Data Communication Accessories

Multiplexing: simultaneous sharing of a single communications link

Two of the most widely used multiplexing techniques in communications are Time Division Multiplexing (TDM) and Statistical Multiplexing (STM or STDM). In TDM, the communications link (main channel) bandwidth is apportioned according to fixed timeslots (predetermined parts) between the various users (subchannels). In STM, there is no pre-assigned splitting of bandwidth, and the time is shared dynamically in direct proportion to the demand of each subchannel. STM is a more efficient way of utilizing the common resource, since it allows assigning more global bandwidth to the subchannels than the available capacity of the aggregate. Unfortunately, STM introduces delay, which is not acceptable in some applications.

Local multiplexers are basically time division multiplexers that include built-in short range modems. They are used to connect several terminals to computer ports in-house or between buildings, over twisted pairs or fiber optic cables.

A service based on statistical multiplexing techniques, such as Frame Relay, enhances the capacity of the network to support multiple data types, with maximum efficiency and minimal delay. Frame Relay capitalizes on data communications techniques, such as traffic prioritization and fragmentation, reduced header, reduced number of operations and fast Data Link layer switching, to increase network performance.

Selecting the right multiplexer for your application

• To provide connection in-house or between buildings in close proximity, a local multiplexer should be used. The number of channels, the type of media and the communications format (async or sync) will determine which multiplexers should be chosen: FLM-1, FLM-2 or FLM-3.
• If the application spans a long distance, e.g., via a modem link, the first decision is whether to use STM or TDM. Generally, if the application is asynchronous, use of statistical multiplexing is preferable. In applications sensitive to delay, it is more common to use TDM multiplexers, since no delay is introduced by these devices (see Kilomux and Megaplex families). However, if a small delay is acceptable, and the equipment responds to flow control or changing clock speeds, synchronous STM multiplexers and packet switching devices provide better utilization of the network.
• In applications that require integration of data, voice and LAN, or where different data types are combined (low and high speed, sync and async, etc.), both TDM multiplexers (such as RAD’s Kilomux or Megaplex) and the packet switching devices such as the SPS or FPS families can be used.

Data solutions over X.25/Frame Relay networks

RAD’s packet switching family includes Packet Assemblers/Disassemblers (PADs) and multiprotocol packet switches. All are designed to the latest ITU specifications and ANSI recommendations. They support various protocols including HDLC, async, SLIP, PPP, IP and ISDN. The products can be used to construct high performance, private X.25/Frame Relay networks or efficient extensions of public X.25/Frame Relay networks. In addition, RAD offers a wide range of interchangeable interfaces, such as built-in CSU/DSUs and BRI ISDN terminal adapters.

Sharing devices in a multipoint environment

As its name implies, a sharing device enables several clients to share a common network resource in a multipoint environment. For example, a sharing device permits several DTEs to use a common modem. The main function of the sharing device is to manage client contention for the resource, to ensure that only one client has a connection to the resource at any one time and to ensure that all clients have an opportunity to access the resource.

The sharing device operates at the physical level and is only responsible for the physical connection between the resource and its clients. The actual addressing and polling is administered by the host and clients.

There are two types of resource sharing: polled and non-polled. In the non-polled environment, a client can access the resource if no other client is currently connected to it, e.g., printer sharing. In the polling environment, the host manages access to the resource by polling each of the clients, e.g., IBM FEP (Cluster Controller communication). When a sharing device receives a poll from the host, it broadcasts the poll to all connected clients. A client who wants to transmit data contends for the link to the host by raising the Request To Send signal (RTS contention) or by initiating data transmission (Data Contention). The sharing device enables the connection until the client signals that it is no longer needed.

The signalling is performed by dropping the RTS or ceasing data transmission.

RAD offers a wide range of sharing devices for various data rates, interfaces, channels and applications. Applications include modem sharing, printer sharing and polled environments. Sharing devices are available either as full-sized powered devices or as miniature products that operate without power.