The invention relates to network independent clocking in a telecommunication system, particularly in data transmission where the maximum data rate of a traffic channel is equal to one of the user data rates at a terminal interface.
Mobile systems generally mean different telecommunication systems that enable private wireless data transmission for subscribers moving within the system. A typical mobile system is a Public Land Mobile Network (PLMN). The PLMN comprises fixed radio stations (base stations) located in the service area of the mobile network. The radio coverage areas (cells) of the base stations provide a uniform cellular network. A base station provides a radio interface (air interface) in the cell for communication between a mobile station and the PLMN.
Another area of mobile systems includes satellite-based mobile services. In satellite system, radio coverage is obtained by satellites instead of terrestrial base stations. The satellites orbit round the earth and transmit radio signals between mobile stations (or User Terminals (UT)) and Land Earth Stations (LES).
Subscriber mobility requires similar solutions in satellite mobile systems as in the PLMNs, i.e. subscriber data management, authentication and location management of mobile subscribers, handover, etc. The satellite systems should also support similar services as the PLMNs.
One way of meeting the above requirements in satellite mobile systems is to use existing PLMN solutions. In principle this alternative is very straightforward since a satellite system can basically be compared to a base station system of a mobile system having a different radio interface. In other words, it is possible to use conventional PLMN infrastructure, where the base station system(s) is (are) a satellite system. In such a case, the same network infrastructure could, in principle, even contain both conventional PLMN base station systems and satellite xe2x80x98base station systemsxe2x80x99.
There are many practical problems, however, in adaptation of the PLMN infrastructure and a satellite system. A problem apparent to the Applicant is that a PLMN traffic channel and a traffic channel of a xe2x80x98radio interfacexe2x80x99 in a satellite system differ considerably. Let us examine an example where the PLMN is the Pan-European digital mobile system GSM (Global System for Mobile Communication), and the satellite mobile system is the Inmarsat-P system that is currently being developed.
At present, a GSM traffic channel supports data transmission at user rates 2400, 4800, 7200 and 9600 bit/s. In addition to user data, status information on the terminal interface (control signals of a V.24 connection) is transmitted on the traffic channel in both directions. In transparent High Speed Circuit Switched Data (HSCSD) data service, it is also necessary to transmit synchronization information between subchannels. In synchronous transparent bearer services, the clocking information of network independent clocking (NIC) must also be transmitted through a transmission channel from transmitting terminal equipment to receiving terminal equipment via a transmission network, when the transmission network and the transmitting terminal equipment are not in sync with each other, i.e. the terminal equipment uses network independent clocking (e.g. internal clock). The above-mentioned additional information raises the bit rate at the radio interface to be higher than the actual user rate. The GSM radio interface rates corresponding to user rates 2400, 4800 and 9600 bits are 3600, 6000 and 12000 bits. These signals are subjected to different channel coding operations, which raise the final bit rate to about 22 kbit/s.
The Inmarsat-P satellite system requires that standard data rates up to 4800 bit/s (e.g. 1200, 2400, 4800 bit/s) can be transmitted on one traffic channel, and that standard data rates exceeding 4800 bit/s (e.g. 9600, 14400, 19200 bit/s, etc.) can be transmitted by using several parallel traffic channels, like in the HSCSD service of the GSM system.
In the Inmarsat-P satellite system, the data rate of one traffic channel at the radio interface is at most 4800 bits, which equals the user data rate of 4800 bits at the terminal interface. In a data service employing two traffic channels, the data rate at the radio interface equals the user data rate of 9600 bits at the terminal interface. A problem arises when not only the user data but also the above-described terminal interface status information and any inter-subchannel synchronization information should be transferred over the radio interface. Therefore the protocol data unit, i.e. frame structure, used by the satellite system at the radio interface should be defined to carry the above mentioned control and synchronization information over the radio interface.
One approach would be to use a GSM solution, i.e. a V.110-based frame structure, for the transmission of the status also at the radio interface of the satellite system. However, this would be a very complicated solution, and it would significantly reduce the user data rates available. A single traffic channel could not support the user data rate of 4800 bit/s since a V.110 frame structure and the terminal interface status information raise the actual data rate (radio interface rate) to be higher than 4800 bit/s. Therefore the highest standard user data rate on one traffic channel would be 2400 bit/s. For the same reason, a two-traffic-channel data service could not support the user rate of 9600 bit/s, but the highest standard user data rate would be 4800 bit/s (or in some systems 7200 bit/s). A corresponding decrease in the available data rates would also occur in data services employing more than two traffic channels. Such a solution, where the overhead information causes a significant loss of capacity, would not be satisfactory.
A similar problem can also arise when other types of radio interfaces, such as wireless telephone systems, are connected to the PLMNs.
A similar problem can also arise on other types of connections in which the radio interface rate is to be used as effectively as possible. For example, a new 14400 bids traffic channel has been planned for the GSM. In order that the terminal interface statuses and any other control information may be transferred over the radio path in addition to the 14400 bit/s user data, the radio interface rate, implemented on the present principles, must be higher than 14400 bit/s, about 18 kbit/s. A higher radio interface rate requires that the existing radio networks should be re-designed and the intermediate rate Transcoder/Rate Adaptor Unit (TRAU) increased so that only two subchannels could be put in a single 64 kbit/s timeslot in the HSCSD service (i.e. efficiency of a TRAU transmission link is impaired). The radio interface rate of 14400 bit/s, which does not cause such problems, can be formed, for example, from the present radio interface rate of 12000 kbit/s by enhancing the puncturing that follows channel coding. The actual user data rate, however, would then be below 14400 bit/s, if the new traffic channel were implemented on the same principles as the existing GSM traffic channels. It would thus be preferable to implement a user data rate of 14400 bids at a radio interface rate of 14400 bit/s.
The parallel Finnish patent applications 955,496 and 963,455 by the same Applicant describe a data transmission method in which the terminal interface status information and any other control or synchronization information are transmitted through a traffic channel in the redundant data elements of end-to-end protocols, such as the redundant parts of the protocol data units of user data or the start and stop bit positions of asynchronous data characters. The overhead information thus does not increase the number of the bits to be transmitted, so the transmission capacity of the traffic channel can be exactly the same as the user data rate at the terminal interface. In high-rate data transmission (HSCSD) a data link may comprise a group of two or more traffic channels, whereby the total capacity of the group of traffic channels can be the same as the user data rate at the terminal interface.
Network independent clocking NIC may be used in synchronous transparent bearer services for the transmission of clocking information from a transmitting terminal equipment to a receiving terminal equipment via a transmission network, when the transmission network and the transmitting terminal equipment are not in synch with each other, i.e. the terminal equipment uses network independent clocking (e.g. internal clock). This is also known as a plesiosynchronous case, since the clocks are nominally the same but originate from different clock sources. In a plesiosynchronous case, data bits are lost now and then during a long transmission session due to the overflow or underflow of the transmission buffer, if no NIC coding is used between the transmitter and the receiver.
In the following, the use of the NIC in the GSM in MS-originated data transmission will be illustrated with reference to FIG. 1A. The MS-terminating data transmission is otherwise the same, but an interworking function (IWF) operates as a transmitter, a terminal adaptation function (TAF) as a receiver, and terminal equipment is replaced by a modem MODEM in the interworking function IWF of the mobile network, as shown in FIG. 1B.
The transmitter (TAF) receives user data DATA and a transmission clock ClkTX, from the terminal equipment (TE). The transmitter (TAF), using the NIC (in a GSM network), compares the transmission clock Clk-rx of the terminal equipment TE with the network clock ClkGSM and detects a slip of the duration of a data bit.
If the slipping results from an overrate of the clock ClkTX of the terminal equipment TE (or modem MODEM) in respect of the network clock ClkGSM, the transmitter (TAF) deletes one data bit from a predetermined bit position from the bit stream DATA (thereby reducing the number of the data bits to be transmitted toward the receiver (IWF)) and transmits the deleted bit outside the data bit stream DATA in a 5-bit NIC code, which also informs the receiver (IWF) of the event.
If, on the other hand, the slipping results from an underrate of the clock ClkTX of the terminal equipment TE (or modem MODEM) in respect of the network clock ClkGSM, the transmitter (TAF) adds a fill bit to a predetermined bit position in the data bit stream DATA (thereby increasing the number of the data bits to be transmitted toward the receiver (IWF)) and transmits a NIC code outside the data bit stream DATA, the NIC code informing the receiver (IWF) of the event.
The receiver (IWF), which uses the NIC (in the GSM), monitors inbound NIC codes and operates accordingly:
If the received NIC code is neutral, the receiver (IWF) interprets the data in the inbound data bit stream in the normal manner.
If the received NIC code contains a data bit, the receiver (IWF) puts the data bit in a predetermined bit position in the inbound data bit stream and speeds up the transmission clock ClkTX toward the modem MODEM (or the terminal equipment TE) for a while so as to make room for the added bit in the bit stream.
If the received NIC code indicates that there is a fill bit in the inbound bit stream, the receiver (IWF) deletes the fill bit and slows down the transmission clock ClkTX toward the modem MODEM (or the terminal equipment TE) for a while so as to delete the xe2x80x98gapxe2x80x99 from the bit stream.
The receiving modem (or terminal equipment) thus obtains data at (approximately) the same frequency as data was transmitted in the transmitting terminal equipment TE (or modem), although the network clock ClkGSM is used on a GSM traffic channel.
The use of network independent clocking NIC is defined both in the ISDN network (ITU-T Recommendation V.110) and in the GSM network (GSM Recommendation 04.21). The principle in the two types of networks is the same except for the following exception: in the GSM it is possible to indicate only a slip that is of the duration of a full bit, whereas in the ISDN it is also possible to indicate a slip that is of the duration of a fraction of a bit.
If network independent clocking (NIC) is used on a traffic channel where there is no room for the control information to be transmitted outside the user data stream, the NIC codes must also be transmitted in the redundant parts of the user data protocol units, for example in accordance with the methods disclosed in the above-mentioned Finnish Patent Applications 955,496 and 963,455.
The use of the existing NIC codings in a case like this would set strict restrictions on the maximum frame length of the user data protocol for example for the following reasons:
The maximum frame length is determined by the fact that only 1-bit compensation is possible in the frames even when the whole 5-bit NIC code is transmitted in one frame. If the whole 5-bit NIC code could be transmitted in one user protocol data frame, the maximum frame length would be restricted to 1250 bytes.
In practice, the maximum frame length would be even shorter, since there is not enough room for the whole NIC code in the redundant bit positions in the most usual cases, i.e. when the user protocol uses a High-Level Data Link Control (HDLC) frame in which the redundant part is a 1-byte long address field. In a case like this, there are six redundant bits available in the address field. Two or three bits must be reserved for the transmission of terminal interface statuses. In addition, two or three bits are needed for in-band numbering of the subchannels and/or frames in high-rate data calls.
To eliminate this problem associated with the lack of bit positions in a frame, it would be possible to use the same mechanism as is currently used in V.110-frame-based transmission in the GSM: the NIC codes are split and sent in successive frames. This, however, would mean a strict restriction to the maximum length of the user data protocol frame, so that the NIC-code repetition frequency would remain sufficiently high. The NIC used in the GSM is described in European Telecommunications Standard Institute (ETSI) recommendation 04.21, version 4.4.0, paragraph 5.1. For example, if two bits were available, one bit would be used to form a superframe structure and the other to carry a 5-bit NIC code in five successive user protocol frames. This would restrict the maximum length of the user protocol frame to 250 bytes (Fame headers and flags included).
It is an object of the invention to alleviate or eliminate the above problems.
The invention relates to a method for network independent clocking in a telecommunication network. The method is characterized by
receiving user data and an associated network independent clock for a transmitter,
comparing the network independent clock with a network clock that is used as the transmitter""s transmission clock,
performing compensation of the network independent clock in the transmitter by changing the number of redundant bits in the user data,
transmitting the user data through a traffic channel or a group of traffic channels to a receiver,
performing compensation of the network independent clock in the receiver by restoring the original redundancy,
temporarily adjusting the outbound network independent clock in the receiver so as to compensate for the change caused by the restoration of the redundancy in the outbound user data stream.
The invention also relates to transmission and reception equipment according to claim 9.
The basis of the present invention is that the network independent clocking is adjusted through a traffic channel in the redundant data elements of end-to-end protocols, such as the redundant parts of the protocol data units of user data. The overhead information thus does not increase the number of the bits to be transmitted, so the data rate on the traffic channel can be the same as the user data rate at the terminal interface. In high-rate data transmission the data frame can comprise a group of two or more traffic channels, whereby the total data rate of the group of traffic channels can be the same as the user data rate at the terminal interface.
In the invention the transmitter compares the inbound network independent clock and the network clock in order to detect slipping. The slipping can be detected in many different ways. In a preferred embodiment of the invention, the transmitter counts the number of the bit sequences (total durations of data bits) that the first clock (e.g. network clock) skips in respect of the second clock. In a second embodiment of the invention, the transmitter monitors the fill level of the transmission buffer. In the present application, a positive skip refers to a case where the inbound network independent clock is faster than the network clock, which means that within a certain period of time the number of network clock bit sequences is lower than the number of network independent clock bit sequences, i.e. the network clock skips network independent clock bit sequences and the transmission buffer fills up. In the present application, a negative skip refers to a case where the network independent clock is slower than the network clock, which means that within a certain period of time the number of network clock bit sequences is higher than the number of network independent clock bit sequences, i.e. the network independent clock skips network clock bit sequences and the transmission buffer is emptied. The corresponding NIC compensations are called positive and negative NIC compensations.
When the transmitter has counted a predetermined number n of skipped bit sequences or the transmission buffer has filled up to a certain level, it drops n redundant bits from the currently transmitted user data protocol frame in positive NIC compensation (positive skip) or adds n redundant bits in the currently transmitted user data protocol frame in negative NIC compensation (negative skip). In addition, the transmitter can indicate the NIC compensation by means of one or more redundant bits remaining in the user data protocol frame. This kind of indication, however, is not needed, if the receiver is able to detect the addition or deletion of redundant bits in some other way.
In another embodiment of the invention, when the transmitter has counted the predetermined number n of skipped bit sequences or the transmission buffer has been emptied to a certain level, the transmitter drops n redundant bits from the next m user data protocol frames to be transmitted where positive NIC compensation is concerned (positive skip), or the transmitter adds n redundant bits to the next m user data protocol frames to be transmitted where negative NIC compensation is concerned (negative skip). In addition, the transmitter indicates by means of one or more remaining redundant bits of the user data protocol frame to be transmitted that a predetermined amount of redundancy has been deleted from the next m frames or that a predetermined amount of redundancy has been added. In this embodiment, the indication, if necessary, is sent only in every mth frame. The more effective indication supports longer frames.
Upon detecting the positive NIC compensation in the received user protocol data frame, the receiver restores the known original redundancy in the positions of the bits deleted by the transmitter. In addition, the receiver temporarily speeds up the outbound network independent clock to make room for the added bits in the outbound bit stream.
Upon detecting negative NIC compensation in the received user protocol data frame, the receiver deletes the known bit stuffing added by the transmitter. In addition, the receiver temporarily slows down the outbound network independent clock to fill the gap caused by the deletion of bits in the outbound bit stream.
The invention makes it possible to increase the maximum length of the user data protocol frame. For example, if four redundant bits could be dropped from the frame for the positive NIC compensation, the average available frame length during a transmission session (xc2x1100 ppm clock tolerance) would be 40000 bits, i.e. 5000 bytes. The more bits to be dropped there are in the frame, the greater the average length of the frames. If the same four bits were used by the present NIC coding of the GSM, two frames would be needed for compensating for a slip of one bit. The average allowable frame length during a transmission session (xc2x1100 ppm clock tolerance) would be 5000 bits, i.e. 625 bytes.