WAN TRANSMISSION EQUIPMENT
WAN transmission equipment is designed to work over POTS and leased telephone lines such as T-carrier or ISDN lines. It may have an analog component, as is true for modems, or it may be completely digital, as for ISDN communications. Most WAN transmission equipment either converts a signal for long-digital communications or creates multiple channels within a single communications medium for higher bandwidth. Common examples of basic WAN transmission equipment are:
Multiplexers
Channel banks
PBXs,
PABXs, and PAXs
 Modems
 Access servers
 Routers.
Multiplexers are network devices that can receive multiple inputs and transmit them to a shared network medium. X.25, ISDN and frame relay use multiplexers for packet-switched communications. The multiplexer is connected to a single cable medium, which is divided into channels or virtual circuits. The multiplexer stores the received packets until it can open the intended channel. The multiplexer simply switches from channel to channel. Each packet is stored until the multiplexer opens it channel for transmission.
Channel banks are devices that convert multiple incoming voice signals into a single line, and multiplexer-converted, multiple data signals into one line for transmission. The need for voice, data, and video communications has led to rapid developments in telecommunications channel banks, so that they now combine the capacity of voice communications with the functionality of a multiplexer for handling voice, data and video signals. Thus, a channel bank is a large scale multiplexer that aggregates telecommunications channels in a centralized location.
Some organizations set up their own telephone service called Private Branch Exchanges (PBXs). PBXs were private, manually operated switchboards that required an operator to make connections inside the organizations. Innovations have resulted in the use of automated private telephone systems called Private Automatic Exchanges (PAXs) and Private Automatic Branch Exchanges (PABXs). PABX still includes the switchboard and a manual capacity as well as automatic switching, whereas a PAX has no switchboard. The PAX or PABX may also carry voice, video, and data communications.
Modems have long played a role in making WANs possible. The term modem is a shortened version of the full name, modulator/demodulator. A modem converts a computer's outgoing digital signal to an analog signal that can be transmitted over telephone line. It also converts the incoming analog signal to a digital signal that the computer can understand.
An access server combines several types of WAN communications into one device. For example, one access server might combine transmission capabilities for modem, X.25, T-l, T-3, ISDN and frame relay. Some access servers are designed for small to mid-sized applications. Those servers have one Ethernet or token ring NIC to connect to the network. They also have a combination of synchronous and asynchronous communication ports, for terminal modem, public telephone, ISDN and X.25 connectivity. Smaller access servers typically have 8 or 16 asynchronous ports and one or two synchronous ports. Larger access servers are modular with slots (perhaps 10 to 20) for communication cards.

A remote router enables networks to be connected into WANs over long distances. Remote routers connect ATM, ISDN, frame relay, high-speed serial, and X.25 networks. Like a local router, a remote router can support multiple protocols, enabling communication with many kinds of distant network. Also like a local router, remote routers are modular, so that different kinds of interferences can be installed in expansion slots; say, interface for ISDN and another one for frame relay.

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