ATM communications system and method

Details ATM communications system and method ATM communications system and method

1999-09-03

2003-07-08

Chin, Wellington (Department: 2664)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S536000

Reexamination Certificate

active

06590909

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to digital communications networks, and particularly, although not exclusively to an arrangement and method for transmitting multiplexed multi-user asynchronous transfer mode (ATM) traffic across such communications networks.
BACKGROUND TO THE INVENTION
The known asynchronous transfer mode (ATM) transmission technique is a modem telecommunications switching technique which is able to switch connections for a wide range of different data types at a wide range of different bit rates. ATM technology provides a flexible form of transmission which allows various types of service traffic data, e.g. voice data, video data, or computer generated data to be multiplexed together onto a common physical means of transmission. Currently, several trends are encouraging the widespread introduction of ATM; for example the availability of high speed, low error rate communication links between switching centers, an availability of technology to digitize video and speech, and pressure to reduce operating costs by integrating previously separate telephony and data networks. ATM technology allows speech data, video data and inter-computer data to be carried across a single communications network. The information carried in each of these services is reduced to digitized strings of numbers which are transmitted across such a communications network from point to point.
A method of switching synchronous transfer mode cells in a circuit emulated ATM switch using a layered protocol model is described in specification No. WO-95-34977. A method of transferring ATM microcells in a telecommunications system is described in specification No. WO-96-34478.
Referring to
FIG. 1
herein, there is shown schematically a portion of a communications network comprising first and second node devices
100
,
101
respectively linked by a communications link
100
. Transport of ATM data communications traffic is made across the communications link
102
between the first and second node devices, which may be for example switches
101
,
102
. Digitized data is received from customer equipment such as telephones, computers, faxes, modems and video broadcast apparatus in the form of frames of digitized signals at transmitting node, e.g. switch
100
. The frames can either be of variable length or fixed length, and may arrive at the switch at a variable rate; or at a fixed rate. The frames of data arriving at the switch are packaged into ATM data cells
103
, which have a fixed number of bytes. Transport of ATM cells between node devices is handled by the node devices operating in accordance with the ATM protocol corresponding to the International Standards Organization (ISO) Open Systems Inter-connexion (OSI) architecture, layers two and three
(1)
. Packaging of the incoming data frames received asynchronously from the customer equipment is handled by the switches operating in accordance with ATM adaptation layer (AAL) protocols which segment the arriving frames of data into payload data of the ATM cells at the transmission node, and reassemble the payload data into frames at the destination node
102
. The ATM adaptation layer corresponds to layer four of the OSI model. Equipment operating in accordance with the ATM adaptation layer protocols are capable of structuring incoming data in different ways, to suit different service types, e.g. video data, computer generated data, voice data. Many different service types can be implemented by the ATM adaptation layer simultaneously.
The basic reason for having ATM cells is that they have a fixed length. Fixed length cells are easier for hardware to handle than variable length frames. The ATM adaptation layer packages various types of data of variable length or fixed length frame type into the fixed length ATM cells for transport between physical devices. Because the ATM cell length was historically selected to accommodate various types of traffic, fixing the length of the ATM cell involved difficult decisions, and the final length of ATM cell selected is not perfect for each type of data carried. The ATM cell comprises a header portion which carries routing information and other housekeeping information necessary for the operation of the ATM network, and a payload portion which carries the actual data traffic. To transfer delay sensitive services such as speech, it is important that the ATM cell be reasonably short in order to avoid unacceptably long delays in filling the cell payload portion before transmitting the cell across the network. On the other hand, for other types of traffic such as computer to computer file transfers longer cells are more efficient, since the proportion of available transmission bandwidth taken up by the ATM cell header compared to the data payload of the cell is reduced. For delay insensitive traffic, the overhead of the housekeeping information sent in the header of each ATM cell would be relatively large if short cells were to be used. Thus, the choice of ATM cell size is a compromise and is settled at a length of 53 octets, comprising 48 octets of data payload (the ATM Service Data Unit, ATM-SDU) and a 5 octet header for transmission of housekeeping protocol information, as shown schematically in
FIG. 2
herein. The protocol header in the ATM cell constitutes approximately 10% of the whole cell. This size of ATM cell introduces a delay in transmission of data which is significant for types of data having a low data rate, for example speech data. For example for a conventional 64 kilobits per second (kbit/s/s) voice data traffic, normal speech data samples are converted into one octet of digital data every 125 microseconds (&mgr;s). Thus, 48×125 &mgr;s=6000 &mgr;s are required to fill the 48 data octets of an ATM cell payload. This introduces a 6 millisecond (ms) delay to each cell transmitted, in addition to two network switching delays one from each switch, and transmission delays across the network. For speech services, it is important to have an effectively constant delay between source and destination of a call, and the delay must be reasonably short. Large variations in delay produce broken sound effects, and make voice signals unintelligible to a service user. Long delays, for example those sometimes encountered on transatlantic satellite links, make two-way conversation awkward. In general, a conventionally accepted maximum one-way delay for speech data is 25 ms. Delays longer than this, as well as making the speech service unacceptable to users, also require complicated and expensive echo suppression equipment, which has the additional disadvantage of introducing noise. Thus, the conventional 53 octet ATM cell is not ideal for 64 kbit/s voice data traffic. However, with the advent of mobile telecommunications systems, normal 64 kbit/s sampled voice signals are compressed using code compression algorithms, resulting in transmission data rates as low as 4 kbit/s. Under these circumstances, the delay introduced in filling a full ATM cell may be as high as 96 ms, an unacceptably high delay.
In view of the above delays and to accommodate different data traffic types, the ATM adaptation layer (AAL) is split into a number of sub-layers. A first sub-layer, the known AAL-type 1 layer is aimed at constant bit rate services. The currently developing, and not yet finalized AAL-type 2 layer (formerly known in Europe as AAL-type 6, and elsewhere as AAL-CU) allows multiple variable length sub-cells, called mini-cells to be carried within one ATM cell. An object of AAL-type 2 is to support all services which require the multiplexing of information from multiple user data sources into a single ATM connection. The AAL-type 2 protocol, which breaks the basic rule of ATM that all cells be of fixed length, is aimed at being expedient for carrying low speed data where the delay caused by waiting for a full ATM cell to fill is too long, and the overhead of carrying an incomplete ATM cell is too great. However, the implementation of this layer is incomplete and there still remains a requirement for a

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