Business Data Communications

Chapter Seven

Wide Area Networking Fundamentals

Primary Learning Objectives

Understand the need for varying WAN connectivity options

Define the term switching network Describe circuit switching networks Describe packet switching networks Differentiate X.25, Frame Relay, and ATM Explain Point-to-point protocol Identify common trunk carrier services Distinguish between symmetric and asymmetric

Digital Subscriber Line services

WAN Connectivity Options

There are several ways in which WAN connections can be established

Many connection options are provided by common carrier infrastructures (AT&T, MCI, Sprint, for example)

WAN connectivity options include: Circuit switching Packet switching: X.25, Frame Relay, ATM Point-to-point protocols Trunk carrier services Digital subscriber lines

WAN Connectivity Options

Switching Networks

Composing a switching network are switches, which: Are a series of interlinked devices

Interlinks between two communicating devices are temporary

Can be hardware, software, or a combination of both Can be connected to multiple links

The switching network infrastructure is referred to, and graphically illustrated as, a “cloud” Edge switches and edge routers connect the enterprise

to the cloud

Switching NetworkCloud

Circuit Switching Networks

Are well-suited for voice, but not data, communications Voice communications are continuous; data communications

are bursty Create a direct connection between two communicating

devices: This direct connection is referred to as a “path” While established, no two other devices can use the circuit path The circuit path uses a constant rate determined by the slower

of the two devices Circuit-switched paths can be temporary or permanent

Circuit Switching Networks

Temporary paths: Require a call setup Are less expensive than permanent paths Do not guarantee that a circuit will always be available

Permanent paths: Do not require a call setup Are more expensive than temporary paths Guarantee that a circuit will always be available

Neither temporary nor permanent circuit switching prioritizes communications

Packet Switching Networks

Break data into units called “packets”: Packets traverse the packet switching cloud through a series of

links Links are connected by packet switching nodes

Improve line efficiently significantly because many packet streams, from differing communications, can use the same links between packet switching nodes

Deliver packets even when the network is busy, although the packets may be delayed

Provide for data-rate link conversion between packet switching nodes

Data-Rate Link Conversion

Packet Switching Networks

Use the network implementation to determine packet size Packets contain not only core data, but control

information as well Temporarily store and then forward packets

as they pass through the packet switching cloud

Allow for prioritization of communications May be based on datagram or virtual circuit

delivery

Packet Switching Networks

Datagram delivery: Is best for small messages Treats each packet independently Does not guarantee packet delivery Does not sequence packets Permits packets to be dynamically rerouted Does not require a setup procedure between

sender and receiver Requires little overhead

Packet Switching Networks

Packet Switching Networks

Virtual circuit delivery: Is best for data-sensitive messages Requires a set-up procedure between sender and receiver Requires all packets in a message to take the same route Guarantees packet delivery Sequences packets Cannot dynamically reroute packets Has significant overhead Takes one of two forms:

Switched virtual circuit Permanent virtual circuit

Packet Switching Networks

Switched virtual circuits: Are similar in concept to a dial-up circuit switched

connection Temporarily set up the route of links a circuit

takes, meaning only for the duration of the communication

Require a setup procedure for each communication

Packet Switching Networks

Permanent virtual circuits: Are similar in concept to a leased circuit switched

connection Once defined, require no additional set-up

procedures, meaning that the route of links does not have to be recreated

Are always available as they are permanent Require virtual circuit identifiers for each

communication

Packet Switching Networks

Packet Switching Networks

Three common forms of packet switching network are: X.25 – The oldest Frame Relay ATM – The newest

X.25

Uses data terminal equipment and data-circuit terminating equipment

Uses virtual circuits and statistical time division multiplexing Supports variable length packets Has a three-layer design: physical, frame, and packet Uses Link Access Procedure-Balanced protocol (based on

HDLC) at the frame layer Provides for significant error checking, a drawback given

today’s much improved transmission media Was not designed with multimedia data in mind

Frame Relay

Utilizes two layers: physical and data link Supports variable length packets Requires Frame Relay Access Devices (FRADs) to

disassemble and reassemble packets Requires that subscribers negotiate a committed

information rate with a common carrier A committed burst rate can also be negotiated

Uses the Link Access Procedure-Function protocol (based on HDLC)

Does not provide for flow and error control These are left to higher-level services

ATM – Asynchronous Transfer Mode

Is a universal integrated carrier of voice, data, audio, and video

Has a significant implementation cost and high degree of configuration complexity

Utilizes two layers: physical and data link Is a point-to-point solution Uses virtual path and virtual channel identifiers Requires fixed-length packets, called cells, of 53

bytes 5 bytes of header, 48 bytes of data

ATM – Asynchronous Transfer Mode

Point-to-Point Protocol

Was developed by the IETF Creates a physical serial link between two devices Is inexpensive, but provides limited bandwidth capacity Is the most common protocol used by dial-up modems Replaced Serial Link Internet Protocol (SLIP) Requires no media access control Uses protocols such as PAP and CHAP for authentication Has a five-phase sequence

Point-to-Point Protocol – Five Phases

Trunk Carrier Services

Most commonly used forms include: T-1, T-3, and Fractional T-1; the “T” stands for trunk services

The higher the “T” level designation, the greater the possible number of communication channels

Significantly more costly than point-to-point protocol, but offers much higher bandwidths

Digital leased lines have a range of bandwidths referred to as digital signal (DS) speeds

Subscribers negotiate with common carriers the level of service quality, for a cost

Trunk Carrier Services

Trunk Carrier Services

Usually used in conjunction with a: Channel service unit (CSU):

The end-point of the digital link Keeps the link open and active

Data service unit (DSU): Converts signals from a connecting device into the

type of signal required by the leased line Connecting devices may be switches, routers, or

Private Branch Exchanges

Trunk Carrier Services

Usually used in conjunction with a: Private Branch Exchange (PBX):

A common connecting device used with a CSU and a DSU

An on-site switching facility used to interconnect telephones to a Public Switched Telephone Network

Trunk Carrier Services

DSL – Digital Subscriber Line

Supports simultaneous voice and data communications Has a relatively low cost, with high transmission speeds Is technically referred to as “xDSL”, whereby the “x” indicates

the type of DSL service provided, for example: ADSL HDSL SDSL IDSL VDSL

DSL – Digital Subscriber Line

Two terms associated with DSL are Downstream and Upstream: Downstream describes the transmitting of data from a

remote location to a local device Upstream describes the transmitting of data from a local

device to a remote location Most users of DSL primarily utilize downstream

transmission Downstream and upstream transmission rates can vary or

be the same, depending on the DSL service Asymmetric versus Symmetric DSL

DSL – Digital Subscriber Line

The most common form of DSL is ADSL, or Asymmetric Digital Subscriber Line

With ADSL, downstream transmissions are faster than upstream transmission, for example: Up to 1 Mbps downstream Up to 512 Kbps upstream

Users located less than 18,000 feet from a local provider’s point-of-presence are good candidates for ADSL

DSL users also need to select between static and dynamic IP addresses

DSL – Digital Subscriber Line

DSL – Digital Subscriber Line

In Summary

There are a variety of WAN connectivity solutions Circuit switched networks are most appropriate for voice Packet switched networks are most appropriate for data Common packet switched networks include X.25, Frame

Relay, and ATM Point-to-Point Protocol, while inexpensive, offers low

bandwidth Trunk services, particularly T-1, T-3, and Fractional T-1, are

popular business WAN solutions Forms of Digital Subscriber Line are becoming increasingly

implemented as home and business solutions