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Metropolitan and Wide Area Networks. Chapter 8. Introduction. Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs. Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.

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Metropolitan and Wide Area Networks

Chapter 8

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Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs.

Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.

Most organizations cannot afford to build their own MANs and WANs, so they rent or lease circuits from common carriers such as AT&T, MCI, BellSouth, PACTEL or NYNEX.

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How to connect LANs/BNs to WAN/MAN?

Two approaches to building up a WAN/MAN:

  • Build up a private network for LANs/BNs

    • e.g. banking network. You need to rent circuits to connect networks distributed in different cities. This is a question of selecting NSPs.

  • Connect LANs/BNs to the Internet

    • How to connect LANs/BNs to the Internet. This is a question of selecting ISPs.

      A combination is to build up a Virtual Private Network (VPN) using the Internet.

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Network Services Available for MAN and WAN

  • Dialed Circuit Services

    • Direct Dialing (DD) & Wide Area Telephone Services (WATS)

  • Dedicated Circuit Services

    • Voice-grade circuits

    • Wideband Analog Services

    • T-Carrier Circuits

    • Synchronous Optical Network (SONET)

  • Circuit-Switched Services

    • Integrated Services Digital Network (Narrowband & Broadband)

  • Packet-Switched Services

    • X.25, Frame Relay, ATM, SMDS, and Ethernet/IP

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Dedicated Circuit Services

There are two main problems with dialed circuits.

  • Each connection goes through the regular telephone network on a different circuit, which may vary in quality.

  • The data transmission rates on these circuits are usually low 28.8 to 56 Kbps.

    One alternative is to establish a private dedicated circuit, which the user leases from the common carrier for their exclusive use, 24 hrs/day, 7 days/week.

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Dedicated Circuit Services

Dedicate circuits are billed at a flat fee per month and the user has unlimited use of the circuit.

There are six types of dedicated circuits:

  • Voice grade circuits

  • Wideband analog services

  • T Carrier circuits

  • SONET circuits

  • Digital subscriber line circuits

  • Cable modem

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Voice Grade Circuits

Voice grade circuits are analog circuits that work in exactly the same manner as traditional telephone lines, except that you do not dial them.

Dedicated voice grade channels often have conditioning (or equalization) done on them to improve data transmission quality by reducing noise and distortion.

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Wideband Analog Services

Wideband analog services are similar to voice grade circuits but they provide much greater bandwidth.

Typically wideband analog services provide one 48,000 hertz bandwidth channel for use with frequency division multiplexing or as 12 individual voice grade channels (4000 Hz each).

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T Carrier Circuits

T Carrier circuits are dedicated digital circuits and are the most commonly used form of dedicated circuit services in North America today.

Instead of a modem, a channel service unit (CSU) or data service unit (DSU) are used to connect the circuit into the network.

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T Carrier Circuits

T-1 circuit (a.k.a. a DS-1 circuit) provides a data rate of 1.544 Mbps. T-1’s allow 24 simultaneous 64 Kbps channels (with TDM) which transport data, or voice messages using pulse code modulation. (64Kbps x 24 = 1.536Mbps)

T-2 circuit (6.312 Mbps) is basically a multiplexed bundle of four T-1 circuits.

T-3 circuit (44.376 Mbps) is equal to the capacity of 28 T-1 circuits (672 64Kbps channels).

T-4 circuit (274.176 Mbps) is equal to the capacity of 178 T-1s.

Fractional T-1, (FT-1) offers portions of a 1.544 Mbps T-1 for a fraction of its full costs.

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T Carrier System

T-Carrier DesignationDS DesignationSpeed







DS-1 (24 DS-0)

DS-2 (96 DS-0)

DS-3 (672 DS-0)

DS-4 (178 T-1)

64 Kbps

1.544 Mbps

6.312 Mbps

44.375 Mbps

274.176 Mbps

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*Digital signal X (DS-x)

A term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel.

Both the North American T-carrier system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.

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*E Carrier Circuits (European Standard)

  • E1 - 2.048 Mbps (32 DS-0). E1 carries at a higher data rate than T-1 because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use.

  • E2 - 8.448 Mbps.

  • E3 - 16 E1 signals, 34.368 Mbps.

  • E4 - four E3 channels, 139.264 Mbps.

  • E5 - four E4 channels, 565.148 Mbps.

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Synchronous Optical Network (SONET)

  • An Optical Network for Dedicated Connection Services.

  • SONET has been accepted by the U.S. Standards Agency (ANSI) as a standard for optical (fiber) transmission at gigabits per second speed.

  • The International Telecommunications Standards Agency (ITU-T) also standardized a version of SONET under the name of synchronous digital hierarchy (SDH). The two are very similar and can be easily interconnected.

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Synchronous Optical Network (SONET)

SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of 51.84 Mbps. Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1.

Several common carriers (e.g. MCI) now use OC-12 circuits at 622.08 Mbps to carry digitized voice traffic.

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SONET Designation SDH Designation Speed










51.84 Mbps

155.52 Mbps

466.56 Mbps

622.08 Mbps

933.12 Mbps

1.244 Gbps

1.866 Gbps

2.488 Gbps

9.952 Gbps








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NY Information Technology Center

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Circuit Switch vs. Packet Switch

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Circuit Switched Services

The major problem with dedicated circuit services it that the user must carefully plan all circuits needed.

In contrast, switched circuits work much like dialed circuits. The user buys a connection into the common carrier’s network from the end points of the WAN, without specifying all the interconnecting circuits needed.

The primary differences from dialed circuits is that the circuits are entirely digital and that they offer higher data transmission rates.

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Circuit Switched Services

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Narrowband Integrated Services Digital Network

The first generation of Integrated services digital network (ISDN), commonly called narrowband ISDN, combines voice, video, and data over the same digital circuit.

ISDN has long been more of a concept than a reliable service in North America.

Acceptance has been slowed because equipment vendors and common carriers conflicting interpretations of ISDN standards.

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Narrowband Integrated Services Digital Network

Narrowband ISDN offers two types of service:

  • Basic rate interface (BRI, basic access service or 2B+D) provides two 64 Kbps bearer (B) channels and one 16 Kbps control signaling (D) channel.

    One advantage of BRI is it can be installed over existing telephones lines. (if less than 3.5 miles).

  • Primary rate interface (PRI, primary access service or 23B+D) provides 23 64 Kbps ‘B’ channels and one 64 Kbps ‘D’ channel. (basically T-1 service)

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  • “I Still Don’t kNow”

  • “ I Still Don’t Need it”

  • “It Still Does Nothing”

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Broadband Integrated Services Digital Network

The second generation of ISDN is called Broadband ISDN (B-ISDN).

  • Actually uses ATM to move data

  • A circuit switched service and is backwardly compatible with ISDN.

  • Currently offered in three services:

    • Full duplex channel at 155.2 Mbps.

    • Full duplex channel at 622.08 Mbps.

    • Asymmetrical service (Upstream at 155.2 Mbps, downstream at 622.08 Mbps).

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Packet Switched Services

Packet switched services enable multiple connections to exist simultaneously between computers.

With packet switching users buy a connection into the common carrier network, and connects via a packet assembly/ disassembly device (PAD).

Packet switching splits messages into small segments called packets.

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Packet Switched Services

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Packet Switched Services

Packets from separate messages are interleaved with other packets for transmission.

Although the packets from one data stream may mix (interleave) with several other data streams during their journey, it is unlikely that packets from two different data streams will travel together during the entire length of their transmission.

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Packet Switched Services

There are two methods used to route packets:

  • A Datagram is a connectionless service which adds a destination and sequence number to each packet, in addition to information about the data stream to which the packet belongs. Packets may follow a different route, and are reassembled at the destination.

  • In a Virtual circuit the packet switched network establishes an end-to-end circuit between the sender and receiver. All packets for that transmission take the same route over the virtual circuit that has been set up for that transmission.

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Packet Switched Services

Packet switched services are often provided by different common carriers than the one from which organizations get their usual telephone and data services.

Therefore, organizations often lease a dedicated circuits from their offices to the packet switched network point-of-presence (POP).

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The oldest packet switched service is X.25, a standard developed by ITU-T. X.25 offers datagram, switched virtual circuit, and permanent virtual circuit services (Data link layer protocol: LAPB (Link Access Procedure-Balanced), network layer protocol PLP).

Although widely used in Europe, X.25 is not widespread in North America. The primary reason is transmission speed, now 2.048 Mbps (up from 64 Kbps).

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X.25 Protocol Suite

It maps to the lowest three layers of the OSI model.

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Frame Relay

Frame relay is a newer packet switching technology that transmits data faster than X.25. It differs from X.25 and traditional networks in three important ways:

1. Frame relay only operates at the data link layer.

2. Frame relay networks do not perform error control.

3. Frame relay defines two connection data rate that are negotiated per connection and for each virtual circuit as it is established: Committed information rate (CIR) and Maximum allowable rate (MAR).

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Frame Relay

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Frame Relay

Different common carriers offer frame relay networks with different transmission speeds: 56 Kbps to 45 Mbps.

At present, frame relay suffers from the same problems as ISDN - a lack of standards.

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Traditional Packet Switching

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Frame Relay Operation

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Frame Relay Architecture

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Frame Relay - How was it going on?

  • Frame relay services grew 46% from 1998 to 1999.

  • Users have long been satisfied with cost benefits and networking stability frame relay provides, particularly for LAN-to-LAN traffic.

  • In carrier backbones, frame relay is giving way to other technologies, such as ATM, Multiprotocol Label Switching (MPLS).

    (See Joanie Wexler, BCR, July 1999)

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Asynchronous Transfer Mode (ATM)

Asynchronous transfer mode (ATM) is one of the fastest growing new technologies, and is similar to frame relay.

All data are packet-switched, and there is no error control at the intermediate computers within the network; error control is the responsibility of the source and destination.

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Asynchronous Transfer Mode (ATM)

ATM has four important difference from frame relay:

  • ATM uses fixed packet lengths of 53 bytes (5 bytes of overhead and 48 bytes of user data), which is more suitable for voice transmissions.

  • ATM provides extensive quality of service information that enables the setting of very precise priorities among different types of transmissions (i.e. voice, video & e-mail; services include CBR, VBR, ABR & UBR).

  • ATM is scaleable. It is easy to multiplex basic ATM circuits into much faster ATM circuits.

  • ATM provides connection-oriented services only.

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ATM Bit Rate Services

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*ATM Cell Format

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Virtual Channels & Virtual Paths

  • Logical connections in ATM are virtual channels

    • analogous to a virtual circuit in X.25 or a frame relay logical connection

    • used for connections between two end users, user-network exchange (control signaling), and network-network exchange (network management and routing)

  • A virtual path is a bundle of virtual channels that have the same endpoints.

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Advantages of Virtual Paths

  • Simplified network architecture

  • Increased network performance and reliability

  • Reduced processing and short connection setup time

  • Enhanced network services

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Figure 8-12 Digital Island’s WAN

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Home Depot Network

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Nortel’s ATM WAN Backbone

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Switched Multimegabit Data Service (SMDS)

  • Uses ATM-like 53-byte cells, but a different address format.

  • Provides datagram-based transmission services.

  • Data unit is large enough to encapsulate frames of Ethernet, token ring and FDDI.

  • An unreliable packet service like ATM and frame relay. Like ATM and frame relay, SMDS does not perform error checking; the user is responsible for error checking.

  • Most RBOCs and MCI offer MSDS services.

  • Speed ranging 56kbps - 44.375Mbps.

  • Not yet a widely accepted standard.

  • Its future is uncertain.

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*SMDS Network Components

SNI: Subscriber network interface

CPE: Customer premises equipment

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*SMDS Interface Protocol (SIP)

SIP is used for communications between CPE and SMDS carrier equipment

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Ethernet/IP Packet Network

  • A MAN/WAN service started in 2000

  • X.25, ATM, frame relay and SMDS use traditional PSTN and thus provided by the common carrier such as AT&T and BellSouth. ISP with Ethernet/IP packet service laid their own gigabit Ethernet fiber-optic networks in large cities.

  • All traffic entering the network must be Ethernet using IP.

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Multiprotocol Label Switching (MPLS)

  • MPLS is a standards-approved technology for speeding up network traffic flow and making it easier to manage.

  • MPLS sets up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to.

  • MPLS is called multiprotocol because it works with the IP, ATM, and frame relay network protocols.

  • MPLS allows most packets to be forwarded at the layer 2 (switching) level rather than at the layer 3 (routing) level.

  • In addition to moving traffic faster overall, MPLS makes it easy to manage a network for quality of service (QoS).

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MPLS Services in the Market

In January 1999, AT&T announced the first VPN services to be based on MPLS --- its IP-Enabled Frame Relay service.

Cable & Wireless and Cisco Systems conducted a trial of IP-VPN service based on MPLS with Hongkong Telecom in March, 1999.

MCI/Worldcom Started to offer MPLS-based IP-VPN service in March, 1999.

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Selecting MAN/WAN Services

A 1995 survey of network managers found that:

  • 45 percent of WAN costs were for network management (primarily support staff salaries).

  • 35 percent was spent on services (leasing data circuits from common carriers).

  • Only 20 percent was spent on equipment.

    The most expensive part of the WAN will be the people required to plan, install, and operate it, so pick one that is easy to manage.

    It costs more to lease services from common carriers than to buy hardware, so selection decisions should be driven more by the services.

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*Internet Backbone Networks-- Major Providers

AT&T Network Services (

BBN Planet (GTE)

Cable & Wireless USA


UUNET, a part of MCI WorldCom

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AT&T Network Service

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GTE BBN Planet

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Cable & Wireless USA

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*Cable & Wireless USA

  • Offers a world-wide voice, data, Internet and messaging services.

  • Its Internet backbones connects to 70+ countries.

  • Service area includes switched services from most of US cities to all 50 states, Puerto Rico, the Virgin Islands and more than 200 countries.

  • Private line and managed data services are available between most major US metropolitan areas and key business centers around the world.

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Key Issues in MAN/WAN Design

  • Vendor capabilities

  • Capacity

  • Flexibility

  • Control

  • Reliability

    A fundamental issue:

  • A Private Network or a Virtual Private Network?

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Value Added Networks and Virtual Private Networks

Several companies offer value added networks (VANs) that are alternatives to building networks by leasing circuits from common carriers. VANs provide additional services over and above those provided by common carriers.

A new type of VAN, called a virtual private network (VPN), or software defined networks, provide circuits that run over the Internet but appear to the user to be private networks.

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What is a VPN

A virtual private network (VPN) is an extension of an enterprise’s private intranet across a public network such as the Internet, creating a secure private connection, essentially through a private tunnel. VPN provides cost-effective data transmission with high security.

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VPN Tunnel


VPN Tunnel




Figure 8-13 VPN Network

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Packet from the client computer

Packet in transmission through the Internet





















Packet from the VPN










VPN Tunnel

Figure 8-14 VPN encapsulation of packets


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*VPN is a cost-effective solution

According to industry analyst Forrester Research Inc., when comparing the cost of traditional leased line network versus today's Internet-based VPN, the cost differences for 1,000 users are eye-popping.

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*Monthly costs for leased-line network and Internet VPN

CityDistance (mi.) T1 Fees Internet VPN Fee



Chicago-NY 807$11,235$1,900

SF-LA 384$ 5,520$1,900

Denver-Salt Lake 537$ 6,285$1,900

Denver-Dallas 794$ 7,570$1,900

NY-DC 235$ 4,775$1,900

NY-Boston 194$ 4,570$1,900

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Virtual Private Networks

There are two important disadvantages of VPNs:

  • Traffic on the Internet is unpredictable.

  • There are several competing standards for Internet-based VPN, so not all vendor’s equipment and services are compatible.

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Typical VPN implementation

Extranet VPNs between a corporation and its strategic partners, customers, and suppliers.

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Typical VPN implementation

Intranet VPNs between internal corporate departments and branch offices

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Typical VPN implementation

Remote Access VPNs between a corporation and remote or mobile employees

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* Technologies in VPNs

  • Tunneling and Security Protocols

    • IP Security (IPSec)

    • Point-to-Point Tunneling Protocol (PPTP)

    • Layer2 Tunneling Protocol (L2TP)

    • SOCKS (a layer 3 VPN protocol)

  • Cryptography Key Management

    • ISAKMP/Oakley (Internet Security Association and Key Management Protocol)

  • VPN Hardware

    • Security policy server

    • Certificate authority

    • Security gateway

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* VPN Solution Providers

  • IBM - eNetwork

  • AT&T - WorldNet VPN service

  • Checkpoint -VPN-1

  • Microsoft - PPTP by Windows NT 4.0

  • FreeGate - Virtual Services Management

  • TradeWave - TradeVPI

  • MultiVPN - Ascend

  • VTCP/Secure - InfoExpress

  • SmartGate - V-ONE

  • Countless VPN solutions:

    • 3Com, Bay, Lucent, ADI, Aventail, PSINet, RedCreek, Shiva, TimeStep, VPNet

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