Networking

X.25

One of the older WAN technologies is X.25, which is a packet-switching technology. Today, X.25 is not as widely implemented as it once was. X.25's veteran status is both its greatest advantage and its greatest disadvantage. On the upside, X.25 is a global standard that can be found in many places. X.25 had an original maximum transfer speed of 56Kbps, which, when compared to other technologies in the mid-1970s, was fast but almost unusable for most applications on today's networks. In the 1980s a digital version of X.25 was released increasing throughput to a maximum 64kbps. This too is slow by today's standards.

Because X.25 is a packet-switching technology, it uses different routes to get the best possible connection between the sending and receiving device at a given time. As conditions on the network change, such as increased network traffic, so do the routes that the packets take. Consequently, each packet is likely to take a different route to reach its destination during a single communication session. The devices that make it possible to use X.25 service are called packet assemblers/disassemblers (PADs). A PAD is required at each end of the X.25 connection. Table 4 compares the various WAN technologies reviewed in this Chapter.

Table 4 Comparing WAN Technologies

WAN Technology

Speed

Supported Media

Switching Method Used

Key Characteristics

ISDN

BRI: 64Kbps to 128Kbps

PRI: 64Kbps to 1.5Mbps

Copper/fiber-optic

Can be used for circuit-switching or packet-switching connections

ISDN can be used to transmit all types of traffic, including voice, video, and data. BRI uses 2B+D channels, PRI uses 23B+D channels. B channels are 64Kbps. ISDN uses the public network and requires dial-in access.

T-carrier (T1, T3)

T1: 1.544Mbps T3: 44.736Mbps

Copper/fiber-optic

Circuit switching

T-carrier is used to create point-to-point network connections for private networks.

FDDI

100Mbps

Fiber-optic

N/A

Uses a dual-ring configuration for fault tolerance. Uses a token-passing media-access method. Uses beaconing for error detection.

X.25

56Kbps/64Kbps

Copper/fiber-optic

Packet switching

X.25 is limited to 56Kbps. X.25 provides a packet-switching network over standard phone lines.

SONET/Ocx

51.8Mbps

Fiber-optic to 2.4Gbps

N/A

SONET defines synchronous data transfer over optical cable.

by BrainBellupdated
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