procurve wan technologies

Rev. 5.21 HP Restricted 1 –1

Overview of WAN Connections Module 1

Objectives This module introduces the basic elements of WAN connections and describes the

role each element plays in creating that connection. After completing this module,

you should be able to:

Describe the three basic elements of a WAN connection

Describe how public carrier networks are used to create a WAN connection

Identify the three types of circuits used to create a WAN connection

Describe how local loops connect the subscriber’s premises to public carrier

networks

Identify the electrical signaling specifications and related technologies used

in public carrier networks

Explain the differences and similarities between T-, E-, and J-carrier WAN

connections

Sample

ProCurve WAN Technologies

1 –2 HP Restricted Rev. 5.21

Introduction

Companies that have multiple offices need a cost-effective, efficient means to

exchange data between those offices. Many companies have created intranets or

extranets, which enable users at different locations to view, upload, and download

information. However, intranets and extranets are only a partial solution to the

problem because the sharing of data is limited to what can be posted on the

intranet or extranet. Each office must maintain its own database, and users cannot

access data stored at other locations. For example, the accounting department at

each office must have a separate database, which cannot be shared over an

intranet.

Security is also an issue because the intranet must be connected to the Internet, in

order to serve multiple locations. The various offices connected through the

intranet can be protected by firewalls, but firewalls are not impervious to attacks.

For many companies, a Wide Area Network (WAN) is a better and more cost-

effective solution for connecting multiple branch offices to a main office. A WAN

allows companies to exchange all types of information, including voice and data.

Combining voice and data traffic can reduce operating expenses for many

companies.

Overview of WAN Connections


This course focuses on WAN connections created using public carrier networks.

Businesses, organizations, and government entities use public carrier networks to

create WAN connections for three primary reasons:

Using public carrier network infrastructure is almost always more cost

effective than using privately owned infrastructure. Public carrier networks

allow many subscribers to share the costs of installing, managing, and

maintaining the infrastructure required to create WAN connections.

Using privately owned infrastructure to create long-distance and international

WAN connections is impractical, sometimes even impossible, and cost

prohibitive. WAN connections that use privately owned infrastructure are

generally limited to relatively short distances, and installing them is beyond

the capacity of all but the largest organizations.

WAN connections created through public carrier networks are substantially

similar to WAN connections created using privately owned infrastructure in

terms of security and performance. Public carrier networks also provide

levels of reliability and redundancy that privately owned infrastructure

typically cannot provide.

WAN routers connect the LANs at each location, identify the traffic addressed to

another LAN, and route the traffic to the next hop. As explained throughout this

course, WAN routers support a variety of WAN connection types, including:

Dedicated T-, E-, and J-carrier lines

Integrated Services Digital Network (ISDN)

Digital Subscriber Line (DSL)



A WAN Connection Defined

In the most general sense, a WAN is a geographically dispersed

telecommunications network. For the purposes of this course, however,

a WAN is defined as a network created to connect two or more LANs.

WAN connections can connect LANs located in the same city or around the

world. As the figure shows, a public carrier network is commonly used to

create WAN connections between LANs in different parts of the world.

Public carrier networks include the Public Switched Telephone Network

(PSTN), which serves the United States and Canada, and Public Telephone

and Telegraph (PTT) companies, which serve Mexico, Europe, Asia, South

America, and other parts of the world.



Basic Elements of a WAN Connection

All WAN connections consist of three basic elements:

The physical transmission media.

Electrical signaling specifications for generating, transmitting, and receiving

signals through various transmission media.

Data-link–layer protocols that provide logical flow control for moving data

between peers in the WAN. (Peers are the devices at either end of a WAN

connection.)

As the figure shows, physical transmission media and electrical specifications are

part of the physical layer (which is layer one) of the Open Systems Interconnection

(OSI) model, and data-link–layer protocols are part of the data-link layer (which is

layer two).

This module focuses on the physical transmission media, the electrical signaling

specifications, and the related OSI layer-one technologies that are used to create

WAN connections through public carrier networks.

Data-link–layer protocols are explained in detail in Module 2: Data-Link–Layer

Protocols.



Physical Transmission Media and Infrastructure

The first basic element of a WAN connection is the physical transmission medium.

The most common physical transmission medium used in public carrier networks

is twisted-pair copper wire, originally installed for Plain Old Telephone Service

(POTS) connections. Twisted pair is currently used in the last mile of 90 percent of

all WAN connections.

Other physical transmission media include coaxial copper cable, fiber optic cable,

and the Earth’s atmosphere, which carries signals by such means as infrared and

microwave transmissions.

The physical transmission media are a large part of what is commonly called

infrastructure. Infrastructure also includes telecommunications switching and

routing equipment.

WAN connections can be created using public carrier network infrastructure,

privately owned infrastructure, or a combination of the two.



Types of WAN Circuits

As the figure shows, three types of circuits are used to create WAN connections

through public carrier networks:

Dedicated circuits

Permanent virtual circuits (PVCs)

Switched virtual circuits (SVCs)

Dedicated Circuits Dedicated circuits are permanent circuits dedicated to a single subscriber. The

connection is always active. The subscriber purchases dedicated time slots, or

channels, that provide a specific amount of bandwidth that is always available for

the subscriber to use. The channels in a dedicated circuit are created using time

division multiplexing (TDM), which is discussed later in this module.

In addition to providing guaranteed bandwidth at all times, dedicated circuits

provide the most secure and reliable WAN connections available.



Dedicated circuits are used to create the following point-to-point WAN

connections:

Carrier lines (which are explained later in this module and in

Module 3: Carrier Line WAN Connections)

DSL connections (which are explained in Module 5: DSL WAN Connections)

Permanent Virtual Circuits (PVCs) PVCs are also permanent circuits dedicated to a single subscriber. The connection

is always active. However, because multiple virtual circuits share a physical

circuit, there is no guarantee that any specific amount of bandwidth will be

available at any specific time. Sometimes there may not be any bandwidth

available on the physical circuit because the physical circuit is saturated.

When the physical circuit is saturated, the traffic is temporarily stored at a

switching point until bandwidth becomes available. When bandwidth becomes

available, the stored traffic is forwarded to its destination. This process is referred

to as store-and-forward processing, or packet switching, which is the same

processing method used on LANs.

PVCs provide an average bandwidth guarantee through statistical multiplexing

(STM), which underlies packet switching technology.

Because PVCs are more cost effective for the public carrier, PVCs are usually less

expensive for the subscriber than dedicated circuits. PVCs are commonly used for

Frame Relay, which is explained in detail in Module 6: Frame Relay.

Switched Virtual Circuits (SVCs) SVCs are identical to PVCs in all respects, except that they are temporary physical

circuits. SVCs are activated when a subscriber initiates a connection to transmit

data. When all data have been transmitted, the connection is deactivated, and the

physical circuit resources are made available to other subscribers.

SVCs are used to create dial-up WAN connections, including ISDN WAN

connections, which are explained in Module 4: ISDN WAN Connections.



PSTN (United States and Canada)

In the United States and Canada, most WAN connections are created through the

PSTN. As the figure shows, the PSTN consists of local exchange carriers (LECs)

and interexchange carriers (IXCs). (LECs are also referred to as telcos.)

Local Exchange Carriers LECs operate the infrastructure that provides access to the PSTN in a limited

geographic area. The area served by a LEC is referred to as a local access and

transport area (LATA). LECs include incumbent local exchange carriers (ILECs)

and competitive local exchange carriers (CLECs).

ILECs are the Regional Bell operating companies (RBOCs) that provide service in

a specific LATA. For example, SBC is the current ILEC in California. ILECs were

created in 1983 when the U.S. government deregulated the telecommunications

industry and mandated the breakup of AT&T.

Deregulation also led to the creation of CLECs, which provide the same services

as ILECs and compete with ILECs in specific geographic areas. For example,

Covad Communications is a CLEC that competes with SBC in California.



Interexchange Carriers IXCs aggregate voice and data traffic from numerous LECs. They operate the

infrastructure that connects LATAs to the interLATAs that move traffic

throughout the United States and Canada. AT&T, Sprint, and MCI are all IXCs

based in the United States. IXCs are commonly referred to as long-distance

carriers.

IXCs also provide the infrastructure that enables PSTN subscribers to create WAN

connections to PTT networks in Europe, Asia, South America, and other parts of

the world.



Public Telephone and Telegraph (PTT) Companies

In most countries outside of the United States and Canada, the public telephone

network is owned and operated by government-owned monopolies called PTTs.

As the figure shows, a PTT operates the entire telecommunications infrastructure

within a country’s borders. For example, British Telecom (BT) provides border-to-

border service in the United Kingdom, while Deutsche Telecom (DTAG) provides

this service in Germany.

PTTs provide both the local-access and long-distance transport infrastructure

needed to create WAN connections through the public carrier network. As the

figure shows, carrier interconnects link individual PTTs to provide an international

public carrier system.



The Local Loop

The connection between a subscriber’s premises and the public carrier’s nearest

central office (CO) is referred to as the local loop. The local loop includes the

entire telecommunications infrastructure—such as repeaters, switches, cable, and

connectors—required to connect a subscriber’s premises to the CO.

A line of demarcation (demarc) separates a subscriber’s wiring and equipment

from that of the public carrier. Each party owns, operates, and maintains the wiring

and equipment on its side of the demarc.

Public carrier networks were originally designed to carry analog voice calls.

Therefore, copper wire is the most common physical transmission medium used on

the local loop. Because of the limits in the signal-carrying capacity of copper wire,

local loops that use copper wire are the slowest, least capable component of a

WAN connection. Public carriers are beginning to install coaxial and fiber optic

cable in local loops to meet ever-increasing bandwidth demands.

Local loop connection types include carrier lines, which are described in

Module 3: Carrier Line WAN Connections. Local loop connection types also

include ISDN and DSL. ISDN and DSL are digital technologies designed to

maximize the limited capabilities of existing local loop copper wiring. ISDN and

DSL are discussed briefly in the next two sections.



ISDN Local Loops ISDN provides integrated voice and data services by means of a fully digital local

loop. An ISDN connection requires Category-3 (CAT-3) or higher twisted pair and

is delivered by means of an SVC.

ISDN is a local loop-only technology. When ISDN traffic reaches the public

carrier’s nearest CO, it is converted for transport through existing public carrier

infrastructure.

ISDN is available in two levels of service: Basic Rate Interface (BRI) and Primary

Rate Interface (PRI). BRI service provides 128 Kbps of bandwidth. PRI service

provides 1.544 Mbps in total bandwidth in T-carrier systems and 2.048 Mbps in

total bandwidth in E-carrier systems.

ISDN is discussed in-depth in Module 4: ISDN WAN Connections.

DSL Local Loops DSL is a digital service that exists only in the local loop. DSL provides a digital

connection between the subscriber and the public carrier’s CO.

Like ISDN, DSL requires CAT-3 or higher twisted pair wiring. Unlike ISDN, DSL

uses PVCs (rather than SVCs), so DSL connections are always active. A DSL

modem or WAN router connects the subscriber’s premises to the public carrier

network.

Different types of DSL are available. Each public carrier determines the types of

DSL that are available in a local service area. The following are some examples of

the types of DSL:

Asymmetric DSL (ADSL)

High bit rate DSL (HDSL)

Symmetric DSL (SDSL)

Very high bit rate DSL (VDSL)

DSL is discussed in-depth in Module 5: DSL WAN Connections.



Local Loop Transmission Media

CAT-3 and CAT-5 Unshielded Twisted Pair (UTP) are the most common types of

copper wire used in the local loop. In some applications where signal interference

is an issue, Shielded Twisted Pair (STP) is used. In some areas, including parts of

the United Kingdom and the Netherlands, a pair of coaxial cables is used instead

of twisted pair to complete local loop connections.

Other transmission media can be used to complete local loops if transmission

speed is a primary consideration. For example, fiber optic cable and coaxial cable

are both used to create T3 and E3 WAN connections, as discussed in Module 3:

Carrier Line WAN Connections.



Electrical Specifications and Related Technologies

An electrical specification defines a set of communication parameters, or rules,

that determine the transmission speed through a WAN connection. When

engineers create an electrical specification, their objective is to find the best way to

reliably transport traffic, as rapidly as possible, through a given transmission

media.

The electrical specifications used for public carrier networks are based on

cooperative standards developed by the American National Standards Institute

(ANSI), the International Standards Organization (ISO), the Conference of

European Postal and Telecommunications (CEPT), ITU-T, and ITU-T’s

predecessor, the Consultative Committee for International Telegraph and

Telephone (CCITT).

Electrical specifications enable both synchronous and asynchronous

communications over a WAN connection. Synchronous communications use a

clock signal to precisely coordinate signal transport through the transmission

media. Asynchronous communications use start and stop bits, rather than a clock,

to coordinate signals.

The remainder of this module focuses on the synchronous electrical specifications

and related technologies that define the basic unit of bandwidth (the DS0 channel)

used in copper-based public carrier networks.



Digital Signal Zero (DS0)

DS0 is a digital channel operating at 64 Kbps, the amount of bandwidth required to

transmit a single analog voice call through a digital telecommunications network.

Based on the ANSI T1.107 specification, DS0 was originally created in the mid

1960s by Bell Laboratories to transport voice traffic over T-carrier systems. PTTs

subsequently adopted a modified version of ANSI T1.107, the ITU-T G.703

specification, which is the basis of European and international E-carrier systems.

J-carrier systems are also based on a modified version of T1.107 and are similar to

T-carrier systems.

DS0 is the fundamental unit of bandwidth—the fundamental channel—in all

copper-based T-, E-, and J-carrier systems. In E-carrier systems, DS0 is called E0,

and in J-carrier systems, DS0 is called J0. However, the basic signal is virtually

identical in all three carrier systems.

DS0, E0, and J0 channels all use a process called Pulse Code Modulation (PCM)

to convert analog (voice) signals into digital signals.



Pulse Code Modulation (PCM)

PCM is the basis of a standard DS0, E0, and J0 channel. PCM converts a

continuously variable analog signal, such as a voice telephone call, into a stream

of digital bits.

As the figure shows, the PCM sampling process creates a digital signal that

represents the original analog waveform. The analog signal is converted

(modulated) into a digital signal that is sent over the WAN connection. On the

receiving side, the digital signal is demodulated (converted) back to an analog

signal that closely approximates the original analog waveform.

In the PCM sampling process, the analog signal is sampled 8,000 times per

second. Each sample is converted into an 8-bit binary code that represents the

voltage of the analog waveform at the time the sample was taken. Thus, the PCM

process is the mathematical basis for the bandwidth required for a standard DS0,

E0, or J0 channel:

8 bits per sample x 8,000 samples per second = 64 Kbps



Time Division Multiplexing (TDM)

As the figure shows, TDM creates a high-bandwidth channel by combining, or

multiplexing, multiple DS0 signals into a larger, more complex signal. Each DS0

receives an equal time slice within the complex signal in a rotating, repeating

sequence, and thus receives an equal amount of bandwidth. On the receiving end,

TDM is used to recover the original DS0 signals through a reverse process called

demultiplexing.

T-carrier and J-carrier systems use TDM to provision 24 DS0 channels for a T1 or

J1 WAN connection. E-carrier systems use TDM to provision 32 DS0 channels for

an E1 WAN connection. TDM is also used to provision larger channels that use

T1/J1/E1 channels as base multiples, as described in the next section.



Digital Signal Hierarchies

Digital signaling hierarchies define the signal multiplexing used in each type of

physical carrier and determine the transmission speed for each carrier. Digital

signaling hierarchies use small bandwidth channels as base multiples for creating

larger bandwidth channels, or carrier signals, in a carrier system.

DS0, E0, and J0 channels serve as the base multiples for creating T1, E1, and J1

carrier signals. T1, E1, and J1, in turn, serve as the base multiples for creating the

more complex, higher-bandwidth carrier signals used in T2, E2, J2, and higher

carrier systems.

T-, E-, and J-carrier systems use similar, but not identical, digital signaling

hierarchies. T-carrier systems use Digital Signal X (DSX), E-carrier systems use

the CEPT digital signal hierarchy, and J-carrier systems use the Japanese signal

hierarchy. These signaling hierarchies are described in the following sections.



Digital Signal X (DSX)

DSX is the digital signal hierarchy that defines the signal multiplexing used in

T-carrier systems.

As the figure shows, DSX specifies that 24 DS0s are multiplexed to create the DS1

carrier signal used in a T1 carrier. A T1 carrier provides a total transmission rate of

1.544 Mbps (24 x 64 Kbps = 1,536 Kbps + 8 Kbps for framing bits and timing

signal synchronization).

Similarly, DSX specifies the following:

Four DS1 signals are multiplexed to create the DS2 signal used in T2

carriers, which provide a transmission rate of 6.312 Mbps.

28 DS1 signals are multiplexed to create the DS3 signal used in T3 carriers,

which provide a transmission rate of 44.736 Mbps.

168 DS1 signals are multiplexed to create the DS4 signal used in T4 carriers,


336 DS1 signals are multiplexed together to create the DS5 signal used in T5




As the figure shows, DSX specifies the physical carriers used at each level in the

hierarchy. (DSX does not define the physical carrier; ANSI T1.107 defines the

physical components of T-carrier systems.) When combined, the physical carrier

and the DSX hierarchy specify a usable physical layer for each type of carrier in a

T-carrier system.

DSX defines Digital Signal Designators (DSDs), or signaling methods, used to

create the carrier signals used at each level of the hierarchy. DSX also defines

DSX interfaces, which describe the physical connections (pinouts) and signaling

logic (send timing, receive timing, send data, and receive data) necessary for

connected devices to communicate.



CEPT Digital Signal Hierarchy

Like the DSX digital signal hierarchy used in T-carrier systems, the CEPT digital

signal hierarchy defines the signal multiplexing used to create the signals carried

in each E carrier. Unlike DSX, CEPT DSDs are identical to the physical carrier

designator.

As the figure shows, the CEPT hierarchy multiplexes 32 E0 channels to create the

signal that is carried within an E1 physical carrier. An E1 carrier provides a total

transmission rate of 2.048 Mbps.

Similarly, the CEPT hierarchy specifies the following:

Four E1 signals are multiplexed to create the E2 signal used in E2 carriers,


16 E1 signals are multiplexed to create the E3 signal used in E3 carriers,


64 E1 signals are multiplexed to create the E4 signal used in E4 carriers,


256 E1 signals are multiplexed together to create the E5 signal used in E5




Japanese Digital Signal Hierarchy

The Japanese digital signal hierarchy defines the signal multiplexing used to create

the signals carried in each J carrier. Unlike DSX, Japanese DSDs are identical to

the physical carrier designator.

As the figure shows, the Japanese hierarchy multiplexes 24 J0 channels to create

the J1 carrier signal that is carried within a J1 physical carrier. A J1 carrier

provides a total transmission rate of 1.544 Mbps.

Similarly, the Japanese hierarchy specifies the following:

Four J1 signals are multiplexed to create the J2 signal used in J2 carriers,


30 J1 signals are multiplexed to create the J3 signal used in J3 carriers, which

provide a transmission rate of 32.064 Mbps.

240 J1 signals are multiplexed to create the J4 signal used in J4 carriers,


In Japan, most PTTs use the T1 standard for data; the J1 standard is used for voice.

The reasons for using the T1 standard will be discussed in Module 3: Carrier Line

WAN Connections.



Encoding Schemes

Encoding schemes define how digital signals are configured for transport through

a physical transmission medium. Encoding schemes use electrical signals to

represent the logical 0 and 1 bits in a data stream.

The public carrier that provides the local loop service determines the encoding

scheme for the WAN connection. All of the subscriber’s equipment must be

configured to use the public carrier’s encoding scheme. Three encoding schemes

are widely used in T-, E-, and J-carrier systems.

Alternate mark inversion (AMI)

Bipolar 8-zero substitution (B8ZS)

High-density bipolar of order 3 (HDB3)

AMI AMI uses alternating positive and negative voltage (referred to as alternating

polarity or bipolarity) to represent logical 1s, and zero voltage to represent logical

0s. Because AMI uses zero voltage for logical 0, it can cause synchronization loss

between peers at each end of a WAN connection when a data stream contains a

long string of logical 0s.



B8ZS B8ZS is a modified version of AMI. B8ZS prevents the synchronization loss

associated with AMI by limiting the number of consecutive 0s in a data stream to

eight. When eight zeros are detected, B8ZS replaces them with two successive

logical 1s of the same polarity in a process referred to as a bipolar violation. B8ZS

is the predominant encoding scheme used in T-carrier systems.

HDB3 HDB3 is based on AMI and prevents synchronization loss in a manner similar to

B8ZS. HDB3 limits the number of consecutive zeros in a data stream to four, and

it replaces them with three logical 0s and a violation bit with the same polarity as

the last AMI logical 1 detected. HDB3 is the predominant encoding scheme used

in E-carrier systems.



Data-Link–Layer Protocols

Data-link–layer protocols are the third and final element of a basic WAN

connection.

Data-link–layer protocols are found at layer two of the OSI model. They enable

flow control, synchronization, integrity checking, and validation for data streams

passing between the physical layer and the network layer (layer three in the OSI

model).

Module 2: Data-Link–Layer Protocols explains data-link–layer protocols in detail.



Module 1 Summary

In this module, you learned about the following:

Three basic elements of a WAN connection:

Physical transmission media

Electrical signaling specifications

Data-link–layer protocols

Local loops and the public carrier networks that provide them

Three types of circuits used to create a WAN connection:

Dedicated circuit

Permanent virtual circuit

Switched virtual circuit

Electrical specifications and related technologies:

Digital signal hierarchies: DSX, CEPT Digital Signal Hierarchy, and the

Japanese Digital Signal Hierarchy

Pulse code modulation

Time division multiplexing



Learning Check Module 1



1. What are the three basic elements of a WAN connection?

______________________________________________________________

______________________________________________________________

______________________________________________________________

2. Which type of circuit is used to create T-, E-, and J-carrier lines?

a. Switched virtual circuit

b. Permanent circuit

c. Permanent virtual circuit

d. Switched circuit

3. Which digital signaling hierarchy forms the basis of E-carrier lines?

a. DSX

b. JSX

c. CEPT

d. EPT

4. How many DS0s are multiplexed into a T1-carrier line?

a. 16

b. 24

c. 20

d. 32

5. How many E0s are multiplexed into an E1-carrier line?

a. 16

b. 24

c. 20

d. 32

6. How many E1 signals are multiplexed to create the E3 signal used in

E3-carrier lines?

a. 16

b. 24

c. 20

d. 32

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