Sending Data Across a WAN

If the technologies discussed above for data transmission are inadequate, the network administrator can consider using any of the advanced WAN technologies. Some of these technologies are:

• X.25 Packet Switching
• Asynchronous Transfer Mode (ATM)
• Integrated Services Digital Network (ISDN)
• Fiber Distributed Data Interface (FDDI)

X.25 Packet Switching

X.25 is a set of protocols included in a packet-switching network. An X.25 network uses switches, circuits and the available routes for routing at anytime.

Asynchronous Transfer Mode (ATM)

ATM is also an advanced form of packet-switching. It is a broadband method that transmits data in 53-byte cells. Each cell consists of 48 bytes of data and five bytes of header information. Therefore, the packets that are generated are consistent and uniform. ATM can be used with any media.

ATM switches are multiport devices that can act as either hubs or routers. In architectures, such as Ethernet and Token Ring where only one computer at a time may transmit, ATM uses switches as multiplexers to permit several computers to put data on a network.

Note: In asynchronous transmission, data is transmitted as a stream. Each character is converted into a string of bits. Each string is separated from the other strings by a start bit and a stop bit. In this method, there is no device or method to coordinate transmission between the sender and the receiver.

Integrated Services Digital Network (ISDN)

Basic Rate ISDN uses digital transmission that divides its bandwidth into three channels. Two of these transmit data at 64 Kbps, and the third at 16 Kbps. The 64 Kbps channels can carry voice, data or images. The slower 16 Kbps channel carries signaling data.

A computer connected to an ISDN service can use both the 64 Kbps channels to have a 128 Kbps data stream.

Fiber Distributed Data Interface (FDDI)

FDDI is a high-speed token-passing ring network that uses fiber-optic media and was developed for high-end computers.

FDDI uses a dual-ring topology. In this, there are two data streams flowing in opposite directions around two counter-rotating rings. One ring is called the primary ring and the other is called the secondary ring. The secondary ring acts as a backup. Computers may connect to one or both FDDI cables in a ring.