Introduction to Data Communications
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62. X.25

X.25 is an analog packet switching network. It can be considered Slow Packet Switching. The transfer speeds are typically 56 kbps to 2.08 Mbps. There is a world-wide set of Public X.25 Networks and it is possible for an organization to have its own private X.25 network.

X.25 is over 20 years old and an established technology. There are many multi-vendor solutions and dissimilar technologies in an organization are allowed to access the X.25 network. In Canada, the main X.25 network is called Datapac which is a public offering of X.25. You pay either a flat rate or by the packet.

X.25 is used to connect LANs together. Due to its slow transfer speed, it is used for:

Host terminal emulations: low data
Client/Server applications such as E-mail: small files, bandwidth
File Server: large amount of data & real-time traffic (doesn't work well)
Databases: usually large databases but queries are small inbound and medium size outbound.

X.25 has a high protocol overhead compared to other networks. This reduces the transfer speed and bandwidth utilization - means its not as efficient.

Overhead Example:

Truck A represents X.25, it has a heavy empty weight of 5 tons (overhead). The bridge (medium) only allows 6 tons of weight, this means that Truck A can only carry 1 Ton of cargo (Data). Truck B is a smaller truck and weighs 3 tons empty, this means that it can carry up to 3 tons of cargo (data) across the bridge (medium). Truck B makes better use of its weight when crossing the bridge - utilizes its bandwidth better (it is more efficient).

Introduction to Data Communications
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