switched 56

Definition of switched 56 in The Network Encyclopedia.

What is switched 56?

A digital switched-data communication technology that provides full-duplex dial-up connections at a speed of 56 Kbps. Switched 56 is essentially the dial-up version of digital data service (DDS) and is generally cheaper than leased-line services.

How It Works

A device called a data set, which is a type of Data Service Unit (DSU), provides switched 56 services to customer premises. For a typical local area network (LAN) connection, a router on the LAN is attached to the data set by using a V.35 serial interface. The data set is then connected over the customer’s local loop twisted-pair wiring to access equipment located at the telco’s central office (CO).

Switched 56 uses the same communication channels as DS0. You can establish circuits by manually entering the destination number on a numeric keypad or (more typically) by using in-band signaling when connecting bridges or routers to the service. Depending on the wiring at the customer premises and the equipment at the CO, you can use one of three configurations for this service:

  • Type I service:
    Uses a two-pair (4-wire) connection and is supported to up to 5500 meters from the CO over standard 26-gauge copper twisted-pair wiring.


  • Type II service:
    Uses a one-pair (2-wire) connection with in-band signaling. This type is not widely implemented.


  • Type III service:
    Uses a one-pair (2-wire) connection with out-of-band signaling and is supported to up to 5500 meters from the CO on 22-gauge or 24-gauge copper twisted-pair wiring and includes forward error correction for enhanced data transmission. Although Type III service appears to be full-duplex, in reality time-compression multiplexing (TCM) rapidly switches half-duplex communication at 160 Kbps to simulate full-duplex at 56 Kbps.

    Graphic S-21. Switched 56.


Some carriers offer other higher speed versions of switched 56. For example, some carriers offer switched 56 as a 64-Kbps service under the name switched 64. Other higher dial-up services include switched 384 and switched 1536, although these are not widely offered anymore.


Switched 56 is a data-only service that is often available where Integrated Services Digital Network (ISDN) is not available. However, switched 56 does not support advanced ISDN features such as caller ID and has greater latency for establishing a connection. The cost is typically billed in the same way that ordinary telephone calls are - that is, local calls are free and long distance is billed by the minute.

Dial-up switched 56 can be a good service to use as a backup wide area network (WAN) link between two networks connected by expensive T1 lines. It is being phased out in most places in favor of ISDN.

See also: