Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1997-06-12
2001-03-20
Pham, Chi H. (Department: 2731)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S336000, C370S345000, C370S508000
Reexamination Certificate
active
06205157
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to propagation delays within transport networks of telecommunication systems, and more particularly, to a method of controlling propagation delays in a transport network by synchronizing an external node to a synchronous air interface.
2. Description of Related Art
In a cellular communication network, the synchronized air interface and an external network node, such as PLMN, PSTN, ISDN, or Packet Data Network are interconnected via a separated node such as an interworking function (IWF) or packet control unit (PCU) and a base transceiver station (BTS) over an associated transport network. However, the separation between the IWF and base transceiver station invites propagation delays over the transport network between these devices. The delays create problems with reassembling data transmitted over the transport network. For transport networks using only a single traffic channel for calls, these delays must be minimized. For transport networks using more than one traffic channel, the delays must be minimized and independent delay variations within the system must be determined in order to recreate the transmitted data stream.
One solution for combating independent time delay on different sub-channels in a fixed cellular network involves the use of a terminal adaptation function (TAF) at the mobile station, and the use at the IWF of a multi frame structure in conjunction with sub-channel memory. An inband signal is generated using redundant control bits in the CCITT V.110 frames. One bit is used for each multiframe structure and three bits are used for sub-channel numbering. This sequence resolves a delay variation of up to (n−1)/2 (where n=number of bits used in sequence) V.110 frames. However, this solution has several drawbacks.
The maximum sub-channel delay variation cannot be determined from only the algorithmic delays. Considerable delay variations may arise within the transport networks where the sub-channels may be independently routed. Furthermore, the inband signaling is transferred over the air interface where bit error rates can be very high. The error rate on the air interface and the length of the multi frame structure cause long synchronization and resynchronization times. There is also a risk of false detection. Furthermore, each of these proposed solutions have been generated with respect to the use of high-speed circuit-switched data and does not provide solutions for other types of implementations such as GPRS. Thus, alternative solutions are necessary.
SUMMARY OF THE INVENTION
The present invention overcomes the foregoing and other problems with a method for maintaining synchronization between a first node, preferably comprising a base transceiver station (BTS), and a second node, preferably comprising either an interworking function (IWF) or packet control unit (PCU), within a cellular communications network. Initially, an uplink synchronization frame is transmitted from the base transceiver station to an interworking function. This causes the IWF to transmit a downlink synchronization frame to the BTS. The downlink synchronization frame contains downlink synchronization data useful in synchronizing the connection between the BTS and IWF. Upon receipt of the downlink synchronization data by the BTS, the BTS marks the received downlink sequence number SeqD with the associated adjusted frame number downlink (aFNd), and a time margin between the downlink sync frames and the air interface may be determined.
The SeqD marked with aFNd, the time margin and other uplink synchronization data are transmitted to the interworking function within an uplink synchronization frame. Receipt of the uplink sync data at the interworking function initiates a determination of the unpacking sequence for the frames transmitted to the IWF. Determination of the unpacking sequence consists of determining the phase of frames from different subchannels that have the same adjusted frame number uplink (aFNu) and uplink sequence number. The determined phases gives the unpacking sequence.
Once the interworking function receives downlink synchronization information from the uplink synchronization frames, the delays in the downlink direction for each individual subchannel may be determined. The delays are used to arrange the transmission in the downlink direction, such that the frame numbers and timeslots are received in increasing downlink sequence number order at the BTS.
The communications link between the base transceiver station and the interworking function may be monitored to determine whether synchronization is maintained in both the uplink and downlink directions. If synchronization is lost in the downlink direction, the synchronization procedures discussed above may be reinitiated to recover synchronization.
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International Search Report, Jun. 10, 1998.
Galyas Johan Karoly Peter
Jung Stefan Wilhelm
Duong Frank
Jenkens & Gilchrist P.C.
Pham Chi H.
Telefonaktiebolaget L M Ericsson (publ)
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