Coordinated cell discharge from ATM queue

Details Coordinated cell discharge from ATM queue Coordinated cell discharge from ATM queue

1998-11-09

2003-07-22

Kizou, Hassan (Department: 2662)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S395700, C370S412000

Reexamination Certificate

active

06597698

ABSTRACT:

BACKGROUND
1. Field of the Invention
The present invention pertains to communications systems, and particularly to communications systems which employ ATM technology.
2. Related Art and Other Considerations
Asynchronous Transfer Mode (ATM) is becoming increasingly used in communication networks. ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size.
As shown in
FIG. 1
, an ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a “payload” or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
Between termination points of an ATM network a plurality of nodes are typically situated, such as switching nodes having ports which are connected together by physical transmission paths or links. The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a cells ultimately to travel from an ingress side of the switch to an egress side of the switch.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa. There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5.
AAL2 is a standard defined by ITU recommendation 1.363.2. An AAL2 packet is shown in
FIG. 2
as comprising a three octet packet header, as well as a packet payload. The AAL2 packet header includes an eight bit channel identifier (CID), a six bit length indicator (LI), a five bit User-to-User indicator (UUI), and five bits of header error control (HEC). The AAL2 packet payload, which carries user data, can vary from one to forty-five octets
FIG. 3
shows how plural AAL2 packets can be inserted into a standard ATM cell. In particular,
FIG. 3
shows a first ATM cell
20
1
and a second ATM cell
20
2
. Each ATM cell
20
has a header
22
(e.g., cell
20
1
has header
22
1
and cell
20
2
has header
22
2
). The payload of the ATM cells
20
begin with a start field
24
(e.g., cell
20
1
has start field
24
1
and cell
20
2
has start field
24
2
). After each start field
24
, the ATM cell payload contains AAL2 packets. For example, the payload of ATM cell
20
1
contains AAL2 packets
26
1
and
26
2
in their entirety, as well as a portion of AAL2 packet
26
3
. The payload of cell
20
2
contains the rest of AAL2 packet
26
3
, and AAL2 packets
26
4
and
26
5
in their entirety. In addition, the payload of cell
20
2
has padding
28
.
The start field
24
, shown in
FIG. 3A
, facilitates one AAL2 packet bridging two ATM cells. Start field
24
includes a six bit offset field (OSF), a one bit sequence number (SN), and one parity bit (P). The six bit offset field (OSF) contains a value, represented by offset displacement
29
in
FIG. 3
, indicative of the octet in the payload whereat the first full AAL2 packet begins. For ATM cell
22
1
, the value of the offset field (OSF) is one, since AAL2 packet starts just after start field
24
1
. For ATM cell
22
2
, the value of the offset field (OSF) is the sum of one (in view of start field
24
1
) and the number of octets of AAL2 packet
26
3
protruding into cell
22
2
.
AAL2 advantageously allows multiplexing of data from many users within a single ATM VCC. In such multiplexing scheme, each user's data is carried in a separate AAL2 packet, but AAL2 packets of differing users are carried in the same ATM cells or cells borne on the same ATM VC. Thus, assuming each user has a different channel identifier (CID) value, as many as 248 user channels can be multiplexed onto one ATM VC. AAL2 thus allows more efficient utilization of low speed links than standard ATM while still maintaining low delay properties.
A problem with using AAL2 arises when terminating AAL2 channels at different nodes or different addresses in the same node. Since the individual AAL2 channels may be multiplexed into one ATM-VCC, it is not possible to direct the individual AAL2 channels (e.g., the AAL2 packets on which the channel data is carried) to different destinations using conventional ATM switches.
One approach for switching AAL2 packets is set forth by Mauger and Rosenberg, “QoS Guarantees for Multimedia Services on TDMA-Based Satellite Network”, IEEE Communications Magazine July 1997). In that approach, fixed-cell ATM switches work in conjunction with separate variable-cell ATM switches for handling AAL2 packets.
BRIEF SUMMARY OF THE INVENTION
An ATM switching node has an ATM switch to which at least one inter-node physical link is connected. The ATM switch node has a centralized queuing resource connected to an ATM switch core. The queuing resource provides centralized queuing for ATM cells destined for routing through the ATM switch core to plural output links. Preferably the queuing resource is provided on one or more boards connected to a port(s) of the ATM switch core, the ATM switch core also having plural terminal boards connected between plural ones of its other ports and respective ones of the plural output links.
The queuing resource is part of a cell handling unit. The queuing resource queues ATM cells which are to be routed through the ATM switch for transmission from the node on the physical link. ATM cells destined for the physical link from the queuing resource are discharged from the queuing resource in accordance with a parameter of the physical link. The parameter is preferably the capacity of the physical link.
In the illustrated embodiment, the node is one of the following nodes of a cellular telecommunications network: a base station; a base station controller; a mobile switching center.


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Mau

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