#)A variety of traffic control functions have been defined to maintain the QOS of ATM connections.
#)These include
1)Network resource management
2)Connection admission control
3)Usage parameter control
4)Priority control
5)Fast resource management
#)We examine each of these in turn.
1)Network Resource Management
3)The essential concept behind network resource management is to allocate network resources in such a way as to separate traffic flows according to service characteristics.
#)So far, the only specific traffic control function based on network resource management deals with the use of virtual paths.
#)A virtual path connection (VPC) provides a convenient means of grouping similar virtual channel connections (VCCs).
#)The network provides aggregate capacity and performance characteristics on the virtual path, and these are shared by the virtual connections.
#)There are three cases to consider:
i)User-to-user application:
*)The VPC extends between a pair of UNIs.
*)In this case, the network has no knowledge of the QOS of the individual VCCs within a VPC.
*)It is the user's responsibility to assure that the aggregate demand from the VCCs can be accommodated by the VPC.
User-to-network application:
*)The VPC extends between a UNI and a network node.
*)In this case, the network is aware of the QOS of the VCCs within the VPC and has to accommodate them.
Network-to-network application:
*)The VPC extends between two network nodes. Again, in this case, the network is aware of the QOS of the VCCs within the VPC and has to accommodate them.
#)The QOS parameters that are of primary concern for network resource management are cell loss ratio, cell transfer delay, and cell delay variation, all of which are affected by the number of resources devoted to the VPC by the network.
#)If a VCC extends through multiple VPCs, then the performance on that VCC depends on the performances of the consecutive VPCs, and on how the connection is handled at any node that performs VCC-related functions.
#)Such a node may be a switch, concentrator, or other network equipment.
#)The performance of each VPC depends on the capacity of that VPC and the traffic characteristics of the VCCs contained within the VPC.
#)The performance of each VCC-related function depends on the switching/processing speed at the node and on the relative priority with which various cells are handled.
#)The diagram below gives an example. VCCs 1 and 2 experience a performance that depends on VPCs b and c and on how these VCCs are handled by the intermediate nodes; this may differ from the performance experienced by VCCs 3,4, and 5.
#)There are a number of alternatives for the way in which VCCs are grouped and the type of performance they experience.
#)If all of the VCCs within a VPC are handled similarly, then they should experience similar expected network performance, in terms of cell-loss ratio, cell-transfer delay, and cell-delay variation.
#)Alternatively, when different VCCs within the same VPC require different QOS, the VPC performance objective agreed upon by network and subscriber should be suitably set for the most demanding VCC requirement.
#)In either case, with multiple VCCs within the same VPC, the network has two general options for allocating capacity to the VPC:
1. Aggregate peak demand:
#) The network may set the capacity (data rate) of the VPC equal to the total of the peak data rates of all of the VCCs within the VPC.
#)The advantage of this approach is that each VCC can be given a QOS that accommodates its peak demand.
#)The disadvantage is that most of the time, the VPC capacity will not be fully utilized, and, therefore, the network will have underutilized resources.
2. Statistical multiplexing:
#)If the network sets the capacity of the VPC to be greater than or equal to the average data rates of all the VCCs but less than the aggregate peak demand, then a statistical multiplexing service is supplied.
#)With statistical multiplexing, VCCs experience greater cell-delay variation and greater cell-transfer delay.
#)Depending on the size of buffers used to queue cells for transmission, VCCs may also experience greater cell-loss ratio.
#)This approach has the advantage of more efficient utilization of capacity, and is attractive if the VCCs can tolerate the lower QOS.
#)When statistical multiplexing is used, it is preferable to group VCCs into VPCs on the basis of similar traffic characteristics and similar QOS requirements.
#)If dissimilar VCCs share the same VPC and statistical multiplexing is used, it is difficult to provide fair access to both high-demand and low-demand traffic streams.
2)Connection Admission Control
#)Connection admission control is the first line of defense for the network in protecting itself from excessive loads.
#)In essence, when a user requests a new VPC or VCC, the user must specify (implicitly or explicitly) the traffic characteristics in both directions for that connection.
#)The user selects traffic characteristics by selecting a QOS from among the QOS classes that the network provides.
#)The network accepts the connection only if it can commit the resources necessary to support that traffic level while at the same time maintaining the agreed-upon QOS of existing connections.
#)By accepting the connection, the network forms a traffic contract with the user.
#)Once the connection is accepted, the network continues to provide the agreedupon QOS as long as the user complies with the traffic contract.
#)For the current specification, the traffic contract consists of the four parameters defined in the table below: peak cell rate (PCR), cell-delay variation (CDV), sustainable cell rate (SCR), and burst tolerance.
#)Only the first two parameters are relevant for a constant bit rate (CBR) source; all four parameters may be used for variable bit rate (VBR) sources.
#)As the name suggests, the peak cell rate is the maximum rate at which cells are generated by the source on this connection.
#)However, we need to take into account the cell-delay variation.
#)Although a source may be generating cells at a constant peak rate, cell-delay variations introduced by various factors will affect the timing, causing cells to clump up and gaps to occur.
#)Thus, a source may temporarily exceed the peak cell rate due to clumping. For the network to properly allocate resources to this connection, it must know not only the peak cell rate but also the CDV.
#)The exact relationship between peak cell rate and CDV depends on the operational definitions of these two terms.
#)The standards provide these definitions in terms of a cell rate algorithm.
#)Because this algorithm can be used for usage parameter control, we defer a discussion until the next subsection.
#)The PCR and CDV must be specified for every connection.
#)As an option for variable-bit rate sources, the user may also specify a sustainable cell rate and burst tolerance.
#)These parameters are analogous to PCR and CDV, respectively, but apply to an average rate of cell generation rather than to a peak rate.
#)The user can describe the future flow of cells in greater detail by using the SCR and burst tolerance as well as the PCR and CDV.
#)With this additional information, the network may be able to more efficiently utilize the network resources.
#)For example, if a number of VCCs are statistically multiplexed over a VPC, knowledge of both average and peak cell rates enables the network to allocate buffers of sufficient size to handle the traffic efficiently without cell loss.
#)For a given connection (VPC or VCC), the four traffic parameters may be specified in several ways, as illustrated below.
#)Parameter values may be implicitly defined by default rules set by the network operator.
#)In this case, all connections are assigned the same values or all connections of a given class are assigned the same values for that class.
#)The network operator may also associate parameter values with a given subscriber and assign these at the time of subscription.
#)Finally, parameter values tailored to a particular connection may be assigned at connection time.
#)In the case of a permanent virtual connection, these values are assigned by the network when the connection is set up.
#)For a switched virtual connection, the parameters are negotiated between the user and the network via a signaling protocol.
#)Another aspect of quality of service that may be requested or assigned for a connection is cell-loss priority.
#)A user may request two levels of cell-loss priority for an ATM connection; the priority of an individual cell is indicated by the user through the CLP bit in the cell header.
#)When two priority levels are used, the traffic parameters for both cell flows must be specified; typically, this is done by specifying a set of traffic parameters for high-priority traffic (CLP = 0) and a set of traffic parameters for all traffic (CLP = 0 or 1). Based on this breakdown, the network may be able to allocate resources more efficiently.
3)Usage Parameter Control
#)Once a connection has been accepted by the Connection Admission Control func-tion, the Usage Parameter Control (UPC) function of the network monitors the connection to determine whether the traffic conforms to the traffic contract.
#)The main purpose of Usage Parameter Control is to protect network resources from an overload on one connection that would adversely affect the QOS on other connections by detecting violations of assigned parameters and taking appropriate actions.
#)Usage parameter control can be done at both the virtual path and virtual channel levels.
#)Of these, the more important is VPC-level control, as network resources are, in general, initially allocated on the basis of virtual paths, with the vir- tual path capacity shared among the member virtual channels.
#)There are two separate functions encompassed by usage parameter control:
- Control of peak cell rate and the associated cell-delay variation (CDV)
- Control of sustainable cell rate and the associated burst toleranceLet us first consider the peak cell rate and the associated cell-delay variation.
#)In simple terms, a traffic flow is compliant if the peak rate of cell transmission does not exceed the agreed-upon peak cell rate, subject to the possibility of cell-delay variation within the agreed-upon bound.
#)I.371 defines an algorithm, the peak cellrate algorithm, that monitors compliance.
#)The algorithm operates on the basis of two parameters: a peak cell-rate R and a CDV tolerance limit of T.
#)Then, T = 1/R is the interarrival time between cells if there were no CDV. With CDV, T is the average interarrival time at the peak rate.
#)The algorithm uses a form of leakybucket mechanism to monitor the rate at which cells arrive in order to assure that the interarrival time is not too short to cause the flow to exceed the peak cell rate by an amount greater than the tolerance limit.
#)The same algorithm, with different parameters can be used to monitor the sus-tainable cell rate and the associated burst tolerance.
#)In this case, the parameters are the sustainable cell-rate R, and a burst tolerance T,.
#)The cell-rate algorithm is rather complex: details can be found in [STAL95a].
#)The cell-rate algorithm simply defines a way to monitor compliance with the trafficcontract.
#)To perform usage parameter control, the network must act on the results of the algorithm.
#)The simplest strategy passes along compliant cells and discards noncompliant cells at the point of the UPC function.
#)At the network's option, cell tagging may also be used for noncompliant cells.
#)In this case, a noncompliant cell may be tagged with CLP = 1 (low priority) and passed. Such cells are then subject to discard at a later point in the network.
#)If the user has negotiated two levels of cell-loss priority for a network, then the situation is more complex.
#)Recall that the user may negotiate a traffic contract for high-priority traffic (CLP = 0) and a separate contract for aggregate traffic (CLP 0 or 1).
#)The following rules apply:
1) A cell with CLP = 0 that conforms to the traffic contract for CLP = 0 passes.
2) A cell with CLP = 0 that is noncompliant for (CLP = 0) traffic but compliant for (CLP 0 or 1) traffic is tagged and passed.
3) A cell with CLP = 0 that is noncompliant for (CLP = 0) traffic and noncompliant for (CLP 0 or 1) traffic is discarded.
4) A cell with CLP = 1 that is compliant for (CLP = 1) traffic is passed.
5) A cell with CLP = 1 that is noncompliant for (CLP 0 or 1) traffic is discarded.
4)Priority Control
#)Priority control comes into play when the network, at some point beyond the UPC function, discards (CLP = 1) cells.
#)The objective is to discard lower priority cells in order to protect the performance for higher-priority cells.
#)Note that the network has no way to discriminate between cells that were labeled as lower-priority by the source and cells that were tagged by the UPC function.
5)Fast Resource Management
#)Fast resource management functions operate on the time scale of the round-trip propagation delay of the ATM connection.
#)The current version of I.371 lists fast- resource management as a potential tool for traffic control that is for further study.
#)One example of such a function that is given in the Recommendation is the ability of the network to respond to a request by a user to send a burst.
#)That is, the user would like to temporarily exceed the current traffic contract to send a relatively large amount of data.
#)If the network determines that the resources exist along the route for this VCC or VPC for such a burst, then the network reserves those resources and grants permission.
#)Following the burst, the normal traffic control is enforced.