Congestion control of AAL2 connections

Details Congestion control of AAL2 connections Congestion control of AAL2 connections

2000-01-03

2004-03-23

Chin, Wellington (Department: 2731)

Multiplex communications

Data flow congestion prevention or control

C370S428000, C370S429000, C370S235000, C370S468000, C370S395210, C370S395610, C370S395630, C370S395640, C370S395650, C709S235000

Reexamination Certificate

active

06711126

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the field of data transfer in the form of asynchronous packets in radio or cable communication networks. When packets are transferred asynchronously, there is a risk of congestion at the network nodes if the instantaneous flowrate of incoming packets exceeds the maximum throughput capacity of the multiplex. In this context, the invention concerns more specifically—but not exclusively—a protocol for managing such congestions in buffers which multiplex network connections known as AAL2 (ATM (asynchronous transfer mode) adaptation layer 2) connections.
AAL2 is used amongst others in UTRAN (UMTS (universal mobile telecommunication system) terrestrial radio access network). UTRAN is the terrestrial radio access network for UMTS. UMTS constitutes the third generation mobile system (3GPP) defined by the standardisation organisation.
The AAL2 protocol and its implementation in data transmission systems are well established and are now standardised for example in the publication by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), Recommendation I366.1 (June 1998) entitled “Series I: Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2″ and, also from ITU-T, recommendation I.363.2 entitled “B-ISDN ATM Adaptation layer type 2 Specification” whose contents are considered as included in the state of the art.
FIG. 1
is a schematic diagram showing an example of part of a communication network linking a core network (CN) to two controllers (RNCs) which are themselves connected to base stations referred to as Node B. By analogy, a Node B is equivalent to a base transceiver station (BTS) in the GSM system. The Node B and RNC are connected together by an interface known as Iub (interface UMTS bis, equivalent to the Abis interface of the GSM). The RNC is connected to the core network CN via an interface Iu (UMTS interface).
The RNCs are connected together by an interface Iur (UMTS interface between two RNCs). These interfaces are based on ATM. ATM is used as a transport layer for the Iu, Iur and Iub interfaces.
Interfaces Iur and Iub receive radio frames or multiple radio frames. At radio level, frames have a 10 ms duration. The time interval between frame transmissions on the Iub and Ur interfaces is typically 10 ms, 20 ms, 40 ms or 80 ms (respectively equivalent to 1, 2, 4 or 8 radio frames).
UMTS serves to transport data and voice, and possibly several other types of service, some of which can have a baud rate of up to 2 Mbits/s.
The transport of data shall be illustrated with an example of a message containing voice at a baud rate of 8 kbits/s. Accordingly, to send frames every 10 ms, 80 bits or 10 octets will be used. Sending voice frames every 20 ms requires 160 bits, or 20 octets (bytes) With a data service at 144 Kbits/s, frames can be formed in groups of four or eight. Transmission time intervals are generally 40 or 80 ms. For 80 ms frames, 1440 octets per frame will be available on the Iub interface. With 40 ms frames, 720 octets will be available per frame on the Iub interface.
The ATM system uses cells of 53 octets (bytes): 5 octets for connection identification information and 48 octets forming the payload.
Information (for example the above-mentioned 10 or 1440 octets) must therefore be transported by ATM cells each having 48 octets available.
To this end, an ATM adaptation layer is used, namely the AAL2. This adaptation layer comprises a number of sub-layers: a service specific segmentation and reassembly sublayer (SSSAR) forming part of SEG-SSCS (optional) which allows data packets exceeding 45 octets to be segmented. Another sublayer, known as the common part sublayer (CPS), allows the SSSAR service data units (SDUs) to be multiplexed into the payload of an ATM cell (the payload of the ATM cell is always 48 octets).
FIG. 2
is a diagram showing the different AAL2 sublayers and the ATM layer which come into play when a frame is sent on the Iub or Iur interface. Right at the bottom of the figure are shown the ATM cells comprising the 48 octet payload plus a 5 octet header.
The figure is based on an example of a 150 octet long AAL2 SDU frame which is shown as the topmost layer in the figure. The service specific convergence sublayer (SSCS) used is the SEG-SSCS (segmentation and reassembly service specific convergence sublayer). In the example, it is reduced to the service specific segmentation and reassembly (SSSAR) sublayer. SSSAR SDU frames thus have the same length as AAL2 SDU frames. This length can change over time and depends on the baud rate, the amount of data to be transported, etc. . . .
Two AAL2 layers shall be of interest in the present case: the mandatory common part sublayer (CPS) and the segmentation and reassembly service specific convergence sublayer (SEG-SSCS), which is an optional segmentation and reassembly layer.
As its name indicates, the SSSAR will segment (and reassemble) the SSSAR SDUs (in the present case 1 AAL2 SDU=1 SSSAR SDU) into packets referred to as SSSAR protocol data units PDUs whose maximum size by default is 45 octets. Consequently, the 150-octet AAL2 SDU of the example can be divided up into three 45-octet SSSAR PDUs and the 15 remaining octets can be loaded into a fourth SSSAR PDU.
The SSSAR PDUs are then transferred to the mandatory common part sublayer (CPS), keeping the condition that one SSSAR PDU corresponds to one CPS SDU as regards the format and in particular the number of constitutive octets.
The CPS layer multiplexes the CPS SDUs it receives on 48 octets that form the ATM cell payload.
To this end, the CPS layer will add a 3 octet header to each CPS SDU it receives for identification of each CPS SDU.
These three octets, which are shown in
FIG. 3
, comprise:
a 5 bit error correction field HEC;
an 8 bit CID field which identifies the AAL2 connection number, i.e. the channel user AAAL2 CPS;
a 6 bit LI field which indicates the payload length minus one of the CPS packet. The maximum length of the CPS packet is 45 octets by default, but this value can be increased to 64 octets by signalling;
a 5 bit UUI (user-to-user interface) field. For SEG SSCS, a CPS-UUI is used to implement a “More Data” bit. A UUI value equal to “27” indicates that more data are required to complete the reassembly of an SSSAR-SDU. A value equal “26” indicates the reception of the last data of an SSSAR SDU. Any other value, i.e. between 0 and 26 should not be used for reasons of compatibility with other SSCS specifications.
Thus, in the case of a segmentation, the UUI field has the value “26” to indicate receipt of an end of SSSAR SDU (150 octets in the example), and the value “27” to indicate that more data follows.
In other words, all the while UUI field yields “27”, the received packets are incomplete and as soon as it yields “26”, the packets can be reassembled and will belong to a given SSSAR SDU.
In a classical installation, if the SSSAR detects errors during reassembly of an SSSAR SDU, the complete CPS SDU must be discarded.
These three octets are found again in the CPS packet header.
In the case of only one CPS PDU, there is also a CPS-PDU start field (STF) octet which serves to identify the position of the first CPS packet inside the payload of the ATM cell or CPS PDU.
A CPS PDU corresponds to the payload of an ATM cell.
The size of CPS PDU is always equal to 48 octets (equal to an ATM cell payload). The first CPS PDU octet is always the STF octet. The 47 remaining octets are either CPS packet octets or padding.
Note that in the case where the CPS SDU is 45 octets, the CPS packet has a size of 45+3=48 octets.
Accordingly, the first 47 octets can be inserted into one CPS PDU. The last remaining octet shall go into another CPS PDU.
Thus upon multiplexing CPS SDUs into CPS PDUs, a straddling is created at the level of the payload. As shown in
FIG. 2
, the 45 payload octets of CPS SDU number
1
are divided on two CPS PDUs, with 44 octets in the first CPS PDU (thus filling up entirely the payload of t

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