Telephone Company WAN Technologies: Packet Switching
With the telephone company supplied technology for packet switching, each packet or piece of data can take a different path thru your WAN from source to destination. Switching in this example is similar to switching on your LAN, except it is implemented on a much larger scale.
When implementing a packet-switched solution, you usually work with a virtual circuit, which, like a physical circuit, establishes a connection between two points, but in this case it is a temporary connection.
From the point of view of the devices on either end of the connection, there is only one router hop from one end of the connection to the other, but in reality they go through a huge switched network, being handled by many devices.
Some examples of this type of connection include:
X.25: This represents one of the oldest technologies in use. As a packet-switched solution, it makes use of a packet assembler/disassembler (PAD) (which prepares data packets for transmission and processes them after transmission) device that is connected to the router's serial port. Through this device, you are able to connect to the X.25 network and transfer your data at a rate of 2 Mbps.
Frame relay: This is the replacement for X.25 and implements the same basic solution, but in a purely digital format and allows for speeds as fast as 50 Mbps. This connection can give you the same speed as a T3connection.
Asynchronous Transfer Mode (ATM): A packet-switched network environment that differs greatly from the previous solutions in that it uses a fixed-length packet size referred to as a cell. A cell is the standard 53 bytes in size. ATM is a WAN technology capable of sending large, low-delay types of data (such as video) and can sustain a transfer rate of 622 Mbps.
The small packet sizes are not a disadvantage — because other solutions need to keep checking for the end of a packet, and ATM eliminates this need by using the standard-sized frames. However, partial frames or cells will be padded with empty data.