Browse Prior Art Database

Modem Receiver With External Timing

IP.com Disclosure Number: IPCOM000047467D
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 4 page(s) / 77K

Publishing Venue

IBM

Related People

Ungerbock, G: AUTHOR

Abstract

The possibility to operate a modem receiver with external timing is discussed. External timing means that the timing of signal-processing and digital-interface functions of a receiver is not derived from the analog signal received, but comes from a source external to the receiver. External receiver timing is useful, e.g., when modem receivers operate back-to-back with modem transmitters. For certain configurations of communication networks it is necessary that modem receivers and transmitters operate back-to-back. One example is when the outputs of several lower-speed modem receivers are to be concentrated at some network location in the transmitter of another channelized higher-speed modem.

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Modem Receiver With External Timing

The possibility to operate a modem receiver with external timing is discussed. External timing means that the timing of signal-processing and digital-interface functions of a receiver is not derived from the analog signal received, but comes from a source external to the receiver. External receiver timing is useful, e.g., when modem receivers operate back-to-back with modem transmitters. For certain configurations of communication networks it is necessary that modem receivers and transmitters operate back-to-back. One example is when the outputs of several lower-speed modem receivers are to be concentrated at some network location in the transmitter of another channelized higher-speed modem. Since each modem receiver outputs data with its own internally generated timing signal, additional provisions are needed to buffer and align the individually received bit streams so that they can be accepted in appropriate synchronism and phase at the inputs of the channelized modem transmitter. It is here suggested to provide external timing for modem receivers to avoid these additional provisions and to provide adaptive equalizers with fractional-T-spaced taps (tap spacing smaller than the signaling period T) in the modem receivers. The timing-phase insensitivity of an adaptive equalizer with fractional-T-spaced taps enables a modem receiver to operate with external timing. In this new mode of operation, slow phase changes between received analog signal and external timing are automatically compensated by continuous re-adaptation of the equalizer coefficients. Thus, the equalizer performs the function of a variable-delay buffer. The maximum rate of phase change to which the equalizer can adapt without significant loss of modem performance depends on the number of equalizer taps and the frequency with which coefficients are updated. Possible sources for external receiver Two approaches for specifying external timing to a receiver are proposed. The standard V.24 interface does not provide a signal path for external receiver timing. The first approach is based on extending the definition of this interface. The second approach consists in letting the receiver operate in synchronism with the transmitter of the same modem. In the following, RST means "Receive Signal Element Timing" and TST means "Transmit Signal Element Timing" in accordance with the V.24 specification. Approach 1: Receiver timing is determined by an interface circuit for external RST specified in one of three ways: (a) A new non-standard circuit is introduced for external RST. (b) The present output circuit for internal RST is

optionally used also for external RST. (c) The present input circuit for external TST is optionally used also for external RST. If method (a) for external receiver timing is in effect, the algorithm for internal timing is replaced by a simple phaselocked loop algorithm which synchronizes bit timing of the receiver to the...