Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-11-09
2004-06-08
Nguyen, Chau (Department: 2663)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S466000, C370S474000
Reexamination Certificate
active
06747977
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates, in general, to an adaptable interface between a broadband-narrowband network and in which adaptation is required between time division multiplexed (TDM) communication and asynchronous transmission mode (ATM) communication. More particularly, but not exclusively, the present invention is applicable to an interface provisioned to support numerous different adaptation protocols, especially adaptation layer two (AAL-2), and a method of packetizing information to optimise utilisation of available bandwidth.
SUMMARY OF THE PRIOR ART
Globally, telecommunication systems can generally be considered to be in a transitional phase between first generation narrowband digital networks (such as the global system for mobile (GSM) cellular communication system) and future multi-media digital networks (such as the universal mobile telecommunication system (UMTS)) having broadband capabilities. Indeed, radio frequency (RF) and wireline systems are being merged together to enhance the information transfer mechanism, while still providing some flexibility with respect to mobility within the network. For example, broadband (typically fibre-optic based) infrastructure connections are being utilised to support information (both voice and data) transfer between cellular RF coverage areas. The transition to broadband systems is, in fact, necessarily required to support higher data rate communications, including video and Internet applications that are presently being both considered and made available to subscribers to the service.
A key goal of development of telecommunications networks is to realise the potential integration of real-time and non-real-time services. The key examples of these two types are voice telephony and computer data. Voice telephony is served predominantly by a circuit switched connection orientated network, arranged to deliver a guaranteed quality of service (QoS). Such networks are implemented by transport and switching systems that use a time division multiplexing scheme. Computer data is served predominantly by the Internet that uses a packet forwarding connectionless mode of operation, i.e. a workable paradigm best suited to the burstiness of traffic demand and general non-deterministic nature of this traffic type.
The technologies and protocols that will serve the integration of these two different types of service are presently being decided by network operators. In this respect, the two main contenders for universal transport and switching are ATM and Internet Protocol (IP), although there are many other legacy systems and nascent technologies that may offer specialised solutions to carrying key services.
Present broadband digital networks are characterised in that user and control information is transmitted in fixed “packet” lengths for the duration of a call, with these packets pre-pended with headers that contain bearer channel identification. Such a broadband system is described in the requirement of the ATM Forum Utopia Level 1/2 Interface. In contrast with narrowband systems, user information is relayed across a node via an asynchronous switching fabric that examines each packet in turn (using some kind of fairness algorithm) and directs it to the appropriate output link in response to the input link and bearer channel identification. Routing and control information transmissions are, however, similar to that for the narrowband case and differ only in as much as the signalling schemes are technology specific.
To facilitate use of broadband networks and the migration of communication networks to high data-rate technologies (e.g. the two mega-bit per second rate envisaged within UMTS), there is a need to provide an effective mechanism for interconnecting narrowband networks through a transparent broadband ether. In other words, the broadband ether must accommodate and support narrowband signalling schemes without affecting either data integrity or in any way inhibiting data flow or interconnection. As such, a narrowband-broadband interface must contain adaptation modules that freely translate between TDM and ATM, for example.
ATM has been designed from the outset to adapt to many different types of communications traffic. ATM is a connection orientated network mechanism, allowing dynamic bandwidth configuration and flexibility as a key advantage over circuit switched networks. ATM has adaptation layers for carrying given services over ATM transport and switches. However the fixed length of ATM cells, while suitable for segmentation of long data packets, thereby simplifying and streamlining switching technologies, is still too large for certain compressed voice services, that suffer a ‘cellification’ delay, affecting existing network delay budgets and acceptable voice characteristics. Indeed, when considering the issues of delay, meaningful voice communication across a channel is achieved with a pure delay of less than one hundred and fifty (150) milliseconds. However, since the signal is likely to suffer from echo, telecommunications standard bodies have stipulated that echo cancellation must be applied to all channels having a one-way delay of greater than twenty-five milliseconds. Unfortunately, in relation to cell assembly of a sixty-four kbps PCM voice channel, a delay of six milliseconds is introduced merely by the provision of sampling. Consequently, an allowable path delay is immediately reduced to nineteen milliseconds, which reduced period is easily exceeded in moderate and long distance calls, e.g. a long distance call between Washington D.C. and San Francisco. Furthermore, conversion of the cell into a narrowband component for onward routing of the call further reduces the available nineteen milliseconds assigned to accommodate all additional delays.
There are, in fact, already a plethora of broadband adaptation schemes that are presently employed or which are being developed or evolved to cope with broadband transmissions. Specifically, ATM adaptation layer protocols such as AAL-1 (and structured data transfer, SDT), AAL-2 and AAL-5 impose very different requirements on processing capabilities of a communication network, especially in relation to a narrowband-broadband interface.
AAL-1 is an ATM adaptation protocol targeted at constant bit rate (CBR) traffic, e.g. voice or video, and is applicable to data rates equal to or exceeding sixty-four kbps (64 kbps). More particularly, narrowband voice signals at 64 kbps are packaged into ATM cells having an overall length of fifty-three bytes; five bytes of which are used as a cell header whilst the remaining forty-eight bytes support the payload. The cell header contains control and routing information, such as the virtual circuit identifier (VCI). With respect to the payload in AAL-1 SDT (structured data trasfer), a first byte (or “octet”) is reserved for a sequence number that provides an error correction facility, while the remaining forty-seven octets are allocated to voice samples. Every eight cells, the first byte is stolen from the forty-seven octets allocated to voice samples of that cell, and is stolen byte is used as a pointer to indicate a structure boundary. The pointer field therefore allows multiplexed transmissions of multiple voice channels, as will be appreciated.
ATM cells can also be formatted in 16-bit words, rather than the more frequently used eight bit word structures described in the immediately preceding paragraph. To accommodate this increase in traffic, each ATM cell is extended to fifty-four bytes, arranged in words with the first three words used for the ATM cell header and twenty-four words sent as traffic in the remaining words of the extended ATM cell.
Eight and sixteen bit buses are particularly suited to byte-based services, and also give backwards compatibility with the ATM Forum Utopia Level 1/2 Interface (see The ATM Forum, “Utopia, AN ATM-PHY Interface Specification Level 1, Version 2.01”, Mar. 21st 1994 (AF-PHY-0017.000), and The ATM Forum Utopia Level 2. Version 1.0, June 1995 (AF-PHY-0039.000)).
In the case of both the
Brueckheimer Simon Daniel
Smith Roger
Tsang Fai
Barnes & Thornburg LLP
Hyun Soon-Dong
Nguyen Chau
Nortel Networks Limited
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