Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Patent
1997-01-06
1999-07-27
Pham, Chi H.
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
370410, H04J 312
Patent
active
059302330
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a data transmission system with sliding-window flow control on a non-transparent data connection whose nominal data transmission rate may vary during the connection.
BACKGROUND OF THE INVENTION
In non-transparent asynchronous data transmission on a circuit-switched connection, data is transmitted from a transmitting party A to a receiving party B in frames or "packets". Besides actual user data, the frames comprise both error-detecting and error-correcting bits to enable the receiving party to detect and possibly correct transmission errors. Each frame is also numbered or the order of the frames is indicated by means of another kind of identifier. The correctness of each received frame is tested at the receiving end. If the frame is found correct, the receiving party acknowledges receipt by transmitting the respective frame number or other identifier. If a frame is not found correct (e.g. due to a transmission error), it will not be further processed (but is "discarded"). A negative acknowledgement (e.g. a retransmission request) is sent, for example in case of discontinuity in frame numbering. Let us assume that the correct frame numbering is, for example, 1,2,3,4,5. However, if frame 3 is followed by frame 5, frame 4 is missing and a negative acknowledgement will be sent for frame 4. Once the transmitting end receives a negative acknowledgement or no acknowledgement at all in respect to a given frame, it retransmits the frame a predetermined number of times. The total number of repetitions is limited, so that endless transmission loops are avoided in a very bad connection.
On such a connection, user data throughput varies with the quality of the connection. Deterioration in the quality of the connection results in an increase in the number of incorrect and lost frames, and consequently the number of repetitions.
The frames have to be stored (buffered) at the transmitting end until they have been acknowledged, so that they are available should retransmission be needed. To limit the amount of necessary buffering, a flow control protocol based on a sliding window may be used in the acknowledgement. In accordance with the flow control protocol, the transmitting party A may send a plurality of data frames before requiring acknowledgement from the receiving party. A window represents a sliding sequence of successive frames that have been sent but have not yet been acknowledged (a transmission window). The maximum number of unacknowledged frames equals, window size WS. Party B is also prepared to receive WS frames in a reception window, which is a sliding sequence of successive frames that may be acceptably received. The frames that fit into the respective window but have not arrived in the correct order are gathered into the reception window. Let us assume that frames 1,2,5,6,7 are received. After frames 1 and 2, the window is slid forward, whereas 5, 6 and 7 are stored in the reception window where they wait for the missing frames 3 and 4. Once frames 3 and 4 arrive, the reception window is slid over 3,4,5,6 and 7. When the receiving end acknowledges one or more frames, the reception and transmission windows are slid forward a corresponding number of frames. By means of a sliding window, the nominal data transmission capacity of a transmission channel may be better utilized and a higher throughput may be achieved than in a case when the transmitting end A does not send a new frame until it has received an acknowledgement of the previous frame from the receiving end.
Usually the receiving end B assumes that the frames arrive in the right order, defined by the frame number or other kind of identifier. If a frame is missing, for example frame N, which is deduced from the reception of frame N+M (M>0) immediately after frame N-1, in conventional systems, party B immediately informs party A about the absence of the frame. As a result, party A retransmits frame N and sometimes also starts the transmission sequence from number N even if some later frames had alre
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Ahopelto Juha-Pekka
Kanerva Mikko
Kari Hannu
Vainikka Jari
Nokia Telecommunications Oy
Pham Chi H.
Yao Kwang B.
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