Metropolitan and Wide Area Networks

Chapter 9

Objectives of Chapter 9

Become familiar with…

• the common carriers and the nature of competition,

• how to improve MAN and WAN performance,

• several factors in selecting MAN and WAN services.

Objectives of Chapter 9

Understand ...

• the role of common carriers in organizational MANs and WANs,

• the four basic categories of MAN and WAN circuits,

• dialed circuit services,

• dedicated circuit services,

• switched circuit services,

• packet network services.

INTRODUCTION

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.

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.

The Telephone NetworkMost countries have a federal agency that regulates

data and voice communications. In the United States, this agency is the Federal Communications Commission (FCC). Each state or province also has its own public utilities commission (PUC) to regulate communications within its borders.

The communications industry in North America operates as a series of private companies that are regulated by the government.

The Telephone Network

A common carrier is a private company that sells or leases communications services and facilities to the public. A common carrier that also provides local telephone services is called a local exchange carrier (LEC), while one that provides long distance services is called an interexchange carrier (IXC).

In the United States, 90 percent of the telephone system used to be run by one common carrier, AT&T.

DIALED CIRCUIT SERVICES

Dialed Circuit ServicesDialed circuit services are the simplest and one of the

most common types of MAN and WAN connections. This type of connection uses the normal telephone network. To use dialed circuit services, the user simply lease connection points into the common carrier’s network, then dials the host computer using a modem, and connects to the host system.

Dialed circuit services may use different circuit paths between the two computers each time a number is dialed.

Direct Dialing

Direct dialing (also called dial-up) is the most commonly used direct circuit service. Every time you call your Internet service provider from your home phone, you are using direct dialing.

Charges for direct dialing are based on the distance between the two telephones (in miles) and the number of minutes the connection is used.

Wide Area Telephone Service (WATS)

Wide Area Telephone Service (WATS) are special rate service that allows direct circuit calls for both voice and data transmission to be purchased in large quantities.

WATS is limited to one direction only; it is either outward dialing or inward dialing. In general, inward WATS uses the toll free 800 and 888 area code series in North America, and similar numbers in other countries.

Synchronous Optical Network (SONET)

The synchronous optical network (SONET) has recently 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 recently standardized a version of SONET under the name of synchronous digital hierarchy (SDH). The two are very similar and can be easily interconnected.

Digital Subscriber Line (DSL)

Digital Subscriber Line (DSL) is one of the most promising proposals now under consideration by the ITU-T to significantly increase the data rates over traditional telephone lines.

The reason for the limited capacity on voice telephone circuits lies with the telephone and the switching equipment at the end office. The actual cable is capable of providing much higher data rates.

Digital Subscriber Line (DSL)DSL services are quite new and not all common

carriers offer them.

Two general categories of DSL services have emerged in the marketplace. – Symmetric DSL (SDSL) provides the same transmission

rates (up to 128 Kbps) in both directions on the circuits.– Asymmetric DSL (ADSL) provides different data rates to

(up to 640 Kbps) and from (up to 6.144 Mbps) the carrier’s end office. It includes an analog channel for voice transmissions.

Digital Subscriber Line (DSL)

A new version of ADSL called Very high rate Digital Subscriber Line (VDSL) has been designed for use over local loops of 1000 feet or less. It uses FDM to provide three channels:– Normal analog channel– Upstream digital 1.6 Mbps channel– Downstream digital 51.84 Mbps channel.

Digital Subscriber Line (DSL)

One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of 1.5-10 Mbps and a downstream rate of 2-30 Mbps.

A few cable companies offer downstream services only, with upstream communications using regular telephone lines.

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 (usually 128 bytes).

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.

Packet Switched ServicesThere 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.

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).

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.

Value Added Networks and Virtual Private Networks

The primary advantage of the VPN is low cost.

There are two important disadvantages of VPNs:– Traffic on the Internet is unpredictable.– There are several competing standards for

Internet-based CPN, so not all vendor’s equipment and services are compatible.

Network Design

Chapter 10

Objectives of Chapter 10

Become familiar with…

• the overall process of design and implementing a network

• commonly used MAN and WAN designs

Understand

• commonly used backbone designs

• commonly used LAN designs

INTRODUCTION

Introduction

The traditional network design approach follows a structured systems analysis and design process similar to that used to build application systems.– The network analyst met with users to determine the

needs and applications.– The analyst estimated data traffic on each part of the

network.– The analyst designed circuits needed to support this traffic

and obtains cost estimates.– Finally, a year or two later, the network is implemented.

Introduction

Three forces are making the traditional design approach less appropriate for many of today’s networks:1. The underlying technology of computers,

networking devices and the circuits themselves if rapidly changing.

2. The growth in network traffic is immense.

3. The balance of costs has changed dramatically over the last 10 years.

Introduction

While some organizations still use the traditional approach, many others use a simpler approach to network design, the building block approach.

Begin by classifying users as “typical” or “high volume.”

The network is then planned by adding circuits using a few standard network designs, also classified as “typical” or “high volume.”

THE NETWORK DESIGN PROCESS

The Network Design Process

The basic process involves three steps that are performed repeatedly:

• Needs analysis• Technology design• Cost assessment

By cycling through these three processes, the network design settles on the final network design.

The Network Design Process

The Network Design Process

Needs Analysis

The goal of needs analysis is to understand why the network and what users and applications it will support.

Much of the work may have already been done monitoring existing systems, which can provide a baseline against future design requirements.

Deliverables

The key deliverables for the needs assessment stage are a set of network maps, showing the applications and the circuits, clients, and severs in the proposed network, categorized as “typical” or “high volume”.

Technology Design

Once the needs have been defined, the next step is to develop a technology design (or set of possible designs) for the network.

Designing Clients and Servers

“Typical” users are allocated the “base level” client computers, as are servers supporting “typical” applications. “High volume” users and servers are assigned some “advanced” computers.

Designing Circuits and Devices

There are two interrelated decisions in designing network circuits and devices: the fundamental technology and protocols

the capacity of each circuit.

Designing the circuit capacity means capacity planning, estimating the size and type of the “standard” and “advanced” network circuits for each type of network.

This requires some assessment of the current and future circuit loading (average vs peak).

Deliverables

The key deliverables at this point are a revised set of network maps that include general specifications for the hardware and software required.

In most cases the crucial part is the design of the network circuits.

Cost Assessment

The purpose of cost assessment is to assess the costs of various network alternatives produced from the previous step.

Some of the costs to consider are:Circuit costs

Internetworking devices

Hardware costs

Software costs

Network management costs

Test and maintenance costs

Request for Proposal (RFP)

Although some network components can be purchased “off-the-shelf”, most organizations develop an RFP before making large network purchases.

Once the vendors have submitted their proposals, the organization evaluates them against specific criteria, and selects the winner(s).

One of the key decisions in the RFP process is the scope (one vendor or multi-?).

Request for proposal

Information in a Typical RFP• Background Information

– Organizational Profile– Overview of current network– Overview of new network– Goals of new network

• Network Requirements– Chose sets of possible network designs (hardware, software, circuits)– Mandatory, desirable, and wish list items– Security and control requirements– Response time requirements– Guidelines for proposing new network designs

Request for proposal • Service Requirements

– Implementation time plan

– Training courses and materials

– Support services (e.g. spare parts on site)

– Reliability and performance guarantees

• Bidding Process– Time schedule for the bidding process

– Ground rules

– Bid evaluation criteria

– Availability of additional information

• Information required from vendor– Vendor corporate profile

– Experience with similar networks

– Hardware and software benchmarks

– Reference lists

Selling the Proposal to Management

One of the main problems in network design is obtaining the support of senior management.

The key to gaining senior management acceptance lies in speaking their language.

A focus on network usage, budgets, and reliability are easily understandable issues.

Deliverables

There are three key deliverables for this step:– An RFP that goes to potential vendors.– After the vendor has been selected, the revised

set of network set of maps with the technology design component complete.

– The business case that provides support for the network design, expressed in business objectives.

COMMON WIDE AREA NETWORK DESIGNS

Common Wide Area Network Designs

Most organizations do not build their own WANs by laying cable, building microwave towers, or sending up satellites. Instead most organizations lease circuits from interexchange carriers, and use those to transmit their data.

Once the major connection points one the WAN have been identified, the next step is to design the circuits that will connect those locations.

Ring-Based WAN Design

A ring-based WAN design connects all computers in a closed loop, with each computer linked to the next, usually with a series of point-to-point dedicated circuits.

One disadvantage is of the ring topology is that messages can take a long time to travel from the sender to the receiver.

In general, the failure of one circuit or computer in the network means that the network can continue to function.

Ring-Based WAN Design

Star-Based WAN Design

A star-based WAN design connects all computers to one central computer that routes messages to the appropriate computer, usually via a series of point-to-point dedicated circuits. It is easy to manage because the central computer receives and routes all messages in the networks.

In general, the failure of any one circuit or computer affects only the one computer on that circuit.

Mesh-Based WAN Design

Mesh-based WAN designs: full or partial mesh.

The effects of the loss of computers or circuits in a mesh network depend entirely on the circuits available in the network.

In general, mesh networks combine the performance benefits of both ring networks, and star networks. The drawback is that mesh networks use decentralized routing so that each computer in the network performs its own routing.

Mesh-Based WAN Design

Cloud-based mesh designs are becoming very popular. With this design all computers are simply connected into a packet switched network provided by a common carrier.

Cloud-based designs are simpler for the organization because they move the burden of network design and management from the organization to the common carrier.

End of Review