a district in a different division would need to go through the root or corporate node. This topology allow for a great deal of network control.


Interconnected (Plex) Network

• This topology provides a high degree of flexibility because many paths are available between nodes if a link should fail, so congestion can be avoided.

• Costs can also be controlled because interconnected topology is capable of the shortest or least expensive configuration.


Combination Networks

• Hierarchical and interconnected networks can be integrated into one network.

• One such combination is a backbone network—for instance, a ring—with spurs attached.

• In widely distributed systems with a large number of nodes, this configuration helps reduce the number of hops, the length of the path, and congestion problems.

• Backbone networks are appearing more often in LAN technology, as well.


Hierarchical Network Topology

Corporate HeadquartersComputer

Regional OfficeComputers

District OfficeComputers

Branch OfficeComputers


Interconnected Network Topology


Backbone Network Topology


Bit Synchronous Protocols

• Synchronous data link control (SDLC) from IBM• Advanced data communications control

procedure (ADCCP), an ANSI standard data link protocol

• High-level data link control (HDLC), a standard of the International Standards Organization (ISO)

• Link access procedure—balanced (LAPB), designated as the data link protocol for X.25 packet distribution networks.


HDLC Frame Format

Flag01111110

Address(1 or more

octets)

ControlField

(8 or 16 bits)

Data(OptionalOctets)

FrameCheck

Sequence16 or 32 bits)

Flag01111110


Expansion of Control Field in HDLC Frame

0Ns

(3 bits)P/FBit

Nr(3 bits)

0Ns

(7 bits)P/FBit

Nr(7bits)

8-Bit Control Field

16-Bit Control Field


Message Routing in a WAN

• Centralized Routing– In centralized routing, one node is designated as the network routing manager to

whom all nodes periodically forward such status information as queue lengths on outgoing and incoming lines and the number of messages processed within the most recent time interval.

– Increased congestion and large processing power requirements are among the disadvantages of centralized routing.

• Distributed Routing– Distributed routing relies on each node to calculate its own best routing table,

which requires each node to periodically transmit its status to its neighbors.– Static Routing

• The purest form of static routing involves always using one particular path between two nodes; if a link I that path is down, then communication between those nodes is impossible.


Message Routing in a WAN (cont.)

– Weighted Routing• When multiple paths exist, some implementations use weighted

routing, in which each path is weighted according to perceived use. The path is then randomly selected from the weighted alternatives.

– Adaptive Routing• Adaptive routing, occasionally called dynamic routing, attempts

to select the quickest or best current route for the message or session.

• Broadcast Routing– Broadcast routing is exemplified by CSMA/CD and token passing. The

message is broadcast to all stations, and only the s6tation to which the message is addressed accepts it.


Centralized Routing

Old Path

New Path


Weighted Routing

20%

30%

50%


Adaptive Routing

VeryBusy

Idle

Idle

Idle

CongestionAvoidance

Path


Packet Distribution Networks

• PDN Terminology– A packet distribution network (PDN) is a packet-switching WAN variously called an X.25, a

value-added network (VAN), or a public data network. The terms packet distribution and packet switching both refer to how data are transmitted (that is, as one or more packets with a fixed length).

• PDN Advantages and Disadvantages– The user is charge for the amount of data transmitted rather than for connect time. The PDN

gives access to many different locations without the cost of switched connections. Access to the PDN is usually via a local telephone call, which also reduces costs.

– But, because the PDN is usually shared, users must compete with each other for circuits. If the number of data packets to be transferred is great, then the cost of using a PDN can exceed that of leased facilities.

• Frame Relay– The overhead of the X.25 protocol results in the transmission speed being limited to 64 Kbps

or less. Consequently, X.25 is not well suited to some forms of transmission such as voice and video, where frames must arrive closely together.


Comparing WANs with LANs

• The primary difference between a WAN and a LAN is distance. A LAN serves a limited geographical are, typically within one building or building complex. A WAN can cover a large geographical area.

• Topology, Protocols, and Routing– LAN topologies are usually bus, ring or star. WAN topologies are typically hierarchical or

interconnected, although rings and stars are also used.

• Media– LAN media are usually twisted-pair wires, coaxial cable, fiber optic cable, or one of the new wireless

media. A single LAN usually uses a single medium type. WAN media are often obtained through a common carrier and may consist of a variety of media, such as telephone wires, fiber optic cable, coaxial cable, microwave radio, and satellite.

• Ownership– A LAN is almost always privately controlled with respect to hardware, software, and media. A WAN,

on the other hand, usually consists of computer hardware and software owned or controlled by the user, together with media and associated data communication equipment provided by a common carrier.


Comparing WANs with LANs (cont.)

• Transmission Speed– LAN nodes are connected via high-speed communication path. The speed

typically is at least 1 Mbps and often higher. Transmission speeds for WANs vary widely and, like those of LANs, are constantly increasing. Common WAN speeds are 9600 bps, 56 Kbps, and 1.54 Mbps. Speeds of 1.54 Mbps result from a transmission service known as T-1.


LAN-WAN Interconnections

• The differences between LANs and WANs just cited present considerable obstacles to LAN-WAN interconnection. When we connect a LAN to a WAN, many of these differences must be reconciled. LAN speeds can be 10,000 times faster than WAN speeds. The format of a LAN message can be different from that of the WAN to which it is connected, and the interconnection must translate messages from one protocol format to the other.

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© 2001 by Prentice Hall13-1 Local Area Networks, 3rd Edition David A. Stamper Part 5: Connecting to Other Systems and Networks Chapter 13 Wide Area Networks