Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-03-30
2003-07-29
Jung, Min (Department: 2663)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S395640, C370S442000, C370S522000
Reexamination Certificate
active
06600746
ABSTRACT:
BACKGROUND
The present invention relates to the transmission of telecommunications data, and more particularly to a novel method of transmitting data on fractional time slots.
Asynchronous Transfer Mode (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 five octet header portion. ATM is well known.
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 AAL2, has been developed for the purposes of carrying compressed voice data on ATM and for improving the efficiency of ATM when employed to transport low bit rate data according to Recommendation I.363.2 (hereinafter “I.363.2”) which has been approved by the International Telecommunications Union (ITU). Referring to
FIG. 1
, AAL2 operates by storing low bit rate data in small, variable length data packets called minicells, for example, minicells 191-197, which are 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
101
. To further improve bandwidth utilization, a minicell, for example minicell
193
, may be segmented so that it overlaps two ATM cells
101
and
102
as illustrated.
Under this standard, ATM is needed as an underlying bearer and is defined to be carried on the E1 line in 30 time slots (
1
-
15
and
17
-
31
) and on the T1 line on all 24 time slots. These standards are according to ITU-T Recommendation G.804.
In some cases, using the whole T1 or E1 can be prohibitively expensive. There are an insufficient number of AAL2 connections to utilize the entire bandwidth. In these cases, fractional T1 or E1 may be utilized. Fractional implies that a number of time slots are concatenated to form a channel, e.g., six concatenated time slots comprising 384 Kbps of bandwidth is quite common. The cost per bit of transported information increases as the amount of bandwidth that is utilized decreases. For instance, using a fractional E1 or T1 with ATM and AAL2 results in a bandwidth penalty of about 10%.
FIG. 1
illustrates a conventional AAL2 multiplexing technique with added resilience against loss of delineation in the form of a start octet.
The basic delineation between cells is provided by fixed size ATM cells
101
,
102
and
103
. 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. An ATM header
111
of five octets contains a header error control (HEC) field
121
that makes it possible for the receiver to check the integrity of the ATM cell header. Under normal practice, if six ATM cell headers in a sequence are received without any errors, 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 the error is repeated a predetermined number of times such as, for example, six times, however, it is considered to no longer be in the synchronized state. The same technique is more difficult to apply to the AAL2 demultiplexing since the minicells
191
to
197
can have variable sizes. A length indicator (LI) field
151
provided in the header of each minicell is used to find the start of the next minicell. The entire minicell is protected by a HEC that is similar to the HEC for the ATM cell. This ensures that the integrity of LI can be checked.
In addition, an offset field
123
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 minicell, at every new ATM cell, regardless of the LI value
151
. The pointer is encapsulated in a start octet
119
. The start octet
119
also includes a sequence number bit
125
, working as a modulo-2 counter, making it possible to detect if ATM cells have been lost or if there is only a single cell. The start octet
119
is protected by a parity bit
127
. If no remaining minicells exist to fill an ATM cell, the remainder of the cell is padded by inserting a zero in every octet to the end of the ATM cell.
FIG. 2
illustrates a minicell according to I.363.2. This packet is made up of a connection identifier (CID) field
205
, a length indicator (LI) field
211
, a user to user indication (UUI) field
215
, a header error control (HEC) field
221
and a payload field
251
.
The CID field
205
is eight bits in length allowing up to 255 connections ranging from CID
1
, to CID
255
. CID
0
is reserved for padding, i.e., if the next octet after the last octet in a previous minicell is zero, then the remainder of the ATM cell is empty. In other words, if the octet where a new minicell is supposed to start is zero, then the remaining octets in the ATM cell are filled with zeroes which is considered to be padding. The receiver, when it detects a zero octet where a new minicell is supposed to start, disregards the remainder of the ATM cell. The LI field
211
is six 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 one to 45 octets. The UUI field
215
is also five bits in length and is transparently conveyed from one end user to the other end user. Transparency, in this context, means that the user may or may not be aware of this activity, in this case, 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. The HEC field
221
, also five bits in length, may be used to verify the integrity of the minicell header.
A copending application, Ser. No. 08/982,425 for “Simultaneous Voice And Data” of Petersen et al., discloses delineation without the support of the underlying ATM connection. The subject matter of this application is hereby incorporated by reference. The described method of delineation, however, is only sufficient for a limited number of active connections and a low bandwidth line, of typically less than 64 Kbps. For larger bandwidths associated with fractional T1/E1, this method of delineation is not resilient as a considerable amount of time is needed to achieve resynchronization. In some instances, this time period may even be indefinite. The use of the disclosed method of delineation is appropriate in a private access line with few users. For public channels with many users expecting a certain quality, however, the time needed to achieve resynchronization is unacceptable. What is needed is another method for achieving re-synchronization in a shorter period of time. This may be accomplished by adapting the AAL2 packets to work directly on fractional time slots.
FIGS. 3
a
and
3
b
illustrate a conventional synchronous frame with T1 time slot structure of 1544 Kbps and E1 time slot structure of 2048 Kbps respectively. The T1 channel structure is divided into 24 consecutive time slots
305
preceded by a F-bit
307
. The time frame is repeated every 125 microseconds. The F-bit
307
is used to indicate, among other things, the start of the frame and multi-frame. The multi-frame structure
309
is repeated after 24 frames.
The E1 channel structure has a similar channel structure based on 32 consecutive time slots
315
. TS
0
is u
Jenkens & Gilchrist P.C.
Jung Min
Telefonaktiebolaget LM Ericsson
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