Figure 7-15 Forming a 64×64 switching fabric with Siemens ASM chips.
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by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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Chapter 8 ATM Traffic Management: Traffic Enforcement and Traffic
Shaping
It is possible that the traffic from user nodes in the ATM network can exceed its capacity, causing memory buffers of the ATM switches to overflow, leading to data loss. In this chapter we shall discuss
issues relating to control of the amount of traffic entering the ATM network, in order to minimize data loss and maximize efficiency. Effective traffic management is key to the success of multi-service
enterprise and public networks.
For an ATM network to provide an acceptable Quality of Service QoS to all the users under times of heavy loading it is essential to implement an efficient and standardized mechanism for control. If
necessary, the ATM network must have the capability of scaling back the rate at which cells enter the ATM network. It should also be able to discard the cells that have already entered the network in order
to reduce congestion.
Traffic management for maintaining QoS includes functions for prevention and control of congestion across an ATM network. Within ATM, traffic management can be divided into the following domains:
• Traffic Control which refers to the set of actions taken by the network to avoid congestion from ever occurring. An additional role of traffic control is to optimize the use of network resources.
• Congestion Control action taken to minimize the intensity, spread and duration of a congestion condition.
The objectives of traffic management are:
• the ability to provide user-friendly, end-to-end quality of service QoS guarantees for existing and new applications.
• optimal use of network resources, including efficient, dynamic bandwidth management; • fair allocation among users and services giving higher priority to mission-critical applications;
• proactive management under congestion while maintaining network robustness and reliability.
Traffic control functions are concerned with congestion avoidance. In certain instances traffic control may fail and congestion may occur. Congestion control mechanisms are invoked at that point. These
functions respond to and recover from congestion. Both fairness and stability must be guaranteed by the congestion avoidance system.
ATM forum has defined a collection of congestion control functions that operate across a spectrum of timing intervals. Table 8-1 lists these functions with respect to the response times within which they
operate. The design of an optimum set of ATM Layer Traffic Controls and Congestion Controls should minimize network and end-system complexity while maximizing network utilization.
A typical ATM call goes through three basic phases with respect to ATM traffic management: call admission control, traffic contract and traffic policing phases. The call admission phase deals with the
bandwidth allocation task for the user. The traffic contract phase defines the agreement established between the end user and ATM network for the traffic characteristics of the call. Finally, the traffic
policing phase deals with the enforcement of the traffic contract for the duration of the call. In the following sections we will elaborate on each phase in more detail.
The problem of effectively controlling the congestion in ATM networks is currently the subject of intense research. A full-blown traffic and congestion control strategy that is widely embraced is yet to
evolve. The ATM forum has published a set of traffic and congestion control capabilities aiming at simple mechanisms and realistic network efficiency for details see ATM Forum User-Network
Interface UNI specification 3.0. The focus in this case is on control schemes for delay-sensitive traffic such as voice and video. The subject of handling bursty traffic is a topic of ongoing research and
standardization efforts.
The user is assured that the offered cell rates will meet the rate specified in the traffic contract based on the means provided by the traffic control. Traffic control ensures that the traffic contract rates are
enforced such that QoS performance is achieved across all users.
This chapter describes several types of Usage Parameter Control UPC, Call Admission Control CAC, Priority Control PC and traffic shaping. Figure 8-1a shows the various control functions and their
placements in the ATM network interfaces.
As illustrated in Figure 8-1a, functions such as Connection Admission Control CAC, Priority Control, resource management, etc. can be employed within a particular network. Figure 8-1b
illustrates the location of various functions with respect to the ATM switch.
Table 8-1 shows the various time scales over which a particular control function is applicable.
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ITU-T Recommendation 1.371 describes a set of traffic control procedures in order to maintain the required ATM network performance:
• Connection Admission Control CAC • Usage Parameter Control UPC and Network Parameter Control NPC
• priority control • traffic shaping
• network resource management • fast resource management
• congestion control
The use of these can be adapted to a particular ATM network environment. The recommendation leaves the usage of these procedures to ATM network providers.
Table 8-1Time scales over which a particular control function is applicable. TRAFFIC CONTROL FUNCTIONS
CONGESTION CONTROL FUNCTIONS
RESPONSE TIME
Network Resource Management Long Term
Connection Admission Control Connection Duration
Fast Resource Management Explicit Notification
Round-Trip Propagation Time Usage Parameter Control Priority Control
Selective Cell Discarding Cell Insertion Time
Traffic management functions are critically important to network providers and users alike, because they are the sources of enhanced QoS, operations simplicity and flexibility, and reduced network cost. In addition, they provide the potential for service
differentiation and increased revenues.
I. The Traffic Contract
At the time of establishing a new ATM connection, the network and the subscriber enter into a traffic contract. The network is responsible for supporting traffic at a certain level on that perticular connection, while the subscriber is responsible for not
exceeding performance limits. Thus the functions related to traffic control are concerned with establishing the necessary traffic parameters and enforcing them. Once the connection is accepted, the network continues to provide the agreed upon QoS as long as
the user complies with the traffic contract.
When a user requests a new Virtual Path Connection VPC or a Virtual Channel Connection VCC, the user must specify the traffic characteristics in both directions for that connection. This is done by selecting a QoS from among the various QoS classes
that the ATM network provides.
Figure 8-1 a Positions of various traffic control functions for an ATM switch.
Figure 8-1 b Location of traffic management functions with respect to an ATM switch.
Before accepting the connection the network must make sure that it can commit the necessary resources for supporting the requested traffic level, without disrupting the QoS for existing connections.
A separate traffic contract exists for every VPC or VCC. This contract deals with following four interrelated aspects of any VPC or VCC ATM cell flow:
1 The traffic parameters necessary for specifying the characteristics of the ATM cell flow. 2 The expected QoS which the ATM network is expected to provide.
3 The conformance checking rules used to interpret the traffic parameters. 4 The definition of a compliant connection which the ATM network imposes on the user.
Traffic parameters describe traffic characteristics of an ATM connection. For a given ATM connection, Traffic Parameters are grouped into a Source Traffic Descriptor, which in turn is a component of a Connection Traffic Descriptor. These terms are defined
as follows ATM Forum 4.0.
Traffic Parameters A traffic parameter describes the inherent characteristics of a traffic source. It may be quantitative or qualitative. Traffic parameters may for example describe Peak Cell Rate, Sustainable Cell Rate, Burst Tolerance, andor source type
e.g., telephone, video phone.
ATM Traffic Descriptor The ATM Traffic Descriptor is the generic list of traffic parameters that can be used to capture the traffic characteristics of an ATM connection.
Source Traffic Descriptor A Source Traffic Descriptor is a subset of traffic parameters belonging to the ATM Traffic Descriptor. It is used during the connection set-up to capture the intrinsic traffic characteristics of the connection requested by a particular
source. The set of Traffic Parameters in a Source Traffic Descriptor can vary from connection to connection.
Connection Traffic Descriptor The Connection Traffic Descriptor specifies the traffic characteristics of the ATM Connection at the Public or Private UNI. The Connection Traffic Descriptor is the set of traffic parameters in the Source Traffic Descriptor, the
Cell Delay Variation CDV Tolerance and the Conformance Definition that is used to unambiguously specify the conforming cells of the ATM connection.
Connection Admission Control procedures will use the Connection Traffic Descriptor to allocate resources and to derive parameter values for the operation of the UPC. The Connection Traffic Descriptor contains the necessary information for conformance testing
of cells of the ATM connection at the UNI.
Figure 8-2 illustrates the various attributes involved in establishing a traffic contract.
Table 8-2Procedures used to set values of traffic contract parameters. The last two rows specify the SVCs and the PVCs,
respectively.
EXPLICITLY SPECIFIED PARAMETERS IMPLICITLY SPECIFIED PARAMETERS
Parameter values set up at circuit- setup time
Parameter values specified at subscription time
Parameter values set using default rules Requested by userNMS
Assigned by network operator Assigned by network operator
Signaling By subscription
Network operator default rules NMS
By subscription Network operator default rules
