Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1999-08-31
2001-11-06
Jung, Min (Department: 2663)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S252000
Reexamination Certificate
active
06314085
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns control of data transmission connections in broadband networks. The invention especially concerns connection admission in an ATM network junction.
TECHNICAL BACKGROUND
In ATM (ATM=Asynchronous Transfer Mode) information is transferred in cells consisting of headers of 5 bytes each and of an information part of 48 bytes. Header fields are e.g. VPI (Virtual Path Indicator) and VCI (Virtual Channel Indicator). The standard size of cells allows quick connecting of the cells. Cell channelling is asynchronous and time-divided. Before the data transfer step proper, a virtual connection is formed through the network, and the cells generated by the user are routed through this virtual connection. Upon completion of the data transfer, disconnection is performed.
A virtual bus level connection and a virtual channel level connection can be distinguished in the network. In the VP (Virtual Path) connection virtual buses are connected between transfer connections. On a virtual bus e.g. a direct connection may be formed between two exchanges, even though the connection passes through other exchanges. The virtual bus contains several virtual channels and in the VC (Virtual Channel) connection virtual channels are connected between virtual buses.
Connection Admission Control or CAC is a set of procedures intended to limit the load caused by sources wishing to enter the network and the load of internal connections in such a way that a guaranteed QOS (Quality of Service) is preserved for each existing connection. The equivalent capacity required by the virtual bus or physical connection must be defined computationally in the connection admission control. If all traffic descriptors of a requested connection are known, the only duty is to determine how much standby capacity there is in the virtual path or physical connection for admission of a new connection without any resulting loss for existing connections.
It is agreed at present that a traffic source requesting connection shall always state at least its PCR (Peak Cell Rate), its SCR (Sustainable Cell Rate), its BT (Burst Tolerance) and the CDVT (Cell Delay Variation Tolerance). In addition, the probability P(on) with which it transmits can be estimated.
However, it is often the case that the “band width” or capacity required by the requested connection is not standard and only depending on the traffic descriptors of this concerned connection, but it depends on the character and volume of the traffic of other virtual channels, which channels share the same virtual bus and physical link. Besides, it is next to impossible to get to know the exact traffic descriptors of sources: it is difficult to characterise the traffic brought about by a work station working in a multiprocessor environment and how the traffic varies with the application.
The equivalent capacity of a virtual channel is that lowest capacity which is needed to ensure QOS for all virtual connections sharing the same virtual bus, should a new request be admitted. Since the equivalent capacity depends on the traffic of all connections, it is difficult to compute the necessary standby capacity and to determine whether there is sufficient standby capacity or not.
FIGS. 1A and 1B
are used to illustrate how difficult it is to determine equivalent capacity. It is assumed that each traffic source has the same traffic descriptors and they transmit cells in bursts: in the “on” state, the source generates information at standard bit rate R bit/s, while in the “off” state it does not generate information, top part of the figure. ATM cells are generated from the information when the source is in the “on” state, bottom part of
FIG. 1A
, so in this state the rate of ATM cells is the said R bit/s. At the ATM access gate the reception rate of cells is S. Thus, the continuous flow of cells generated by the source in the “on” state is seen at the access gate of the network as a quasi periodic sequence of ATM cells while the average time of arrival is one cell for each S/R time slot. In the off state no cells will arrive.
The following is a description of multiplexing of N different sources into a common virtual bus, FIG.
1
B. The input gate has a buffer
11
, which stores such cells in a queue which can not be forwarded at once to the output link having a finite capacity C. If the buffer size is infinite, then the minimum rate of the output link must be equal to the mean rate of cells arriving from N sources, which is N×R×P(on), wherein P(on) is the probability of cells being transmitted by the source. If the buffer is very small, it is safest to use a top rate as the effective band for the source, which would hereby also be an unconditional top limit for the reserved capacity. Thus, the equivalent capacity required for multiplexing N sources at the same time maintaining only a small buffer overflow possibility, N×R (equivalent capacity is N times the top rate of the source). It follows from these, that when using a buffer of a moderate size the equivalent capacity needed for multiplexing N sources is somewhere in a range between N×R×P(on) and N×R.
It is very difficult to determine the equivalent capacity when traffic of different types and having a complicated source model is multiplexed into the same virtual channel.
The connections needed by many traffic sources are bursts, whereby the transfer capacity required at some moment is high while the capacity required at another moment is small. Bursts may be described as a phenomenon where a set of cells (a burst) arrives at short intervals and the following set (burst) arrives after a relatively long time. Since different connections need different capacities and the need varies quickly, statistic channelling is used in an ATM network. The statistic characteristics of burst-like traffic sources is utilised in the channelling: when combining a large set of traffic sources the combined traffic will behave in a more stable manner than individual sources and although the transmission speed of individual sources varies, the combined transmission speed of mutually independent individual sources is almost standard. By using statistic channelling it is possible with the same resources to serve more connections, that is, the utilisation rate of the network can be raised.
In spite of statistic channelling, congestion and overloads will occur in the network. Congestion in a broadband network means such a state of network elements, wherein the network is unable to fulfil required performance objectives. Overload again means a situation where performance objectives can still be achieved even though the performance has decreased. Congestion and overload are caused both by unpredictable statistic variations in the traffic and by failure situations occurring in the network. Since it is still impossible to know with sufficient exactness network services, the volume of traffic brought about by them and the exact characteristics of traffic sources, the occurrence of congestion situations is unavoidable in the network. The purpose of traffic control and congestion situation control is to protect the network and the user so that the desired quality of network service is achieved. Traffic control exists when the activity is preventive and it is intended to prevent the occurrence of congestion situations. Control of congestion situations for its part reacts to congestion situations observed in the network. Most significant from the viewpoint of service quality are such traffic control functions which will prevent beforehand congestion situations from occurring. The CAC (Connection Admission Control) belonging to these functions is the most important preventive traffic control method. Functions relating to connection admission control attend to routing of connections, they make decisions on connection admission, they reserve the necessary resources and they set traffic parameters monitored by UPC (Usage Parameter Control) and by NPC (Network Parameter Control). The simplified result
Altera Law Group LLC
Jung Min
Nokia Telecommunications Oy
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