Packet Broadband Network Handbook.pdf

PART

Packet Network

Foundations

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Packet Network Foundations of ARTS

Part 1: Packet Network Foundations

technologies: X.25, frame relay, asynchronous transfer mode

(ATM), and Internet protocol (IP).

Before packet networks, communications technology used circuit-

switched telephone networks with dedicated, analog circuits that func-

tioned on a “always on once activated” basis. A dedicated circuit cannot

be used for other purposes even if no communications are taking place

at the moment. In regard to telephone conversations, it is estimated that on

the average a dedicated circuit carried active traffic only 20 to 25 percent

of the time and is idle the other 75 to 80 percent. Moreover, other services

such as video data streams cannot be efficiently carried on circuit-

switched networks.

Packet networks based on packet switching technologies represent a

radical departure. The key idea behind packet switching is that a mes-

sage or a conversation is broken into independent, small pieces of

information called packets that are either equal or variable in size. These

packets are sent individually to a destination and are reassembled there.

No physical resource is dedicated to a connection, and connections

become virtual, thus allowing many users to share the same physical

network resource.

The concept of packet switching is attributed to Paul Baran who first

outlined its principles in an essay published in 1964 in the journal On Dis-

tributed Communications. The term packet switching itself was coined by

Donald Davies, a physicist at the British National Physical Lab, who

came up with the same packet switching idea independently. It is inter-

esting to note that a few decades earlier, a similar discovery in physics by

Albert Einstein—that waves of light can be broken into a stream of

individual photons—led to the development of quantum mechanics.

Packet networks allow more efficient use of network resources. Each

packet occupies a transmission facility only for the duration of the

transmission, leaving the facility available for other users when no trans-

mission is taking place.

Packet-switched networks are highly fault-tolerant. From the very

start of their development, network survivability was a major design

goal. Because packet networks do not rely on dedicated physical connec-

tions, packets can be routed via alternative routes in case of an outage in

the original communications link.

Packet networks can support bandwidth on-demand and flexible

bandwidth allocation. Bandwidth is allocated at the time of communi-

cation, and the amount of bandwidth allocated is based on need. In

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P art I of this book introduces four widely deployed packet network

Packet Network Foundations of ARTS

Part 1: Packet Network Foundations

Figure P1-1

Packet network

foundations.

Packet-switched

network

Connection-oriented

packet network

Connectionless packet

network

X.25

Frame relay

AT M

contrast, a bandwidth of 64 Kbps is built into the infrastructure of cir-

cuit-switched telephone networks.

Since the very first packet-switched network ARPNET was built in

1969, many packet switching technologies have been developed. Among

them, four have endured and achieved large-scale deployment: X.25,

frame relay, ATM, and IP. The packet network technologies can be gener-

ally divided into the two categories shown in Fig. P1-1: connection-ori-

ented and connectionless.

A connection-oriented packet network provides a virtual connection

for a communications session between a source and a destination either

on a permanent or a temporary basis. Packet networks of this category

include X.25, frame relay, and ATM.

Connectionless packet networks are represented by IP. In a classic IP

network, packets of the same message may travel different routes and

arrive at the destination out of order. The distinction between connec-

tion-oriented and connectionless technologies is not absolute: Connec-

tion-oriented packet networks such as ATM and X.25 can also provide

connectionless service. In addition, the ubiquitous connectionless IP net-

work is moving toward being connection-oriented via new IP network

infrastructures such as multiprotocol label switching (MPLS), as will be

seen in Part 4 of this book.

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Packet Network Foundations of ARTS

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Source: Packet Broadband Network Handbook

CHAPTER

X.25 Networks

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