Packet loss tolerant reshaping method

Details Packet loss tolerant reshaping method Packet loss tolerant reshaping method

1999-07-02

2002-12-03

Cangialosi, Salvatore (Department: 2661)

Multiplex communications

Diagnostic testing

Determination of communication parameters

C370S233000

Reexamination Certificate

active

06490254

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of telecommunications, and specifically to reshaping streams of data packets or cells in a communications network.
2. Background Information
FIG. 7
shows a DLC (Data Link Control) layer and other protocol layers in an exemplary conventional system having a laptop computer communicating with a server via a wireless link between the laptop computer and an AP (Access Point), and an Internet connection between the server and an IP router connected to the AP.
In a WLAN (Wireless Local Area Network) that has a DLC (Data Link Control) layer and uses a TDMA (Time Division Multiple Access) TDD (Time Division Duplexing) frame structure and a packet-oriented delivery technique, the DLC layer can clump or bunch together data packets or cells in a stream of packets, and thus alter a timing of the stream. For example, the clumping or bunching changes time intervals between adjacent packets or cells in a stream. Most real time applications are designed for a fixed network, where packets are expected to arrive with a fairly constant rate that is lower than, or equal to, a peak PR (Packet Rate). To ensure that such real time applications work properly in the WLAN, a need exists for a reshaper to compensate for the clumping introduced by the DLC layer. The clumping can be compensated for by, for example, restoring a time interval or distance between consecutive packets in the stream before delivering the packets to the network layer.
A fixed ATM (Asynchronous Transfer Mode) network and a prototype WATM (Wireless ATM) network are examples of networks that employ reshapers. Conceptually, any packet-and connection-oriented network that supports real-time connections may need reshapers.
In a fixed ATM network, reshaping is performed to restore traffic characteristics of connections. Connection traffic characteristics can generally be expressed in the form of a PCR (Peak Cell Rate), an MCR (Mean Cell Rate) and a CDV (Cell Delay Variation), or variation in a time interval between adjacent cells or packets in a stream or sequence. Due to multiplexing in the ATM network, the traffic characteristics of a connection can be distorted. For instance, ATM cells or packets can be clumped together in time, resulting in a PCR that is too high and/or a CDV that is too large.
In most situations an ATM reshaper is used on interfaces between ATM networks that are managed by different operators, or in other words on inter-carrier interfaces. ATM reshapers can also optionally be used on other interfaces.
An alternative term for an ATM reshaper is “cell spacer”, which describes the function of the reshaper. The ATM reshaper ensures that an amount of time between two consecutive cells or packets is always equal to, or longer, than an amount of time 1/PCR. For a VBR (Variable Bit Rate) connection, the ATM reshaper also monitors the MCR (Mean Cell Rate), but for the sake of simplicity let us consider a CBR (Constant Bit Rate) connection.
FIG. 1
shows an example where an ATM reshaper
105
is used in an ATM network
102
that provides a CBR transport service between first and second end terminals
101
and
104
. The first end terminal
101
is a source of cells or packets, and the second end terminal
103
is a sink or destination for the cells or packets sourced. by the first end terminal
101
. As shown in
FIG. 1
, the first end terminal
101
provides a stream of cells or packets to a first portion
104
of the ATM network. The first portion
104
of the ATM network processes the cells or packets and then forwards them to the ATM reshaper
105
, but with an altered time spacing or time interval between the cells or packets. The reshaper receives the altered stream of cells or packets from the first portion
104
of the ATM network, and “reshapes” or restores the proper time intervals between the cells or packets before forwarding them to the second end terminal
103
.
FIG. 2
further illustrates how the ATM reshaper
105
supports the CBR connection. The CBR source or end terminal
101
forwards ATM cells or packets to the ATM network
102
with an agreed upon PCR, +/− some accepted CDV. As mentioned above, when ATM cells or packets are processed in an ATM network a time spacing or interval between the cells or packets can be altered, so that the ATM cells or packets become clumped together in time, even so close that ATM cells are sent out from the ATM network back to back with no time interval between them. This can occur, for example, due to buffering in the ATM network. The reshaper
105
reshapes the output of the ATM network portion
104
to ensure that the traffic characteristics of cells or packets output from the ATM network
102
comply with the ATM traffic contract when the cells or packets are forwarded to the CBR sink, or second end terminal
103
. In accordance with the traffic contract, the output of the reshaper is allowed to have some CDV. This means that the time between two consecutive cells is allowed to be slightly less than T seconds, where T=1/PCR.
FIG. 2
shows a very fortunate case, where the ATM reshaper
105
perfectly manages to restore the time intervals of the original cell stream as output by the first end terminal
101
or CBR source.
As shown in
FIG. 2
, the output S
1
of the first end terminal
101
or CBR source features cells or packets P
1
-P
7
, evenly spaced in time by a time interval T=1/PCR. When this stream of cells is output from the first portion
104
of the ATM network
102
as the output S
2
, it has a different spacing. For example, as can be seen in
FIG. 2
, P
2
is located very close to P
3
, P
6
is located very close to P
7
, and the interval between P
4
and P
6
is significantly larger than T. The output stream S
2
from the first portion
104
of the ATM network
102
is provided to the ATM reshaper
105
, which reshapes the stream to have the original spacing of S
1
, and outputs the reshaped stream as the output S
3
. The output S
3
has a time spacing that is equal to T=1/PCR, within the allowed CDV.
When the connection supported by the ATM network
102
is a VBR connection, the reshaper
105
also ensures that a mean bandwidth of the connection also complies with the ATM traffic contract. In other words, the smallest time interval between two consecutive cells or packets will still be 1/PCR (plus or minus an allowed CDV), but the time interval can also be much longer when necessary to prevent the mean bandwidth from being exceeded.
Reshaping can also be necessary in a WATM network. The DLC framing and the ARQ (Automatic Repeat Request) function in a WATM network will cause clumping of packets when the WATM network transports cells or packets to an ATM application. Since the ATM application expects the traffic characteristics of the connection to be in accordance with an ATM traffic contract governing the connection the WATM network is supporting, this clumping needs to be compensated for. By including a reshaper in a terminal through which the WATM network communicates with the ATM application, packet clumping introduced by the DLC Layer in the WATM network when transporting packets in downlink to the terminal can be smoothed out or reshaped, before the packets are forwarded to the ATM application.
A reshaper may also be required in a BS (Base Station) or AP (Access Point) to the WATM network, in order to smooth out the packet clumping introduced by the DLC layer in uplink. If the WATM network enforces the connections it is supporting with a UPC (Usage Parameter Control) function, then the BS will need to reshape streams of cells or packets being transported by and through the connections, because the WATM network will discard packets that violate the traffic contract. In addition, cell or packet streams being transported by uplink connections are not reshaped, it might be difficult for the CAC (Connection Admission Control) algorithm in the WATM network to statistically estimate the traffic load in the network, since the D

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