Managing traffic efficiently - theATM way

IntroductionManaging a wide variety of traffic over a network is a constant challenge to public and private network operators alike.There are different traffic characteristics and network performance requirements that must be addressed, as well as the needfor networks to efficiently support multiple applications using multi-vendor network equipment. ATM networks are, by theirvery nature, designed to carry traffic generated by a wide variety of applications.

To efficiently support multiple services in a potentially multi-vendor network environment, the ATM Forum has specifiedtraffic management functions and procedures - such as traffic contract, conformance, policing, shaping, connection admissioncontrol, flow control and congestion control - by way of its TM4.0 specification, which was approved in April 1996. Themain aims of the TM4.0 ATM traffic management specification are:

Protect the network and users to achieve netowrk performance ogbjectives;Optimise the use of network resources;Protect applications against network congestion.

ATM traffic managementThe ATM traffic management specification can be divided into layers of functions and procedures. The following figuredepicts the hierarchy of the TM4.0 ATM layer traffic management functionality. Applications are assigned an ATM layerservice category which best complies with its required quality of service (QoS) and the expected behaviour of theapplication. The service category dictates which connection traffic parameters should be specified, and the QoS objectives.

Once established, the connection is monitored to ensure it conforms to the traffic parameters. Proper scheduling of the

connections at each queuing point in the network ensures that each connection receives its guaranteed QoS. In case ofcongestion - caused by statistical overlapping of bursts or a malfunction in the switching fabric - specific congestion controland flow control techniques can be applied. The TM4.0 specification introduces two optional congestion control techniques:priority discard and frame discard. The implementation of these techniques, including connection admission control,shaping, queuing and scheduling, are not subject to ATM Forum specification.

Diagram 1

Full range of applicationsThe ATM Forum TM4.0 specification currently defines four service categories, which cover the spectrum of current andpotential applications using the ATM network. These services categories include:

Constant bit rate (CBR) service;Variable bit rate (VBR) service;Available bit rate (ABR) service;Unspecified bit rate (UBR) service.

CBR serviceThe CBR service is designed to support real-time applications (such as circuit emulation and voice), and therefore assumes areal-time delay constraint and delay jitter. The allocated rate is defined and the application is assumed to offer trafficconstantly at this rate. The acceptable loss rate, maximum delay and jitter are also specified. The CBR traffic is deterministicand can be characterised by a constant cell inter-arrival which corresponds to a known peak emission rate.

VBR serviceThe VBR service is mainly used for video applications, frame relay, SMDS traffic, or any other application which has knownor predictable bursty traffic characteristics. VBR traffic can be characterised by an ON/OFF source. During the ON period,the cells' inter-arrival is constantly based on a peak emission rate. No cells are generated during the OFF period. Theduration of each ON and OFF period is also well characterised. VBR service is further divided into two subcategories,based on the delay requirements of the applications. The sub-categories are referred to as real-time VBR (RT-VBR) andnon-real-time VBR (NRT-VBR).

ABR serviceThe ABR service is designed to implement a feedback mechanism which allows end systems to adapt their sending rate tothe congestion status of the network, thereby minimising cell loss. The feedback is conveyed via a control loop whichoriginates from an end system. In the case of ATM, the end system's response to the feedback is specified as part of theprotocol. The traffic characterisation of an ABR connection is dynamic. An ABR connection can be allocated/reservedbandwidth corresponding to minimum cell rate (MCR) and extra bandwidth is made available via the flow control protocol.

UBR serviceUBR service does not include bandwidth allocation and does not mandate the enforcement of a traffic parameter. All UBRconnections share the remaining bandwidth with no specific control mechanism at the ATM layer. With a UBR connection,there is no guaranteed QoS, and the user must tolerate whatever capacity and cell loss the network can provide at the instantthe cell goes through the network. End-to-end protocols used in the Internet, such as TCP/IP, regulate the flow of trafficbased on packet loss. This type of traffic can run efficiently using the UBR service enhanced with a frame discard congestioncontrol technique.

Structure of service categoriesThe ATM Forum service categories are structured in such a way that an ATM switch can support several service categoriesand a range of refined QoS priorities within each category in a progressive manner. Typical applications which would use theRT-VBR or CBR service category include real-time video transport, videoconferencing and video-on-demand, while real-time circuit emulation, PSTN access and compressed voice would best fit the CBR category. Non-real-time UBR or ABRapplications would include LAN interconnection, bridging and routing service for Internet service providers.

Quality of service requirementsApplications are differentiated by their QoS requirements, which are expressed in terms of delay and loss requirements. TheATM layers of a network define three performance parameters: cell loss ratio (CLR), maximum cell transfer delay(maxCTD), and peak-to-peak cell delay variation (CDV). The cell loss ratio for an ATM connection is defined as thenumber of lost cells divided by the total number of transmitted cells. The cell transfer delay requirement is expressed in terms

of maximum cell transfer delay for a connection, which is specified as the (1-a) quantile of the CTD measurements. Celldelay variation is a measure of the jitter (positive and negative) in the cell inter-departure pattern of a given connection withrespect to its inter-arrival pattern. The peak-to-peak CDV is the (1-a) quantile of the CTD, minus the fixed CTD that couldbe experienced by any delivered cell on a connection during the entire connection holding time.

Traffic descriptorsThe ATM Forum's TM4.0 specification defines source traffic descriptors for each connection. These represent theconnection's traffic characteristics, and include:

Peak cell rate (PCR);Sustainable cell rate (SCR) and maximum burst size (MBS);Minimum cell rate (MCR);Cell delay variation tolerance (CDVT).

Peak cell rate represents the peak emission rate of the source. The inverse of the PCR represents the theoretical minimuminter-arrival time of cells at a queuing point. PCR can be limited by the physical speed of the source or via ingress trafficshaping. To allow for statistical gains in the bandwidth allocation, connections may specify a sustainable cell rate. Themaximum burst size parameter represents the connection's burstiness factor. It specifies the maximum number of cells thatcan be transmitted at PCR while still being compliant to the negotiated SCR. Theoretically, the MBS represents the

averaging period over which to calculate SCR.

For connections with dynamic bandwidth requirement (ABR), the minimum cell rate represents the minimum requiredbandwidth. The MCR may be specified as zero. The bandwidth available from the network may vary, but shall not becomeless than MCR. The cell delay variation tolerance parameter is used to characterise the delay jitter/cell clumping (CDV) thatcan be encountered by cells in the network. This parameter is used in conjunction with PCR and SCR monitoring to ensurethat cells which were generated at the appropriate interval, but suffered a positive CDV, are still seen as conforming to thetraffic descriptors. Essentially, CDVT allows some number of cells to be generated at link rate while still being compliant withthe PCR descriptor.

Mapping service categories, traffic descriptors and QoSThe set of traffic descriptors, which is conveyed at connection set up, varies depending on the connection service category.The following table shows the common source traffic descriptors associated with each service category. PCR and CDVTare always specified.

1. The value of CLR is not signalled. CLR is low for sources that adjust cell flow in response to control information. Whethera quantitative value for CLR is specified is network specific.

Table 1

Conformance monitoring and enforcementA conformance definition is a theoretical description of how the traffic should behave to comply with the traffic descriptors.The conformance definition consists of a sequence of generic cell rate algorithms (GCRA) which is applied to each set oftraffic descriptors. A connection requests a conformance definition which specifies which combination of parameters it will beusing and whether the CLP bit is meaningful or not (CLP significant or transparent). The conformance definition also specifiesthe action of the policing device, if enabled.

The ATM Forum states that any cells which enter the network conforming to this theoretical algorithm should receive theQoS guaranteed by the service category. Therefore, it is important that the traffic be policed at entry to the network toensure that the non-conforming traffic is either tagged (CLP=1) or discarded, to protect the conforming traffic. The source oftraffic needs to perform traffic shaping to ensure that the cells entering the network are in conformance with the agreed trafficparameters, otherwise there is a very high potential for cells to be discarded at the UPC. Policing ABR connections can bedone using a GCRA algorithm similar to the one implemented for CBR and VBR, but it is modified to take into account thefeedback information sent to the source and two timers reflecting the feedback delay. The policed rate varies dynamically intime.

Diagram 2

Flow controlFlow control protocols are used to avoid congestion, react to impeding congestion and prevent congestion collapse. TheTM4.0 specifies a flow control protocol for the ABR service. The specification is flexible and allows for variousimplementation options. In ABR, the source creates a control loop by generating resource management (RM) cells at a fixedinterval of data cells (the default is every 32 data cells). The destination turns around the RM cells, inserts the congestioninformation, then sends the cells in the backward direction of the connection. The source adjusts the shaping rate of thevirtual connection (VC) - up or down - based on the congestion information contained in the backward RM cells. Switchesin the path indicate congestion status either via a single bit (binary mode) or by inserting an explicit rate value, whichrepresents the share of the bandwidth allocated to the VC (explicit rate mode).

The source adjusts the sending rate, taking into account both the binary and explicit feedback, and accounting for thepotential loss of RM cells. The binary mode is backward-compatible with existing switching elements in the field since it isbased on the EFCI bit, which is specified as part of the ATM Forum UNI 3.1[1] specification. One key parameter whichimpacts on the performance of the protocol is responsiveness. This is mainly driven by the feedback loop delay (transmission+ propagation + queuing). The propagation delay becomes a significant factor when the protocol is used over a WAN. Toalleviate potential performance degradation due to a long feedback loop, ABR defines the concept of virtual sources andvirtual destinations VS/VD) which segment the feedback loop into sub-loops by assigning the functions of source anddestination to intermediate nodes in the network. The coupling between adjacent loops is flexible and allows for productdifferentiation. VS/VD functions can be at the ingress and/or the egress of a node, and be fully coupled, or totallyindependent.

Diagram 3

Congestion controlThe ATM Forum TM4.0 specification defines congestion control in terms of discard priority management and frame discard.Discard priority management allows network elements to discard CLP= 1 cells with higher probability than cells with

CLP=0. This only applies when the CLP significant mode is used. In the CLP transparent case, cells with CLP=1 have thesame priority as cells with CLP=0. The frame discard mechanism consists of discarding complete AAL5-PDUs whencongestion is impending. Frame discard improves the performance of the applications, and optimises network efficiency bypreventing partial packets from using up network resources. These are potential methods of implementing the mechanisms toprovide a range of efficiency and fairness. These methods of implementation are not subject to interoperability agreements(eg, ATM Forum specification).

ConclusionThe ATM Forum TM4.0 specification defines a set of interoperable functions and procedures that allow efficientmanagement of multi-service networks. Using these functions, a network operator can define flexible competitive serviceofferings.

References[1] ATM Forum. UNI 3.1 specification. March 1993.[2] ATM Forum. TM4.0 specification. April 1996.

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Last revised: Saturday, 20 March 1999