Extended AAL2 connection identifier

Details Extended AAL2 connection identifier Extended AAL2 connection identifier

1999-02-09

2003-07-22

Kizou, Hassan (Department: 2662)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S474000, C370S539000

Reexamination Certificate

active

06597697

ABSTRACT:

BACKGROUND
The present invention relates to the transmission of telecommunications data, and more particularly to a method of increasing the data transmitting capacity using the minicells of an asynchronous transmission mode (ATM).
ATM is a standard protocol for transmitting asynchronous telecommunications data. This protocol is based on the transmission of data in fixed size data packets known as ATM cells. Each ATM cell exhibits a singular format comprising a 48 octet payload portion and a 5 octet header portion. ATM is well known in the art.
Unfortunately, ATM does not efficiently transport low bit rate data as the length of a typical low bit rate data packet is significantly less than 48 octets (i.e., the length of an ATM cell payload). Any unused portion of an ATM cell payload is filled with “padding bits”. When padding bits are inserted rather than data, bandwidth is wasted. The insertion of padding bits may also result in unacceptable transmission delays, which may be detrimental, especially when the data being transported is highly sensitive to delays, such as voice-type data.
An ATM adaptation layer, known as AAL
2
, has been developed for the purpose of improving the efficiency of ATM when employed to transport low bit rate data. Referring to
FIG. 1
, AAL
2
operates by storing low bit rate data in small, variable length data packets called minicells
111
(sometimes referred to as microcells or short packets). An improvement in bandwidth utilization is achieved by inserting several minicells into the payload of a single ATM cell, such as ATM cell
121
. In order to further improve bandwidth utilization, a minicell, for example minicell
131
, may be segmented so that it overlaps two ATM cells as illustrated.
More recently, a new standard for carrying compressed voice on Asynchronous Transfer Mode (ATM), Recommendation I.363.2 (hereinafter I.363.2), has been approved by the International Telecommunication Union (ITU). This allows up to 255 connections to be simultaneously multiplexed on an ATM Virtual Channel Connection (VCC). The eight bit connection identifier (CID) field in the ATM Adaptation Layer type
2
Common Part Sublayer (AAL
2
-CPS) packet is used for this purpose. If more than these 255 AAL
2
connections are desired, a new ATM-VCC is needed.
FIG. 2
illustrates an AAL
2
-CPS packet according to I.363.2. This packet is made up of a connection identifier (CID) field
211
, a length indicator (LI) field
221
, a user to user indication (UUI) field
231
, a header error control (HEC) field
241
and a payload field
251
.
The CID field
211
is eight bits in length, allowing up to 255 connections ranging from CID-
1
to CID-
255
. Typically, however, only up to 248 connections are utilized. These range from CID-
8
to CID-
255
. CID-
0
is reserved for padding, i.e., if the next octet after the last octet in a previous AAL
2
-CPS packet is zero, then the remainder of the ATM cell is empty. In other words, if the octet where a new AAL
2
-CPS packet is supposed to start is zero, then the remaining octets in the ATM cell is filled with zeroes and is considered to be padding. The receiver, when it detects a zero octet where a new AAL
2
-CPS packet is supposed to start, disregards the remainder of the ATM cell. The LI field
221
is five bits in length and indicates the number of octets in the payload. It ranges from LI-
0
to LI-
44
which corresponds to payloads of 1 to 45 octets. The UUI field
231
is also five bits in length and is transparently conveyed from one end user to the other end user. That is, the user may or may not be aware of this activity; in this case, the user may or may not be aware of the UUI field being conveyed. It may be considered as a field in which the user may place any type of information as long as that information is not placed in the range of UUI-
26
to UUI-
31
which are reserved for segmentation and OAM usage. By limiting the segmentation and OAM to bits
26
to
31
, CID expansion is facilitated according to exemplary embodiments of the present invention. The HEC field
241
, also five bits in length, may be used for verifying the integrity of the AAL
2
-CPS packet header.
This particular number of CIDs (i.e.,
255
) results from the fact that the ATM multiplexing capability can be used to increase the number of AAL
2
connections. However, the ATM cell header in every cell takes about 10% of the bandwidth. Therefore, for every 48 octets of payload, 5 octets are inserted. In instances where bandwidth is extremely expensive, it is economically beneficial if the AAL
2
-CPS packets are placed directly on the E
1
or T
1
time-division multiplex (TDM) lines. The number of AAL
2
connections that can be indicated by the CID, however, will not be sufficient in such cases.
FIG. 3
illustrates a conventional AAL
2
multiplexing technique with added resilience against loss of delineation in the form of a start octet.
The basic delineation is provided by the fixed size ATM cells
301
,
302
and
303
. The fact that the ATM cells come “back to back” every 53 octets makes it easy to use a receiver state machine that takes this into account. The ATM header
311
of 5 octets contains a HEC field
321
that makes it possible for the receiver to check the integrity of the ATM cell header. If the HEC matches, it is highly probable that the receiver will find the header and if the next header matches, the probability increases even further. Under normal practice, if six headers in a sequence match, the receiver is considered to be synchronized. Furthermore, due to the 53 octet length, the state machine does not have to leave the sync state at a first error in the ATM cell header. If it is repeated a predetermined number of times (e.g., six times), however, it is considered to no longer be in the synchronized state. The same technique is more difficult to apply to the AAL
2
demultiplexing since the AAL
2
-CPS packets
391
to
397
may have variable sizes. The length indicator
351
provided in the header of each AAL
2
-CPS packet is used to locate the start of the next AAL
2
-CPS packet. The entire AAL
2
-CPS packet header is protected by a HEC that is similar to the one for the ATM cell. This ensures that the integrity of LI can be checked.
In addition, an offset field
323
of six binary coded bits is inserted as a first octet in the payload of every ATM cell. The offset field contains a pointer that makes it possible to find the first AAL
2
-CPS packet, at every new ATM cell, regardless of the length indicator value
351
. The pointer is encapsulated in a start octet
331
. The start octet
331
also includes a sequence number bit
325
, working as a modulo-
2
counter, making it possible to detect if ATM cells have been lost, single or odd. The start octet is protected by a parity bit
327
. If no remaining AAL
2
-CPS packets exist to fill an ATM cell, the remainder is padded by inserting a zero in every octet to the end of the ATM cell.
With eight bits allocated to the CID, the indication provided by the CID as well as the number of connections simultaneously multiplexed on an ATM-VCC is limited to 255 although, typically, only 248 connections are used. What is desired, therefore, is a method for increasing the number of multiplexed connections.
SUMMARY
It is, therefore, an object of the invention to overcome the aforementioned limitations of conventional AAL
2
connections for transmitting telecommunications information.
This is achieved by multiplexing a plurality ATM-VCCs into a single channel. Multiplexing a plurality of AAL
2
minicells within an ATM into a VCC is known. The eight bits of the CID field of the minicell are used to identify up to 255 of these connections which have been multiplexed into one VCC. Each VCC containing AAL
2
minicells is identified by a virtual channel indicator (VCI). In order to identify a plurality of ATM-VCCs that are multiplexed into a single channel, the five bits of the UUI field of the minicell are utilized to identify up to twenty five such multiplexed VCCs.
The CID field is used as an indica

Affiliated with

Also associated with

Rating

Extended AAL2 connection identifier does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Extended AAL2 connection identifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Extended AAL2 connection identifier will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3103310