Lesson 6: WAN Basics

In this Lesson, we’ll discuss the WAN. We’ll start by defining what a WAN is, and then move on to talking about basic technology such as WAN devices and circuit and packet switching.
also cover transmission options from POTS (plain old telephone service) to Frame Relay, to leased lines, and more.
Finally, we’ll discuss wide area requirements including a section on minimizing WAN charges with bandwidth optimization features.

The Agenda

- WAN Basics

- Transmission Options

- WAN Requirements & Solutions

WAN Basics

What Is a WAN?

So, what is a WAN? A WAN is a data communications network that serves users across a broad geographic area and often uses transmission facilities provided by common carriers such as telephone companies. These providers are companies like MCI, AT&T, UuNet, and Sprint. There are also many small service providers that provide connectivity to one of the larger carriers’ networks and may even have email servers to store clients mail until it is retrieved.

- Telephone service is commonly referred to as plain old telephone service (POTS).

- WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

Common WAN network components include WAN switches, access servers, modems, CSU/DSUs, and ISDN Terminals.

WAN Devices

A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch traffic such as Frame Relay, X.25, and SMDS and operate at the data link layer of the OSI reference model. These WAN switches can share bandwidth among allocated service priorities, recover from outages, and provide network design and management systems.

A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to analog. At the destination, these analog signals are returned to their digital form.

An access server is a concentration point for dial-in and dial-out connections.

A channel service unit/digital service unit (CSU/DSU) is a digital interface device that adapts the physical interface on a data terminal equipment device (such as a terminal) to the interface of a data circuit terminating (DCE) device (such as a switch) in a switched-carrier network. The CSU/DSU also provides signal timing for communication between these devices.

An ISDN terminal is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232. A terminal adapter is essentially an ISDN modem.

WAN Terminating Equipment

The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DCE is the service provider, and the DTE is the attached device (the customer’s device). In this model, the services offered to the DTE are made available through a modem or channel service unit/data service unit (CSU/DSU).
CSU/DSU (Channel Service Unit / Data Service Unit) Device that connects the end-user equipment to the local digital telephone loop or to the service providers data transmission loop. The DSU adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. Also responsible for such functions as signal timing for synchronous serial transmissions.
Unless a company owns (literally) the lines over which they transport data, they must utilize the services of a Service Provider to access the wide area network.

Circuit Switching

- Dedicated physical circuit established, maintained, and terminated through a carrier network for each communication session

- Datagram and data stream transmissions

- Operates like a normal telephone call

- Example: ISDN

Service providers typically offer both circuit switching packet switching services.
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

Packet Switching

Packet switching is a WAN switching method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. Statistical multiplexing is used to enable devices to share these circuits. Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25 are examples of packet-switched WAN technologies.

- Network devices share a point-to-point link to transport packets from a source to a destination across a carrier network

- Statistical multiplexing is used to enable devices to share these circuits

- Examples: ATM, Frame Relay, SMDS, X.25

WAN Virtual Circuits

- A logical circuit ensuring reliable communication between two devices

- Switched virtual circuits (SVCs)

- Dynamically established on demand
- Torn down when transmission is complete
- Used when data transmission is sporadic

- Permanent virtual circuits (PVCs)

- Permanently established
- Save bandwidth for cases where certain virtual circuits must exist all the time

- Used in Frame Relay, X.25, and ATM

A virtual circuit is a logical circuit created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). Virtual circuits are used in Frame Relay and X.25 and ATM.
SVCs are dynamically established on demand and are torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic.
PVCs are permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time.

WAN Protocols

The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. A protocol implements the functions of one or more of the OSI layers. A wide variety of communication protocols exist, but all tend to fall into one of the following groups:

- LAN protocols: operate at the physical and data link layers and define communication over the various LAN media

- WAN protocols: operate at the lowest three layers and define communication over the various wide-area media.

- Network protocols: are the various upper-layer protocols in a given protocol suite.

- Routing protocols: network-layer protocols responsible for path determination and traffic switching.

SDLC:-
Synchronous Data Link Control. IBM’s SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB.

HDLC:-
High-Level Data Link Control. Bit-oriented synchronous data link layer protocol developed by ISO. Specifies a data encapsulation method on synchronous serial links using frame characters and checksums.

LAPB:-
Link Access Procedure, Balanced. Data link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC.

PPP:-
Point-to-Point Protocol. Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits with built-in security features. Works with several network layer protocols, such as IP, IPX, & ARA.

X.25 PTP:-
Packet level protocol. Network layer protocol in the X.25 protocol stack. Defines how connections are maintained for remote terminal access and computer communications in PDNs. Frame Relay is superseding X.25.

ISDN:-
Integrated Services Digital Network. Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.

Frame Relay:-
Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, and generally replaces it.

Transmission Options or WAN Services

There are a number of transmission options available today. They fall either into the analog or digital category. Next let’s take a brief look at each of these transmission types.

POTS Using Modem Dialup

Analog modems using basic telephone service are asynchronous transmission-based, and have the following benefits:

- Available everywhere
- Easy to set up
- Dial anywhere on demand
- The lowest cost alternative of any wide-area service

Integrated Services Digital Network (ISDN)

ISDN is a digital service that can use asynchronous or, more commonly, synchronous transmission. ISDN can transmit data, voice, and video over existing copper phone lines. Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity—incurring charges only when the line is active.
ISDN provides a high-bandwidth, cost-effective solution for companies requiring light or sporadic high-speed access to either a central or branch office.
ISDN can transmit data, voice, and video over existing copper phone lines.
Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity —incurring charges only when the line is active.
Companies needing more permanent connections should evaluate leased-line connections.

- High bandwidth
- Up to 128 Kbps per basic rate interface
- Dial on demand
- Multiple channels
- Fast connection time
- Monthly rate plus cost-effective, usage-based billing
- Strictly digital

ISDN comes in two flavors, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two “B” or bearer channels of 64 Kbps each and one additional signaling channel called the “D” or delta channel.
While it requires only one physical connection, ISDN provides two channels that remote telecommuters use to connect to the company network.
PRI provides up to 23 bearer channels of 64 Kbps each and one D channel for signaling. That’s 23 channels but with only one physical connection, which makes it an elegant solution- there’s no wiring mess (PRI service typically provides 30 bearer channels outside the U.S. and Canada).
You’ll want to use PRI at your central site if you plan to have many ISDN dial-in clients.

Transmission Options or WAN Services

There are a number of transmission options available today. They fall either into the analog or digital category. Next let’s take a brief look at each of these transmission types.

POTS Using Modem Dialup

Analog modems using basic telephone service are asynchronous transmission-based, and have the following benefits:

- Available everywhere
- Easy to set up
- Dial anywhere on demand
- The lowest cost alternative of any wide-area service

Integrated Services Digital Network (ISDN)

ISDN is a digital service that can use asynchronous or, more commonly, synchronous transmission. ISDN can transmit data, voice, and video over existing copper phone lines. Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity—incurring charges only when the line is active.
ISDN provides a high-bandwidth, cost-effective solution for companies requiring light or sporadic high-speed access to either a central or branch office.
ISDN can transmit data, voice, and video over existing copper phone lines.
Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity —incurring charges only when the line is active.
Companies needing more permanent connections should evaluate leased-line connections.

- High bandwidth
- Up to 128 Kbps per basic rate interface
- Dial on demand
- Multiple channels
- Fast connection time
- Monthly rate plus cost-effective, usage-based billing
- Strictly digital

ISDN comes in two flavors, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two “B” or bearer channels of 64 Kbps each and one additional signaling channel called the “D” or delta channel.
While it requires only one physical connection, ISDN provides two channels that remote telecommuters use to connect to the company network.
PRI provides up to 23 bearer channels of 64 Kbps each and one D channel for signaling. That’s 23 channels but with only one physical connection, which makes it an elegant solution- there’s no wiring mess (PRI service typically provides 30 bearer channels outside the U.S. and Canada).
You’ll want to use PRI at your central site if you plan to have many ISDN dial-in clients.

Transmission Options or WAN Services

There are a number of transmission options available today. They fall either into the analog or digital category. Next let’s take a brief look at each of these transmission types.

POTS Using Modem Dialup

Analog modems using basic telephone service are asynchronous transmission-based, and have the following benefits:

- Available everywhere
- Easy to set up
- Dial anywhere on demand
- The lowest cost alternative of any wide-area service

Integrated Services Digital Network (ISDN)

ISDN is a digital service that can use asynchronous or, more commonly, synchronous transmission. ISDN can transmit data, voice, and video over existing copper phone lines. Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity—incurring charges only when the line is active.
ISDN provides a high-bandwidth, cost-effective solution for companies requiring light or sporadic high-speed access to either a central or branch office.
ISDN can transmit data, voice, and video over existing copper phone lines.
Instead of leasing a dedicated line for high-speed digital transmission, ISDN offers the option of dialup connectivity —incurring charges only when the line is active.
Companies needing more permanent connections should evaluate leased-line connections.

- High bandwidth
- Up to 128 Kbps per basic rate interface
- Dial on demand
- Multiple channels
- Fast connection time
- Monthly rate plus cost-effective, usage-based billing
- Strictly digital

ISDN comes in two flavors, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two “B” or bearer channels of 64 Kbps each and one additional signaling channel called the “D” or delta channel.
While it requires only one physical connection, ISDN provides two channels that remote telecommuters use to connect to the company network.
PRI provides up to 23 bearer channels of 64 Kbps each and one D channel for signaling. That’s 23 channels but with only one physical connection, which makes it an elegant solution- there’s no wiring mess (PRI service typically provides 30 bearer channels outside the U.S. and Canada).
You’ll want to use PRI at your central site if you plan to have many ISDN dial-in clients.

Leased Line

Leased lines are most cost-effective if a customer’s daily usage exceeds four to six hours. Leased lines offer predictable throughput with bandwidth typically 56 Kbps to 1.544 Mbps. They require one connection per physical interface (namely, a synchronous serial port).

- One connection per physical interface
- Bandwidth: 56 kbps–1.544 Mbps
- T1/E1 and fractional T1/E1
- Cost effective at 4–6 hours daily usage
- Dedicated connections with predictable throughput
- Permanent
- Cost varies by distance

Frame Relay

Frame Relay provides a standard interface to the wide-area network for bridges, routers, front-end processors (FEPs), and other LAN devices. A Frame Relay interface is designed to act like a wide-area LAN- it relays data frames directly to their destinations at very high speeds. Frame Relay frames travel over predetermined virtual circuit paths, are self-routing, and arrive at their destination in the correct order.
Frame Relay is designed to handle the LAN-type bursty traffic efficiently.
The guaranteed bandwidth (known as committed information rate or CIR) is typically between 56 Kbps and 1.544 Mbps.
The cost is normally not distance-sensitive.

Connecting Offices with Frame Relay

Companies who require office-to-office communications, usually choose between a dedicated leased-line connection or a packet-based service, such as Frame Relay or X.25. As a rule, higher connect times make leased-line solutions more cost-effective.
Like ISDN, Frame Relay requires only one physical connection to the Frame Relay network, but can support many Permanent Virtual Circuits, or PVCs.

Frame Relay service is often less expensive than leased lines, and the cost is based on:

- The committed information rate (CIR), which can be exceeded up to the port speed when the capacity is available on your carrier’s network.
- Port speed
- The number of permanent virtual circuits (PVCs) you require; a benefit to users who need reliable, dedicated connections to resources simultaneously.

X.25

X.25 networks implement the internationally accepted ITU-T standard governing the operation of packet switching networks. Transmission links are used only when needed. X.25 was designed almost 20 years ago when network link quality was relatively unstable. It performs error checking along each hop from source node to destination node.
The bandwidth is typically between 9.6Kbps and 64Kbps.
X.25 is widely available in many parts of the world including North America, Europe, and Asia.
There is a large installed base of X.25 devices.

Digital Subscriber Line (xDSL)

- DSL is a pair of “modems” on each end of a copper wire pair
- DSL converts ordinary phone lines into high-speed data conduits
- Like dial, cable, wireless, and T1, DSL by itself is a transmission technology, not a complete solution
- End-users don’t “buy” DSL, they “buy” services, such as high-speed Internet access, intranet, leased line, voice, VPN, and video on demand
- Service is limited to certain geographical areas

Digital subscriber line (DSL) technology is a high-speed service that, like ISDN, operates over ordinary twisted-pair copper wires supplying phone service to businesses and homes in most areas. DSL is often more expensive than ISDN in markets where it is offered today.
Using special modems and dedicated equipment in the phone company's switching office, DSL offers faster data transmission than either analog modems or ISDN service, plus-in most cases-simultaneous voice communications over the same lines. This means you don't need to add lines to supercharge your data access speeds. And since DSL devotes a separate channel to voice service, phone calls are unaffected by data transmissions.

DSL Modem Technology

DSL has several flavors. ADSL delivers asymmetrical data rates (for example, data moves faster on the way to your PC than it does on the way out to Internet). Other DSL technologies deliver symmetrical data (same speeds traveling in and out of your PC).
The type of service available to you will depend on the carriers operating in your area. Because DSL works over the existing telephone infrastructure, it should be easy to deploy over a wide area in a relatively short time. As a result, the pursuit of market share and new customers is spawning competition between traditional phone companies and a new breed of firms called competitive local exchange carriers (CLECs).

Asynchronous Transfer Mode (ATM)

ATM is short for Asynchronous Transfer Mode, and it is a technology capable of transferring voice, video and data through private and public networks. It uses VLSI technology to segment data at high speeds into units called cells. Basically it carves up Ethernet or Token ring packets and creates cells out of them.

Each cell contains 5 bites of header information, 48 bites of payload for 53 bites total in every cell. Each cell contains identifiers that specify the data stream to which they belong. ATM is capable of T3 speeds, E3 speeds in Europe as well as Fiber speed, like Sonet which is asynchronous optical networking speeds of OC-1 and up. ATM technology is primarily used in enterprise backbones or in WAN links.

How to choose Service?

Analog services are the least expensive type of service. ISDN costs somewhat more but improves performance over even the fastest current analog offerings. Leased lines are the costliest of these three options, but offer dedicated, digital service for more demanding situations. Which is right?
You’ll need to answer a few questions:

- Will employees use the Internet frequently?
- Will the Internet be used for conducting business (for example, inventory management, online catalog selling or account information or bidding on new jobs)?
- Do you anticipate a large volume of traffic between branch offices of the business?
- Is there a plan to use videoconferencing or video training between locations?
- Who will use the main office’s connection to the Internet - individual employees at the central office, telecommuting workers dialing in from home, mobile workers dialing in from the road?

The more times the answer is “yes”, the more likely that leased line services are required. It is also possible to mix and match services. For example, small branch offices or individual employees dialing in from home might connect to the central office using ISDN, while the main connection from the central office to the Internet can be a T1.
Which service you select also depends on what the Internet Service Provider (is using. If the ISP’s maximum line speed is 128K, as with ISDN, it wouldn’t make sense to connect to that ISP with a T1 service. It is important to understand that as the bandwidth increases, so do the charges, both from the ISP and the phone company. Keep in mind that rates for different kinds of connections vary from location to location.

Let’s compare our technology options, assuming all services are available in our region. To summarize:

- A leased-line service provides a dedicated connection with a fixed bandwidth at a flat rate. You pay the same monthly fee regardless how much or how little you use the connection.

- A packet-switched service typically provides a permanent connection with specific, guaranteed bandwidth (Frame Relay). Temporary connections (such as X.25) may also be available. The cost of the line is typically a flat rate, plus an additional charge based on actual usage.

- A circuit-switched service provides a temporary connection with variable bandwidth, with cost primarily based on actual usage.

How to choose Service?

Analog services are the least expensive type of service. ISDN costs somewhat more but improves performance over even the fastest current analog offerings. Leased lines are the costliest of these three options, but offer dedicated, digital service for more demanding situations. Which is right?
You’ll need to answer a few questions:

- Will employees use the Internet frequently?
- Will the Internet be used for conducting business (for example, inventory management, online catalog selling or account information or bidding on new jobs)?
- Do you anticipate a large volume of traffic between branch offices of the business?
- Is there a plan to use videoconferencing or video training between locations?
- Who will use the main office’s connection to the Internet - individual employees at the central office, telecommuting workers dialing in from home, mobile workers dialing in from the road?

The more times the answer is “yes”, the more likely that leased line services are required. It is also possible to mix and match services. For example, small branch offices or individual employees dialing in from home might connect to the central office using ISDN, while the main connection from the central office to the Internet can be a T1.
Which service you select also depends on what the Internet Service Provider (is using. If the ISP’s maximum line speed is 128K, as with ISDN, it wouldn’t make sense to connect to that ISP with a T1 service. It is important to understand that as the bandwidth increases, so do the charges, both from the ISP and the phone company. Keep in mind that rates for different kinds of connections vary from location to location.

Let’s compare our technology options, assuming all services are available in our region. To summarize:

- A leased-line service provides a dedicated connection with a fixed bandwidth at a flat rate. You pay the same monthly fee regardless how much or how little you use the connection.

- A packet-switched service typically provides a permanent connection with specific, guaranteed bandwidth (Frame Relay). Temporary connections (such as X.25) may also be available. The cost of the line is typically a flat rate, plus an additional charge based on actual usage.

- A circuit-switched service provides a temporary connection with variable bandwidth, with cost primarily based on actual usage.

Wide-Area Network Requirements

- Minimize bandwidth costs
- Maximize efficiency
- Maximize performance
- Support new/emerging applications
- Maximize availability
- Minimize management and maintenance

Manage Bandwidth to Control Cost

Because transmission costs are by far the largest portion of a network’s cost, there are a number of bandwidth optimization features you should be aware of that enable the cost-effective use of WAN links. These include dial-on-demand routing, bandwidth-on-demand, snapshot routing, IPX protocol spoofing, and compression.
Dial-on-demand ensures that you’re only paying for bandwidth when it’s needed for switched services such as ISDN and asynchronous modem (and switched 56Kb in the U.S. and Canada only).
Bandwidth-on-demand gives you the flexibility to add additional WAN bandwidth when it’s needed to accommodate heavy network loads such as file transfers. Snapshot routing prevents unnecessary transmissions. It inhibits your switched network from being dialed solely for the purpose of exchanging routing updates at short intervals (e.g.: 30 seconds). Many of you are familiar with compression, which is also a good method of optimization.

Lets take a close look at a few features that will keep your WAN costs down.

- Dial-on-Demand Routing

Dial-on-demand routing allows a router to automatically initiate and close a circuit-switched session.
With dial-on-demand routing, the router dials up the WAN link only when it senses “interesting” traffic. Interesting traffic might be defined as any traffic destined for the remote network, or only traffic related to a specific host address or service.
Equally important, dial-on-demand routing enables the router to take down the connection when it is no longer needed, ensuring that the user will not have unnecessary WAN usage charges.

- Bandwidth-on-Demand

Bandwidth-on-demand works in a similar way.
When the router senses that the traffic level on the primary link has reached a certain threshold—say, when a user starts a large file transfer—it automatically dials up additional bandwidth through the PSTN to accommodate the increased load.
For example, if you’re using ISDN, you may decide that when the first B channel reaches 75% saturation for more than one minute, your router will automatically dial up a second B channel. When the traffic load on the second B channel falls below 40%, the channel is automatically dropped.

- Snapshot Routing

By default, routing protocols such as RIP exchange routing tables every 30 seconds. If placed as calls, these routine updates will drive up WAN costs unnecessarily, and Snapshot Routing limits these calls to the remote site.
A remote router with this feature only requests a routing update when the WAN link is already up for the purpose of transferring user application data.
Without Snapshot Routing, your ISDN connection would be dialed every 30 seconds; this feature ensures that the remote router always has the most up-to-date routing information but only when needed.

- IPX Protocol Spoofing

Protocol spoofing allows the user to improve performance while providing the ability to use lower line speeds over the WAN.

- Compression

Compression reduces the space required to store data, thus reducing the bandwidth required to transmit. The benefit of these compression algorithms is that users can utilize lower line speeds if needed to save costs. Compression also provides the ability to move more data over a link than it would normally bear.

- Three types
Header
Link
Payload

- Van Jacobson header compression
RFC 1144
Reduces header from 40 to ~5 bytes

- Dial Backup

Dial backup addresses a customer’s need for reliability and guaranteed uptime. Dial backup capability offers users protection against WAN downtime by allowing them to configure a backup serial line via a circuit-switched connection such as ISDN. When the software detects the loss of a signal from the primary line device or finds that the line protocol is down, it activates the secondary line to establish a new session and continue the job of transmitting traffic over the backup line.

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- Summary -

- The network operates beyond the local LAN’s geographic scope. It uses the services of carriers like regional bell operating companies (RBOCs), Sprint, and MCI.

- WANs use serial connections of various types to access bandwidth over wide-area geographies.

- An enterprise pays the carrier or service provider for connections used in the WAN; the enterprise can choose which services it uses; carriers are usually regulated by tariffs.>

- WANs rarely shut down, but since the enterprise must pay for services used, it might restrict access to connected workstations. All WAN services are not available in all locations

- Snapshot Routing

By default, routing protocols such as RIP exchange routing tables every 30 seconds. If placed as calls, these routine updates will drive up WAN costs unnecessarily, and Snapshot Routing limits these calls to the remote site.
A remote router with this feature only requests a routing update when the WAN link is already up for the purpose of transferring user application data.
Without Snapshot Routing, your ISDN connection would be dialed every 30 seconds; this feature ensures that the remote router always has the most up-to-date routing information but only when needed.

- IPX Protocol Spoofing

Protocol spoofing allows the user to improve performance while providing the ability to use lower line speeds over the WAN.

- Compression

Compression reduces the space required to store data, thus reducing the bandwidth required to transmit. The benefit of these compression algorithms is that users can utilize lower line speeds if needed to save costs. Compression also provides the ability to move more data over a link than it would normally bear.

- Three types
Header
Link
Payload

- Van Jacobson header compression
RFC 1144
Reduces header from 40 to ~5 bytes

- Dial Backup

Dial backup addresses a customer’s need for reliability and guaranteed uptime. Dial backup capability offers users protection against WAN downtime by allowing them to configure a backup serial line via a circuit-switched connection such as ISDN. When the software detects the loss of a signal from the primary line device or finds that the line protocol is down, it activates the secondary line to establish a new session and continue the job of transmitting traffic over the backup line.

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- Summary -

- The network operates beyond the local LAN’s geographic scope. It uses the services of carriers like regional bell operating companies (RBOCs), Sprint, and MCI.

- WANs use serial connections of various types to access bandwidth over wide-area geographies.

- An enterprise pays the carrier or service provider for connections used in the WAN; the enterprise can choose which services it uses; carriers are usually regulated by tariffs.>

- WANs rarely shut down, but since the enterprise must pay for services used, it might restrict access to connected workstations. All WAN services are not available in all locations