EP1153524A1

EP1153524A1 - Method and apparatus for synchronizing devices in atm based base station subsystems using special virtual channel connections

Info

Publication number: EP1153524A1
Application number: EP00903725A
Authority: EP
Inventors: Topi Kaaresoja
Original assignee: Nokia Networks Oy; Nokia Oyj
Current assignee: Nokia Oyj
Priority date: 1999-02-09
Filing date: 2000-02-08
Publication date: 2001-11-14
Also published as: JP2002541690A; CA2361279A1; WO2000048422A1; AU2551200A

Abstract

A method and apparatus for synchronizing devices in ATM based base station subsystems using special ATM virtual channel connections is disclosed. The invention provides a high, constant bit rate connection to the BTSs via the ATM cloud that is primarily for synchronization. The method in accordance with the invention includes establishing a high, constant bit rate virtual channel connection to each of the plurality of remote devices from the transmitting device and broadcasting data cells to the plurality of remote devices for processing by the plurality of remote devices to deduce a clock frequency for synchronization thereto. The remote devices comprise base transceiver stations (BTS). The transmitting device (1210) is an interworking function device, which may be a transcoder. The method enables clock synchronization by providing a synchronous residual timestamp, through analysis to deduce the arrival time of the data cells, and by determining an available capacity of the buffer and adjusting the clocks of the plurality of remote devices in response to the available capacity of the buffer.

Description

METHOD AND APPARATUS FOR SYNCHRONIZING DEVICES IN ATM BASED BASE STATION SUBSYSTEMS USING SPECIAL VIRTUAL CHANNEL CONNECTIONS

5 BACKGROUND OF THE INVENTION

Field of the Invention.

This invention relates in general to a method and apparatus for providing synchronicity between network devices, and more particularly to method and 10 apparatus for synchronizing devices in ATM based base station subsystems using special virtual channel connections.

Description of Related Art.

The demand by consumers all over the world for mobile communications

15 continues to expand at a rapid pace and will continue to do so for at least the next decade. Over 100 million people were using a mobile service by the end of 1995, and that number is expected to grow to 300 million by the year 2000. Several factors are contributing to the exciting growth in the telecommunications industry. For example, a combination of technology and competition bring more

20 value to consumers. Phones are smaller, lighter, had a longer battery life, and are affordable now for the mass market. Operators are providing excellent voice quality, innovative services, and roaming across the country or world. Most important, mobility is becoming less expensive for people to use. Around the world, as well as in the United States, governments are licensing additional

25 spectrum for new operators to compete with traditional cellular operators.

Competition brings innovation, new services, and lower prices for consumers.

Wireless personal communication networks (PCN) based on digital technologies have emerged as an important field of activity in telecommunications. This is mainly due to the success of mobile phones,

30 pagers, and notebook computers. However, the day is fast approaching when electronic mail, faxes and other types of data can be sent or received using simple handheld devices. In cellular systems, each geographic cell is served by a separate base station that provides wireless telecommunications services to station sets located within the cell. However, transmissions and receptions by base stations must be synchronized with each other and with a transcoder. Today, this is true with pulse-coded modulation (PCM) networks that are widely used in cellular networks. In the future, these systems will be connected via ATM networks.

When using ATM for transmission in cellular networks, synchronizing the network elements can be a problem. Transcoders (TC), or more generally Interworking Function (IWF) between ATM and PCM, inherit a common network clock from PSTN or MSC. However, delivering this clock to the base transceiver stations (BTSs) may be a problem. This is especially true if a public ATM network is used. Nevertheless, even in more private networks, some ATM switches can lack an ability to lock to a physical transmission clock. Thus, the BTSs belonging to the network must all be synchronized. Clocks, i.e., oscillators, in BTSs are quite accurate. This means that the oscillators can run free without synchronization quite a long time, e.g., a week. Synchronization techniques exist which are able to synchronize two end elements over ATM. For example, in an ATM based cellular network there are a plurality of BTSs in the edge of the ATM cloud. Synchronization is missing in the physical level, so the BTSs should be synchronized by some other method.

Typically, the synchronization techniques are based on time stamps, an investigation of the mean arrival rate of incoming ATM cells or the investigation of the filling level of the buffer for the incoming ATM cells. Still, these synchronization techniques are all based on scanning the cell rate of the traffic channels. However, the use of traffic channels poses a problem, because the cell rate or bit rate of speech traffic channels is rather low and getting even lower, and, in the future, the traffic flow will be variable rather than constant. Therefore, deducing the clock from the low, variable bit rate traffic channel is difficult and takes a long time. One contributing factor is the elimination of the CDV (Cell Delay Variation) which takes too much time, and the duration of a single cell is usually not enough for this. The synchronization can even be impossible because of the variable bit rate connection. It can be seen that there is a need for a method and apparatus for synchronizing BTSs at the edge of an ATM cloud.

SUMMARY OF THE INVENTION To overcome or at least mitigate the limitations in the prior art described above, and to overcome or at least mitigate other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for synchronizing devices in ATM based base station subsystems using special ATM virtual channel connections. The present invention solves the above-described problems by providing a high, constant bit rate connection to the BTSs via the ATM cloud that is primarily for synchronization.

A method in accordance with the principles of the present invention includes establishing a high, constant bit rate virtual channel connection to each of the plurality of remote devices from the transmitting device and broadcasting data cells to the plurality of remote devices for processing by the plurality of remote devices to deduce a clock frequency for synchronization thereto.

Other embodiments of a method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the remote devices comprise base transceiver stations.

Another aspect of the present invention is that the transmitting device is an interworking function device.

Another aspect of the present invention is that the interworking function device comprises a transcoder.

Another aspect of the present invention is that the transmitting device and the remote device are connected via an asynchronous connection, the asynchronous connection preventing the physical transmission of the clock of the transmitting device. Another aspect of the present invention is that the data cells provide a synchronous residual timestamp for allowing the plurality of remote devices to deduce the clock of the transmitting device and to synchronize thereto. Another aspect of the present invention is that the method further includes analyzing at the plurality of remote devices to deduce the arrival time of the data cells, calculating a difference between the arrival time of the cells and an expected arrival time, and adjusting the clocks of the plurality of remote devices according to the calculated difference to synchronize the clocks of the plurality of remote devices to the transmitting device.

Another aspect of the present invention is that the method further includes storing the calculated difference for N cells, deriving a mean arrival time for N cells, and adjusting the clocks of the plurality of remote devices according to the means arrival time for N cells to synchronize the clocks of the plurality of remote devices to the transmitting device.

Another aspect of the present invention is that the method further includes analyzing a buffer in the plurality of remote devices to determine a filling level of the buffer and adjusting the clocks of the plurality of remote devices according to the filling level of the buffer for the plurality of remote devices to synchronize the clocks of the plurality of remote devices to the transmitting device.

Another aspect of the present invention is that a cellular communications system includes a plurality of base transceiver stations, the plurality of base transceiver stations being coupled to an ATM network and an Interworking Unit comprising a transcoder, coupled to the ATM network, for providing interworking functions between the ATM network and a telecommunications system, wherein the Interworking Unit establishes a high, constant bit rate virtual channel connection to each of the plurality of base transceiver stations and broadcasts data cells to the plurality of remote devices over the high, constant bit rate virtual channel connections to the plurality of base transceiver stations to allow the plurality of base transceiver stations to deduce a clock frequency of the Interworking Unit for synchronization thereto.

For a better understanding of the invention, its advantages, and the objects obtained by its use, reference will be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

Fig. 1 illustrates the Open Systems Interconnection (OSI) physical layer; Fig. 2 illustrates an ATM cell;

Fig. 3 illustrates VPI and VCI operations;

Fig. 4 illustrates a telecommunications system, wherein a transcoder is used as an Interworking Function between the ATM cloud and the PCM networks according to the present invention; Fig. 5 illustrates a more detailed block diagram of the BTS and IWF according to the present invention;

Fig. 6 illustrates the AAL1 -PDU;

Fig. 7 illustrates is a diagram of a system device wherein alternative methods for deducing the clock frequency may be demonstrated; Fig. 8 illustrates a system using an ATM connection wherein clock synchronization is needed;

Fig. 9 shows the arrival of two adjacent cells at the receiving end of the connection as seen using the clock of the receiving device;

Fig. 10 illustrates a flow chart of the implementation wherein N cells are observed before adjusting the clock;

Fig. 11 illustrates a phase locked loop for correcting the frequency f- and

Fig. 12 illustrates the use of ATM multicasting to send cell streams from the TC or IWF to BTSs for deducing the clock frequency in BTSs according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention.

The present invention provides a method and apparatus for synchronizing devices in ATM based base station subsystems using a special virtual channel. The present invention provides a high, constant bit rate connection to the BTSs via the ATM cloud that is primarily for synchronization.

Fig. 1 illustrates the Open Systems Interconnection (OSI) physical layer 100. Modern networks must handle multiple types of traffic such as video 110, voice 112, data files 114, and interactive data 116. The ATM Adaptation Layer 120 is a collection of standardized protocols that provide services to higher layers by adapting user traffic to a cell format. The AAL 120 is divided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) sublayer (not shown). The ATM Layer 130 is the second layer of the ATM protocol stack model 100 that constructs and processes the ATM cells. The functions of the ATM layer 130 also include Usage Parameter Control (UPC) and support of Quality of Service (QoS) classes. Finally, the physical layer 140 is the bottom layer of the ATM protocol reference model 100. The physical layer 140 is subdivided into two sublayers, the Transmission Convergence (TC) and the Physical Medium (PM) (also not shown). The physical layer 140 provides the ATM cells transmitted over the physical interfaces that interconnect ATM devices. ATM ( Asynchronous Transfer Mode) has been advocated as an important technology for the wide area interconnection of heterogeneous networks. In ATM networks, the data is divided into small, fixed length units called cells. Fig. 2 illustrates an ATM cell 200. The cell 200 comprises 53 bytes. Each cell contain a 5 byte header 210 which comprises of identification, control priority and routing information. The rest 48 bytes are the actual data 220. ATM does not provide any error detection operations on the user payload inside the cell 200, and also provides no retransmission services, and only few operations are performed on the small header.

ATM switches support two kinds of interfaces: user-network interface (UNI) and network-node interface (NNI). UNI connects ATM end systems (hosts, routers etc.) to an ATM switch, while an NNI may be imprecisely defined as an interface for connecting two ATM switches together. The ITU-T

Recommendation requires that an ATM connection be identified with connection identifiers that are assigned for each user connection in the ATM network.

At the UNI, the connection is identified by two values in the cell header: the virtual path identifier (VPI) and the virtual channel identifier (VCI). Fig. 3 illustrates VPI and VCI operations 300. VPIs and VCIs are used in the ATM network by the switches to determine how to route the cell through the network. For example, the video session's 310 connection is associated with VPI 4 312/VCI 10 314 at a first UNI 320 and VPI 20 330/VCI 33 332 at the other UNI 340. The manner in which VPI/VCI values are established and managed is left to the network administrator. In this example, the VPI/VCI numbers have local significance at each UNI. Of course, the network must assure that these local VPI/VCI values at each UNI are mapped together through the network. Both VPI and VCI combine together to form a virtual circuit identifier.

There are two fundamental types of ATM connections: Permanent Virtual Connections (PVC) and Switched Virtual Connections (SVC). A PVC is a connection set up by some external mechanism, typically network management, in which a set of switches between an ATM source and destination ATM systems are programmed with the appropriate VPI/VCI values. PVCs always require some manual configuration. An SVC is a connection that is set up automatically through signaling protocol. SVCs does not require the manual interaction needed to set up PVCs and, as such, are likely to be much more widely used. All higher layer protocols operating over ATM primarily uses SVCs. ATM adaptation layers (AALs) 120, as shown in Fig. 1 , provide mechanisms for supporting transport protocols over ATM cells. AAL1 and AAL2 have been defined by the ITU-T for use in the wide area for support of constant and variable bit rate services respectively and AAL 3/4 for connection-less data transport. However, AAL5 is proposed for all types of computer oriented multiservice traffic especially for the local area. AAL5 has lower pay-load overhead per cell and relies on quality of service and statistical mechanisms to provide multiservice capability.

Fig. 4 illustrates a telecommunications system 400, wherein a transcoder 410 is used as an Interworking Function between the ATM 420 and the PCM networks 430, 432 according to the present invention. When using ATM for transmission in cellular networks, synchronizing the network elements can be a problem. Transcoders (TC) 410, or more generally Interworking Function (IWF) between ATM 420 and PCM 430, 432, inherit a common network clock from PSTN 432 or MSC 430. The problem is then to deliver this clock to the BTSs 440. This is especially true if a public ATM network is used. Even in more private networks, some ATM switches can lack an ability to lock to a physical transmission clock. In Fig. 4, the base transceiver stations (BTSs) 440 are coupled to the telecommunications network via ATM connections. The Transcoder (TC) 410 is essentially an Interworking Function (IWF) between the ATM 420 and the PCM 430, 432. The IWF 410 provides rate adaptation and data conversion between the ATM 420 and the PCM 430, 432. The PCM world can be a PSTN 432 or a MSC 430 depending of the phase of the ATM based network evolution.

Transcoder Rate Adaptor Unit (TRAU) frames are transmitted over ATM 420 using either one-to-one mapping, i.e., AAL0, AAL1 or AAL2.

The transmitting clock of the BTS 440 operates at the frequency f_BTS and the receiving clock of the TC 410 operates at the frequency f_τc. Because the transmission is now asynchronous, the TC 410 can not know the exact frequency of the BTS transmitter 440. If f_BTS is a little bit higher than f_τc and no DTX (Discontinuous Transmission) is used in the ATM 420, the receiving buffers of the TC 410 will overflow at some moment. Audible clicks will occur on the telephone connected to the PCM 430, 432 when a TRAU frame are discarded because of the overflow of the buffer. The same is true in the opposite direction, as well. Those skilled in the art will recognize that the Interworking Unit 410 may not include a transcoder as shown in Fig. 4. Accordingly, those skilled in the art will recognize that if the transcoder is not included as a portion of the Interworking Unit, the synchronization virtual channel connections are needed either between Interworking Unit and the BTSs or between the transcoder and the BTSs, depending on the location of the transcoder.

Fig. 5 illustrates a more detailed block diagram 500 of the BTS 540 and IWF 540 according to the present invention. In Fig. 5, the BTS 540 includes an ATM IF 542 and synchronous parts 544. Similarly, the IWF 510 includes an ATM IF 512 and synchronous 514. Accordingly, the synchronous parts 514 of a IWF 510 has to be synchronized with the synchronous parts 544 of the BTS 540. If this is not done, clock asynchrony must be solved in some other manner to prevent overflows. Clock synchronization may be accomplished by physically transmitting the clock from PSTN 532 up to BTSs 540 or by deducing the clock frequency in some other manner. Physical transmission of the clock is available, for example, via Plesiochronous Digital Hierarchy (PDH) or Synchronous Digital Hierarchy (SDH) frames. The clock can be propagated from PSTN 530, 532 up to BTSs 540 if all the transmission equipment, i.e., ATM switches, support this kind of clock locking. This method is the easiest way to ensure the synchrony.

Alternatively, deducing the sender's clock frequency may be accomplished using either a timestamp, investigating of the mean arrival rate of incoming ATM cells or investigating the filling level of the buffer for the incoming ATM cells.

The method used in AAL1 is Synchronous Residual Timestamp (SRTS). Fig. 6 illustrates the AAL1 -PDU 600. A four bit field 600 is provided in the AAL1 - PDU which can be used for the SRTS. This process is defined in ITU Recommendation 1.363. The ATM network clock is taken and divided by 2[k], where k = 0,1...11. Also, k is chosen to produce a frequency near the service bit- rate (greater than but less than twice the rate). The difference between the service bit-rate and the network related frequency is produced and the 4 Isbs of this difference are transmitted using 4 CSI bits in the SN field 610 from 8 successive ATM cells. These 4 bits comprise the SRTS. Both transmitter and receiver carry out this process and by comparing the locally produced SRTS with the received far-end SRTS, the local clock can be synchronized to the remote clock

Accordingly, SRTS provides the residual clock between the source and the network clocks that is sent to the other end. Thus, a connection to the master clock source is needed. With this method, the clock can be propagated all over the network. If AAL1 is not in use, this kind of method could be used with AAL0, AAL2 and AAL5. In the case of the AAL0, i.e., one-to-one mapping, an absolute timestamp field could be added to the cell, since there is space for it. Fig. 7 illustrates is a diagram of a system device 700 wherein alternative methods for deducing the clock frequency may be demonstrated. The device 700 includes a buffer 710 for receiving incoming cells 720. A processor 730 monitors and controls the buffer 710 to allow cells in the buffer 710 to be sent. A measurement of the filling level of the buffer 710 for incoming cells may be used. The purpose of measuring the filling level of the buffer 710 for incoming cells is to keep the filling level of the buffer 710 for incoming cells at some constant level. If the buffer 710 is filled, the clock frequency is increased a little until the filling level of the buffer 710 is again at the predetermined level. If the buffer 710 is going to underflow the clock frequency is decreased until the filling level of the buffer 710 is again at the predetermined level.

Yet another method involves the principle of investigating the mean arrival rate of incoming cells, which requires knowledge of the arrival time of cells. For example, a device 700 at one end, end A, sends cells every t_A and a processor 730 in device 700 the other end, end B, knows that the cell should come at the time t_B. The processor calculates the time difference t_B - t_A, which is then used to adjust the clock. In ATM networks, the cell delay variation (CDV) can be diminished by using the median of the arrival time of, for example, ten cells. This method is especially well suited for ATM based GSM systems where one-to-one mapping is used. This method may be more fully illustrated with reference to Figs. 8 and 9.

If the receiving and the transmitting clocks of one end are locked to each other, the other end should be the master clock. The other end detects the rate of the incoming cells and adjusts its clock frequency accordingly. Typically two elements of a Base Station Subsystem (BSS) in a cellular network are connected using a synchronous PCM connection. As stated above, one of the devices is selected as a master and its clock synchronizes the PCM frames sent over the connection. The second device, a slave, synchronizes its own clock to the master. However, this synchronization is lost if two PCM devices are connected using an asynchronous ATM connection which doesn't transfer the synchronization at the physical level. Fig. 8 illustrates a system 800 using an ATM connection wherein clock synchronization is need. In Fig. 8, TC 810 is shown sending data 812 to a BTS 820. TC 810 has a clock frequency of f_t 830. BTS 820 has a clock frequency of f.832. As illustrated in Fig. 8, if the two clocks 830, 832 had exactly the same frequency, no problem would arise since the transmitting and the receiving end operate at the same speed.

Unfortunately, no clock is exact, so without some synchronization, one of the clocks is probably faster than the other. For example, if the clock of the transmitting device 810 at clock frequency f_t 830 is faster than the clock frequency f_r 832 of the receiving device 820, the buffers of the receiver are gradually filled. This can result in lost data and, in the case of time sensitive applications like GSM speech, cumulate delay that can rapidly become irritating. In order to resolve this problem, a method based on the expected arrival of cells and on the averaging procedure is used to eliminate the effect of Cell Delay Variation (CDV). This method may be used whenever the time interval between transmitted ATM cells is a known constant. For example, a system may pack one GSM Full Rate TRAU frame inside one ATM cell. The TRAU-frame contains 20 ms of coded speech. Thus, the ATM cells are transmitted with the interval of 20 ms corresponding to the cell rate r_t = 50 cells/s. The frequency f of the clock can be obtained form the cell rate r using f_k=Rr_k where / =rfor the receiver, k=t for the transmitter and R is a constant. For simplicity, it is assumed below that f?=1 so that the clock of the device has the same frequency as the cell rate, f_k=r_k.

Fig. 9 shows the arrival of two adjacent cells 910, 912 at the receiving end of the connection as seen using the clock of the receiving device. The time interval between the cells is expected to be At_r 920 = 1/r=20 ms and r=50 cells/s is the expected rate of arriving cells. However, the cell / 910 arrives after the interval At, 930 according to the clock of the receiver. This means that the receiver thinks that the transmitter is transmitting at some cell rate that slightly differs from 50 cells/s. This is due to the different clock frequencies so the new, corrected clock frequency of the receiver should become

/-" =/, A (1) Usually, however, we want to know the frequency difference

1 1 At_r - At, C - At, Af - fι - fr - _Atι - _A^ ^~ _AtιAt^ ^~ _AtιC ^■ I²)

Here, it is assumed that the difference Δf_r-Δt, is only due to the difference between the clocks. The time interval At_r 920 is constant from the view point of the receiver and is presented here by Cto emphasize that. This difference should be added to the frequency of the clock at the receiving end so that it starts to run at the frequency of the transmitting device. Thus, from Eq. (2) C - Δt, ^f -^f-⁺-π≠ ⁽³⁾ where the index corr refers to the corrected frequency of the receiver.

However, there is one extra component to the time interval At, 930 seen by the receiver. Namely, an ATM cell carrying a TRAU frame has some Cell Delay Variation (CDV) caused by the ATM network. Thus the time interval is actually a sum of the instantaneous CDV and the difference between the clock frequencies, so that

Δt, =Δt^_DV + (4)

where the index / refers to the Λh cell. If the effect of CDV is large, the clock of the receiver is adjusted according to the more or less random cell delay variation, not according to the source clock. However, it can be assumed that the cell delay variation At_C'_DV is randomly distributed with a Gaussian distribution. Thus, an average Δt,^αve over certain number N of samples should eliminate the effect of CDV. Then At, 930 can be replaced by Δt,^αve , which yields a good estimation of the necessary correction to the frequency of the receiving clock, namely

An implementation may therefore observe N cells before adjusting the clock of the receiving device.

Fig. 10 illustrates a flow chart 1000 of the implementation wherein N cells are observed before adjusting the clock. When the first cell arrives, the receiver starts a timer 1010 and stops it at time At, when the next cell arrives 1020. Then, the difference At-At, is calculated 1030 as being C-Δt₍. Then, C-Δt₍ is stored 1040 and the timer is started 1050. Thereafter, i is incremented and checked to determine whether i is equal to N 1060. This procedure is repeated N times 1070. Then, the average over the measured values At, is calculated 1080. Then, the clock of the receiver is corrected using Eq. (5). Thus,

where the first sum should approach zero when N → ∞.

The frequency f_r of the receiver is corrected with a Phase Locked Loop (PLL) 1100 as illustrated in Fig. 11. The time difference calculator, i.e., subtractor, can be considered as the Phase Detector (PD) 1110 of the PLL 1100. If the result of the subtraction is positive, a positive pulse is generated by the integrator 1120. If the result is negative, a negative pulse is generated by the integrator 1120. These pulses are used to control the Voltage Controlled Oscillator (VCO) 1130 of the PLL 1100. This example shows that only the time difference At_r - At, is needed to correct the frequency of the receiver. Accuracy is dependent upon the value of N. If the frequency difference in

Eq. (2) is very small, this method can make the situation worse if N is too small. So this synchronizing method could be used as an second alternative if the physical synchronization is not available.

Accordingly, several synchronization techniques are available for synchronizing two end elements over ATM. The above mentioned techniques are based on timestamps, the investigation of the mean arrival rate of incoming ATM cells, the investigation of the level of the buffer for the incoming ATM cells. However, all of the above different kinds of adaptive methods are based on scanning the cell rate of traffic channels. The use of traffic channels presents a problem because the cell rate or bit rate of speech traffic channels is rather low and getting even lower. Furthermore, in the future, the traffic flow will be variable rather than constant. Therefore, deducing the clock from the low variable bit rate traffic channel is difficult and takes a long time. This is because the CDV (Cell Delay Variation) takes time. The duration of a single cell is usually not enough for this. The synchronization can even be impossible because of the variable bit rate connection.

Fig. 12 illustrates the use of ATM multicasting 1200 to send cell streams from the TC or IWF to BTSs for deducing the clock frequency in BTSs in accordance with the present invention. In Fig. 12, a plurality of BTSs 1240 are coupled to the PCM 1230 via the TC 1210 and ATM 1220. High, constant bit rate virtual channels 1260-70 are setup to each of the BTSs 1240 for synchronization. The TC uses the broadcast synchronization virtual channels 1260-1270 to send cell streams to the BTSs 1240. Any of the above mentioned methods may be used in conjunction with present invention to deduce the clock frequency in the BTSs using the synchronization virtual channels 1260-1270. Synchronization Virtual Channels 1260-1270 could be set up during the night time, for example, when the traffic load is low. The cell rate for each of the synchronization virtual channels 1260- 1270 is selected to be sufficiently high. While high cell rates can be expensive if public operator are used, the synchronization procedure according to the present invention does not take long because of the high, constant bit rate. Furthermore, the synchronization needs to be performed only once a day, thereby minimizing expense.

For example, as discussed above, a method for eliminating the CDV was introduced. It was assumed that CDV is randomly distributed with a Gaussian distribution, which is not necessarily true, but is still useful as an assumption here. It was shown that the CDV can be eliminated by measuring sufficient amounts of cells, i.e., N, thereby averaging the CDV out. However, it was not defined how big N should be. Nevertheless, if Λ/ = 10⁵, a single GSM traffic channel and AAL5 are used, a connection with a cell rate

I6000bit/ s

R= - -=50cellsls ensues. The time for synchronization is

320bit/cell

N lOOOOOcells , . , . . . ., , , . , . .

— = -— — = 2000s , which is more than half an hour. However, this duration

R 50cellsls is too long compared to usual call duration of mobile phone.

According to the present invention, a high cell rate is used to solve this problem. If, for example, a 155 Mbit/s line is used, a cell rate of

155* 10° R= — — — «4,8 * 10⁵ ce/Z,s/.y ensues. The time for synchronization diminishes to

N lOOOOOcells _r.

— = — — =0,2 = 210ms , which is a significant improvement.

R 4S0000cells/s a Because the variable cell rate of the traffic channel is a problem, a

Constant Bit Rate (CBR) connection for the synchronization virtual channels 1260-1270 is used. In addition, to the synchronization data, software updates, for example, can be contained by the synchronization cells.

In summary, the present invention describes a method for using a Special Virtual Channel for Synchronization in ATM based BSS. Using a special virtual channel for synchronization alleviates the problem of tying-up traffic channels for adaptive synchronization. Accordingly, a dedicated, constant, high cell rate connection is used for synchronization. The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method for synchronizing clock for a plurality of remote devices to a clock of a transmitting device, comprising: establishing a high, constant bit rate virtual channel connection to each of the plurality of remote devices from the transmitting device; broadcasting data cells to the plurality of remote devices for processing by the plurality of remote devices to deduce a clock frequency for synchronization thereto.

2. The method of claim 1 wherein the remote devices comprise base transceiver stations.

3. The method of claim 1 wherein the transmitting device is an interworking function device.

4. The method of claim 3 wherein the interworking function device comprises a transcoder.

5. The method of claim 1 wherein the transmitting device and the remote device are connected via an asynchronous connection, the asynchronous connection preventing the physical transmission of the clock of the transmitting device.

6. The method of claim 1 wherein the data cells provide a synchronous residual timestamp for allowing the plurality of remote devices to deduce the clock of the transmitting device and to synchronize thereto.

7. The method of claim 1 further comprising analyzing at the plurality of remote devices to deduce the arrival time of the data cells, calculating a difference between the arrival time of the cells and an expected arrival time, and adjusting the clocks of the plurality of remote devices according to the calculated difference to synchronize the clocks of the plurality of remote devices to the transmitting device.

8. The method of claim 7 further comprising storing the calculated difference for N cells, deriving a mean arrival time for N cells, and adjusting the clocks of the plurality of remote devices according to the means arrival time for N cells to synchronize the clocks of the plurality of remote devices to the transmitting device.

9. The method of claim 1 further comprising analyzing a buffer in the plurality of remote devices to determine a filling level of the buffer and adjusting the clocks of the plurality of remote devices according to the filling level of the buffer for the plurality of remote devices to synchronize the clocks of the plurality of remote devices to the transmitting device.

10. A cellular communications system, comprising: a plurality of base transceiver stations, the plurality of base transceiver stations being coupled to an ATM network; and an Interworking Unit, coupled to the ATM network, for providing interworking functions between the ATM network and a telecommunications system, wherein the Interworking Unit is arranged to establish a high, constant bit rate virtual channel connection to each of the plurality of base transceiver stations and to broadcast data cells to the plurality of remote devices over the high, constant bit rate virtual channel connections to the plurality of base transceiver stations to allow the plurality of base transceiver stations to deduce a clock frequency of the transcoder for synchronization thereto.

11. The cellular communications system of claim 10 wherein the telecommunications system further comprises a pulse coded modulation communication system.

12. The cellular communication system of claim 11 wherein the pulse coded modulation system comprises a mobile services switching center.

13. The cellular communication system of claim 11 wherein the pulse coded modulation system comprises a public switched telephone network.

14. The cellular communication system of claim 10 wherein the data cells provide a synchronous residual timestamp for allowing the plurality of base transceiver stations to deduce the clock of the transcoder and to synchronize thereto.

15. The cellular communication system of claim 10 wherein each of the plurality of base transceiver stations is arranged to analyze the data cells to deduce the arrival time of the data cells, to calculate a difference between the arrival time of the cells and an expected arrival time, and to adjust a clock in each of the plurality of base transceiver stations according to the calculated difference to synchronize the clocks of the plurality of base transceiver stations to the transcoder.

16. The cellular communication system of claim 15 wherein each of the plurality of base transceiver stations is arranged to store the calculated difference for N cells, to derive a mean arrival time for N cells, and to adjust the clock in each of the plurality of base transceiver stations according to the means arrival time for N cells to synchronize the clocks of the plurality of base transceiver stations to the transcoder.

17. The cellular communication system of claim 10 wherein each of the plurality of base transceiver stations is arranged to analyze a buffer in the plurality of base transceiver stations to determine a filling level of the buffer and to adjust the clocks of the plurality of base transceiver stations according to the filling level of the buffer for the plurality of base transceiver stations to synchronize the clocks of the plurality of base transceiver stations to the Interworking Unit.