#)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.

untitled101

#)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.

untitled102


#)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.

untitled103

#)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.

 


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