Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-07-28
2003-04-01
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S395100
Reexamination Certificate
active
06542501
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to speech transmission in a packet network and especially to transmission between a transcoder and a base station of a mobile communication network.
BACKGROUND OF THE INVENTION
In a digital telephone system a speech signal is encoded in some manner before it is channel coded and sent to the radio path. In speech coding digitalized speech is processed frame by frame at intervals of about 20 ms by using different methods so that it results in a parameter group representing speech for each frame. This information, that is, the parameter group is channel coded and sent to the transmission path. The used speech coding algorithms are RPE-LTP (Regular Pulse Excitation LPC with Long Term Prediction) and various code excited algorithms CELP (Code Excited Linear Prediction) of which VSELP (Vector-Sum Excited Linear Prediction) should be mentioned. Encoders using varying algorithms have it in common that an encoder produces speech frames of 20 ms in duration.
In addition to actual coding, the following functions are also built in for speech processing: a) on the transmitter side Voice Activity Detection VAD with which the transmitter can be instructed to be switched on only when there is speech to be sent (Discontinuous Transmission, DTX), b) on the transmitter side the evaluation of background noise and the generation of respective noise parameters and on the reception side the generation of comfort noise in a decoder from the noise parameters, and c) acoustic echo suppression. Noise during a break makes the connection sound more pleasant than absolute silence.
In known GSM mobile telephone systems the input of a speech encoder is either a PCM signal of 13 bits from the network or an AND converted PCM of 13 bits from the audio part of the mobile station. The speech frame obtained from the output of the encoder is20 ms in duration and comprises 260 audio bits which are formed by encoding 160 of PCM-encoded speech samples. Voice activity detection (SAD) defines from the parameters in the speech frame whether the frame contains speech or not. If speech is detected, the frames transmitted to the radio path as so-called traffic frames are speech frames. After a speech burst, and at specified intervals also during the speech breaks indicated by the VAD, the traffic frames are SID frames (Silence Descriptor) containing noise parameters, in which case the receiver is able to generate from these parameters noise similar to the original noise also during breaks.
A traffic frame thus contains a speech block of 260 bits representing 20 ms of encoded speech/data or noise. Furthermore, the frame has 56 bits available for frame synchronization, speech and data indication, timing and other information, the total length of the traffic frame being 316 bits. Traffic frames in the uplink and downlink direction differ from one another only in these 56 bits.
Referring to
FIG. 1
, which shows a simplified view of the present GSM network from the point of view of transmission. Network Subsystem comprises a mobile service switching centre, the mobile communication network being connected via the system interface of the mobile services switching centre to other networks than Public Switched Telephone Network PSTN. Via A interface the network subsystem is connected to the base station subsystem BSS comprising base station controllers, each of which controls base stations BTS connected thereto. The interface between the base station controller and the base stations connected thereto is an Abis interface. The base stations are in radio communication with mobile stations via the radio interface. Traffic frame forming unit TRAU explained above is in
FIG. 1
placed in association with the base station but is may also be situated in association with the mobile services switching centre.
The mobile services switching centre MSC is shown in a simplified way in FIG.
2
. Control of the base station system BSS is one function of the mobile services switching centre in addition to a call control. The function of the switching matrix is to select, switch and separate speech/data and signaling paths passing through it in the desired way. The switching matrix switches in this way its part of the connection between a mobile subscriber and a subscriber of another network and of the connection between two mobile subscribers. The function of the Network Interworking Functions IWF
1
is to adapt the GSM network into other networks. The PCM trunk line is connected to a PBX system by a terminal circuit trunk interface
3
so that the physical interface of layer
1
between the exchange and the base station controller BSC is a line of 2 Mbit/s, that is, 32 time slots of 64 kbit/s (=2048 kbit/s). The signaling terminal
4
carries out signaling according recommendation CCITT No:7.
The functions of the base station controller indicated with reference
14
in
FIG. 1
include selection of a channel between it and the mobile station, link control and channel release. It carries out mapping from the radio channel to the channel of the PCM time slot of the interconnecting line between the base station and the base station controller. The base station controller shown in a simplified way in
FIG. 3
comprises terminal circuits, trunk interfaces
31
and
32
by means of which the base station controller is connected on the one hand to the mobile services switching centre over the A interface and on the other hand to the base stations over the Abis interface. Transcoder and Rate Adaptation Unit TRAU is part of the base station system BSS and it may be situated in association with the base station controller BSC as shown in
FIG. 1
or also in association with the mobile services switching centre. The transcoders convert speech from one digital format to another, they for example convert the 64 kbit/s A-law PCM from the exchange over the A interface into encoded speech of 13 kbit/s to be sent to the base station line and vice versa. The rate adaptation for the data is carried out in between the rate 64 kbit/s and the rates 3.6, 6 or 12 kbit/s.
The base station controller BSC configures, allocates and supervises the circuits of 64 kbit/s in the direction of the base station. It also controls the switching circuits of the base station by means of the PCM signaling link and allows the circuits of 64 kbit/s to be used efficiently, that is, a switch at the base station, which the base station controller controls, switches transmitter/receivers to PCM links. This switch hence operates as a drop/insert multiplexer which drops a PCM time slot for the transmitter of data or inserts a reception time slot to a PCM time slot of data or links the PCM time slots forwards to other base stations. The base station controller thus sets up and releases connections to the mobile station. The connections from the base stations to the PCM line/lines over the A interface and the procedure in the opposite way are multiplexed in a switching matrix
33
.
The physical interface of layer
1
between the exchange and the base station controller BSC is a line of 2 Mbit/s, that is, 32 time slots of 64 kbit/s (=2048 kbit/s). The base station is totally controlled by the base station controller BSC and it mainly contains transmitter/receivers TRX which implement the radio interface towards the mobile station. Four full rate traffic channels via the radio interface can be multiplexed into one PCM channel of 64 kbit/s between the base station controller and the base station, in which case the rate of the speech/data channel is in this interval 16 kbit/s. In that case one PCM link of 64 kbit/s can transmit four speech/data connections.
FIG. 1
illustrates the transmission rates used in the GSM for a channel. The mobile station sends speech or data information over the radio interface on the radio channel as traffic frames. A base station
13
receives the information and transmits it to the time slot of 64 kbit/s of the PCM line. The other three traffic channels of the same carrier wave are also inserted in t
Hsu Alpus H.
Nguyen Toan
Nokia Telecommunications Oy
Pillsbury & Winthrop LLP
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