Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-06-05
2003-04-29
Nguyen, Steven (Department: 2665)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S503000
Reexamination Certificate
active
06556573
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to network communications systems, and more particularly, to a system and method for providing clock synchronization of source and destination clocking functions, for ATM variable bit rate traffic operating under the AAL2 and AAL5 layered protocols.
BACKGROUND OF THE INVENTION
The modern communications era has brought about a tremendous proliferation of wireline and wireless networks. Computer networks, television networks, and telephony networks in particular are experiencing an unprecedented technological expansion, fueled by consumer demand. The ever-increasing need for transportation, due in part to the expansion of the world-wide market and the popularity of suburbia, has led to an increased use of automobiles and airplanes for business and pleasure. The desire to maintain the ability to communicate, even while away from the home or office, has driven the wireless communication market to a large extent. One response to this demand was the mobile/wireless telephone network.
The demand by consumers all over the world for mobile communications is expanding 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 value to consumers. Phones are smaller, lighter, have 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 spectrum for new operators to compete with traditional cellular operators. Competition brings innovation, new services, and lower prices for consumers.
Cellular telephone communications systems allow users of cellular telephones to be connected to other cellular telephone users, as well as being connected to the conventional landline Public Switched Telephone Network (PSTN). Cellular telephones work by dividing geographical areas into “cells”. Each cell includes a base station, which typically contains a transceiver, antenna, and dedicated lines to a Mobile Telephone Switching Office (MTSO) or Mobile Switching Center (MSC). Adjacent cells utilize different radio frequencies in order to prevent interference between the adjacent cells.
Many standards exist for public mobile systems. One such standard is the Global System for Mobile communications (GSM), which is accepted in Europe and several countries outside Europe as the international standard for modern public mobile systems. GSM is an open system that can be used for both voice and data, and uses digital transmission for voice and signaling. GSM systems also use time-division multiplexing (TDM) that can offer many channels on one frequency.
A variety of data transmission technologies can be used in connection with mobile communications. One transmission technology currently being explored in connection with the present invention is asynchronous transfer mode (ATM) technology as it is used in connection with voice and data transmission in mobile system architectures such as GSM. ATM is a cell-based switching and multiplexing technology designed to be a general-purpose, connection-oriented transfer mode for a wide range of services.
ATM is a transfer mode that uses information “cells” as the means of transferring information. The cell has a fixed length of 53 octets, which includes a 5-octet header and a 48-octet payload. The header includes a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) which identify the cell destination. A transmission path contains one or more virtual paths (VP), while each virtual path may contain one or more virtual channels (VC). The VPI together with the VCI identify the virtual circuit by which ATM switches route the call from source to destination. Other fields in the cell header include the Generic Flow Control (GFC) which allows a multiplexer to control the rate of an ATM terminal, and the Payload Type (PT) field that indicates whether the cell contains user data, signaling data, or maintenance information. The Cell Loss Priority (CLP) bit in the cell header indicates the priority of the cell relative to other cells, and the Header Error Check (HEC) field detects and corrects errors in the header.
In a communication system such as a Base Station System (BSS), a need to interface dissimilar technologies may arise. For example, a cellular network incorporating ATM technology may at some point require interfacing to a public switched telephone network (PSTN) or mobile switching center (MSC) that implements pulse code modulation (PCM). PCM refers to the traditional digital modulation method that encodes a voice signal into an eight-bit word representing the amplitude of each pulse. More specifically, the speech signal is sampled at 8 khz, and each sample is encoded using an 8-bit code to provide 8,000 samplings at 8 bits per sample, resulting in a 64-Kbps bit stream. Because of the dissimilarity between ATM and PCM information transfer technologies, conversion and synchronization may be required.
For example, an ATM-based BSS is designed to work in a cellular environment having an arrangement of cells, each cell having a base transceiver station (BTS) (also referred to as a base station or BS). Cells when grouped together form a cluster, and the MSC is connected to all of the base stations in a cluster. Alternatively, a base station controller (BSC), which is a modular switching platform, can perform the connection and traffic concentration between a number of BTSs and the MSC. A BSC also controls the basic functionality of those BTSs connected to it. Each BTS manages radio traffic with multiple mobile stations (MS). In sum, the BSS is a system of BTSs and BSCs which is viewed by the MSC through a single interface.
In such an ATM-based BSS, a transcoder (TC) is an inter-working function (IWF) between the ATM and the PCM environments. An IWF allows interoperation between a native protocol and an ATM-based end device. The PCM environment can be a PSTN or a MSC depending on the phase of the ATM-based GSM network evolution. A TC is a network element which provides the conversion between GSM-coded voice signals and the standard PCM 64-Kbit/sec signal used in the MSC and PSTN.
However, the interface between an ATM-based BSS and the PCM-based MSC (or PSTN) can bring about some synchronization concerns. The principal concern can be described as follows. The transmitting clock of the BTS operates at the frequency f
BTS
, and the receiving clock of the TC operates at the frequency f
TC
. Because the transmission is asynchronous, the TC will not know the exact frequency of the BTS transmitter. If f
BTS
is slightly higher than f
TC
and no discontinuous transmission (DTX) is used in the ATM, the receiving buffers of the TC may eventually overflow. Audible “clicks” will occur on the telephone connected to the PCM when a TRAU frame is discarded because of the overflow in the buffer. This holds true for communications in the opposite direction as well.
In order to avoid this overflow condition, the PCM interface of the IWF should be synchronized with the PCM component of the BTS. In many cases, the BTS will have some PCM-oriented components in order to synchronize with mobile stations, as well as an ATM interface.
Synchronization has been accomplished in various manners. If the ATM switches can support propagating a physically transmitted clock from the PSTN to the BTS, such as by way of plesiochronous digital hierarchy (PDH) or synchronous digital hierarchy (SDH) frames, synchronization can be accomplished.
Where the ATM switches do not support such physical clock propagation, other methods must be used. For example, the cloc
Altera Law. Group LLC
Nguyen Steven
Nokia Telecommunications Oy
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