This document provides an overview of ATM traffic management. It discusses why traffic management is needed in ATM networks to support different applications and allocate resources fairly. It describes how network congestion can occur and the effects of congestion. It also defines important traffic parameters used in ATM like Peak Cell Rate and Sustainable Cell Rate. Furthermore, it outlines the different ATM service categories including Constant Bit Rate, Real-Time Variable Bit Rate, and Available Bit Rate, and provides examples of applications for each category.

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Rivier College
CS575: Advanced LANs
ATM Traffic Management
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Overview
0 Why Traffic Management?
0 Network Congestion
0 Effects of Network Congestion
0 Traffic Parameters
0 ATM Service Categories
0 Quality of Service (QoS)
0 Quality of Service (QoS) Parameters
0 Traffic Contract
Jain
0 Traffic Control Functions
0 Usage Parameter Control
0 Packet Discarding
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Why Traffic Management?
0 The ATM technology is intended to support a wide variety of services and
applications such as voice, video, and data
0 ATM promises to support all these different requirements with a common
network
0 Within such a network all connections may impact on each other
0 ATM must manage traffic fairly and provide effective allocation of network
resources for these different applications
0 It is the task of ATM traffic control to:
- protect the network and the end-systems from congestion in order to
provide specified and guaranteed levels of Quality of Service (QoS)
- use available network resources efficiently
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Network Congestion
0 Network congestion is a state when the network cannot meet the negotiated
network performance objectives for established connections or for new
connection requests
0 Network congestion can be caused by:
- Unpredictable statistical fluctuation of traffic flows
- Fault conditions within the network
0 ATM layer traffic control is a set of actions taken by the network to avoid
network congestion
0 Traffic control takes measures to adapt to unpredictable fluctuations in
traffic flows and other problems within the network.
0 ATM layer congestion control refers to the set of actions taken by the
network to minimize the intensity, spread, and duration of congestion
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Effects of Network Congestion
Source: Stallings: Data and Computer Communications p316
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Traffic Parameters
0 Traffic parameters describe traffic characteristics of a connection
0 For a given connection, traffic parameters are grouped into a source traffic
descriptor
0 Traffic parameters specified in ATM Forum UNI
- Peak Cell Rate (PCR)
= An upper bound on the rate that traffic can be submit on a
connection
= Measured in cells/second
- Sustainable Cell Rate (SCR)
= An upper bound on the average cell rate of a burst traffic of an
ATM connection
= Measured in cells/second
= Calculated over the duration of the connection
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Traffic Parameters (concluded)
- Maximum Burst Size (MBS)
= The maximum number of cells that can be sent at the peak cell rate
- Minimum Cell Rate (MCR)
= The minimum number of cells that the user considers acceptable
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ATM Service Categories
0 ATM carries a wide range of heterogeneous traffic mix
0 To ensure network resources are fairly allocated for each traffic type, ATM
services are divided into different service categories
0 Each ATM service category represents a class of ATM connections that have
homogeneous characteristics in terms of traffic pattern, QoS requirements,
and possible use of control mechanisms, making it suitable for a given type
of resource allocation
0 The ATM Forum specifies the following five categories of services:
- CBR: Constant Bit Rate
- rt-VBR: Real-Time Variable Bit Rate
- nrt-VBR: Non-Real-Time Variable Bit Rate
- UBR: Unspecified Bit Rate
- ABR: Available bit Rate
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ATM Service Categories (continued)
0 All service categories apply to both VCCs and VPCs.
0 ATM services are characterized by the traffic parameters:
0 Constant Bit Rate (CBR)
- Requires a fixed amount of bandwidth continuously available during
the connection lifetime
- The amount of bandwidth is characterized by a Peak Cell Rate (PCR)
value
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ATM Service Categories (continued)
- The source can transmit cells at the PCR at any time and for any
duration
- Intended to support real-time applications
- Requires tightly constrained delay and delay variation
- Application examples: interactive (real-time) voice, video, and circuit
emulation
0 Real-Time Variable Bit Rate (rt-VBR)
- Intended for real-time applications
- Requires tightly constrained delay and delay variation
- Characterized by a PCR, Sustainable Cell Rate (SCR), and Maximum
Burst Size (MBS)
- The source may transmit bursty traffic, e.g., Motion JPEG or MPEG
compressed video
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ATM Service Architecture (continued)
- Application Example: native ATM voice with compression, interactive
(real-time) compressed video (videoconferencing), and other types of
multimedia communications
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ATM Service Categories (continued)
0 Non-Real-Time Varible Bit Rate (nrt-VBR)
- Intended for non-real-time applications
- Source transmits bursty traffic
- Characterized by a PCR, SCR, and MBS
- Requires low Cell Loss Ratio (CLR)
- May support statistical multiplexing of connections
- No delay bounds are associated with this service category
- Application Example: Critical response time transaction processing
such as airline reservations, banking transactions, processing
monitoring
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ATM Service Categories (continued)
0 Unspecified Bit Rate (UBR)
- Intended for non-real-time, bursty applications
- Does not specify traffic related service guarantees
- No commitment is made about cell transfer delay
- No commitment is made as to cell loss ratio experienced by cells on the
connection
- Best effort service
- Application example: E-mail, LAN traffic, and TCP/IP traffic
0 Available Bit Rate (ABR)
- Intended for bursty traffic whose bandwidth range is known roughly
- End system specifies maximum required bandwidth (PCR) and
minimum usable bandwidth (MCR)
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ATM Service Categories (continued)
- The cell rate provided by the network can change throughout the
connection
- The user gets what’s available
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ATM Service Categories (continued)
- The goal is to provide rapid access to unused network bandwidth at up to PCR
whenever the network bandwidth is available
- Cell loss ratio is minimal provided that the user adapts to the network’s feedback
controls
- Intended for non-real-time applications
- Application example: file transfer, browsing the Web
- No numeric commitment is made about cell transfer delay
- Flow control mechanism specified
- A rate-based service specified by the ATM Forum
- Flow control model
= A source generates forward Resource Management cells (RM-cells)
= RM-cells are turned around by the destination as backward RM-cells
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ATM Service Categories (concluded)
= Backward RM-cells carry feedback information provided by the
network and/or destination to the source
- The source performs dynamic traffic shaping based on feedback
received from the network
unspecified bit-rate
and
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Quality of Service (QoS)
0 QoS is a set of user-perceivable performance parameters that characterize
the traffic over an ATM connection
0 Defined on an end-to-end basis
0 User requests a QoS class for an ATM connection
0 The requested QoS class is a part of the traffic contract
0 The network commits to meet the requested QoS as long as the user complies
with the traffic contract
0 ATM Forum QoS Classes
QoS Class QoS Parameters Application
0 Unspecified Best Effort, At Risk
1 Specified Circuit Emulation, CBR
2 Specified VBR Video/Audio
3 Specified Connection-Oriented Data
4 Specified Conectionless Data
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Quality of Service (QoS) Parameters
0 QoS parameters describe the level of service for each connection
0 ATM Forum specified six QoS parameters
0 Through the use of network signaling to establish an ATM connection, three
of these may be negotiated between the end-system and the network
- Peak-to-peak Cell Delay Variation (peak-to-peak CDV)
- Maximum Cell Transfer Delay (maxCTD)
- Cell Loss Ratio (CLR)
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Quality of Service (QoS) Parameters (continued)
Cell Transfer Delay Probability Density
Model
Source: ATM Forum Traffic Management Specification Version 4.0
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Quality of Service (QoS) Parameters (continued)
0 Peak-to-peak Cell Delay Variation (peak-to-peak CDV)
- Cell delay variation (CDV) is defined as a measure of cell clumping
- It is how much more closely the cells are spaced than the nominal
interval
- Cells may be sent into the network evenly spaced, a variety of factors
may contribute to cell clamping or gaps in the cell stream
- If the network cannot properly control CDV, distortion can occur for
real-time services such as voice, video, and multimedia applications
- If cells arrive too closely together, cell buffers may overflow
- Subscribers of CBR or VBR services need to specify this parameter
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Quality of Service (QoS) Parameters (continued)
0 Maximum Cell Transfer Delay (maxCTD)
- CTD is the elapsed time between a cell’s exit at the source and its entry
at the destination
- It includes both node processing and internode transmission time
- Subscribers of CBR or VBR services need to specify this parameter
0 Cell Loss Ratio (CLR)
- CLR = (Lost Cells) / (Total Transmitted Cells)
- Cells may be lost due to
= network malfunction
= discarded for noncompliance
= discarded in response to network congestion
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Quality of Service (QoS) Parameters (concluded)
0 Higher values of cell loss is dominated by the effects of queuing strategy and
buffer sizes
0 Delay, delay variation, and cell loss are impacted by buffer size and
buffering strategy
0 The error rate is determined by fiber transmission characteristics
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Traffic Contract
0 Agreement between user and network across UNI regarding:
- The QoS that a network is expected to provide
- The Connection Traffic Descriptor, which includes
= Source Traffic Descriptor Traffic Contract
= Cell Delay Variation Tolerance (CDVT)
= Conformance Definition
- Source Traffic Descriptor
= Defines the characteristics of ATM traffic coming into the network
= Includes several negotiable traffic parameters: PCR, SCR, MBS,
and Burst Tolerance (BT)
= Specifies flow for CLP = 0 and/or CLP = 0 + 1
- Cell Delay Variation Tolerance (CDVT)
= The upper bound on the cell clumping measure is CDVT
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Traffic Contract (continued)
= It is the measure of how much cell clumping is acceptable resulting
from network operations such as cell multiplexing or the insertion
of OAM cells
= CDVT controls the amount of variability acceptable using a leaky
bucket algorithm
- Conformance Definition
= Defines what cell rates and streams will be monitored
= Defines the checking rule used to interpret the traffic parameters
= Defines the network’s definition of a compliant connection, i.e.,
what constitutes obeying the rules
= Conformance is determined by the Usage Parameter control (UPC)
at the ingress to the network
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Traffic Contract (concluded)
0 A separate traffic contract for each Virtual Path Connection (VPC) or
Virtual Channel Connection (VCC)
0 Negotiated at connection time
- Signaling message for SVC
- Circuit provision for PVC
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Traffic Control Functions
0 Connection Admission Control (CAC)
0 Usage Parameter Control (UPC)
0 Selective cell discarding
0 Traffic Shaping
0 Explicit Forward Congestion Indication (EFCI)
0 Cell Loss Priority Control
0 Network Resource Management (NRM)
0 Frame discard
0 ABR Flow Control
0 Others
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Connection Admission Control
0 Responsible for determining whether a connection request is admitted or
denied
0 For each connection request, CAC derives the following information from
the traffic contract
- Values of parameters in the source traffic descriptor
- The requested and acceptable values of each QoS parameter and the
requested QoS class
- The value of the CDVT
- The requested conformance definition
- Based on that information and the network’s definition of a compliant
connection to determine
= Whether the connection can be accepted or not
= The traffic parameters needed by UPC
= Allocation of network resource
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Usage Parameter Control
0 What is UPC
- Commonly known as Traffic Policing
- A network traffic control mechanism
- Required at the public UNI
- Detects and stops user traffic violations
- Ensures QoS for other connections
0 UPC Functions
- Monitors cells submitted at the UNI
- Checks for connection compliance
= Is the user sending data too quickly?
= Is the user obeying the traffic contract?
- Checks validity of VPI/VCI values
= Is the user using the correct VPI/VCI?
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Usage Parameter Control (continued)
0 UPC Action
- For non-conforming cells
= Discard or
= Tag as low priority (overwriting CLP bit to 1)
- For conforming cells
= Transparently pass or
= Traffic shape
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Usage Parameter Control (continued)
Discard
cell
No
Yes
Conform to
SCR/BT
0 CLP = 0
Yes Yes
Valid Conform to
In VPI/VCI PCR/CDV CLP Out
CLP = 0+1
1
Conform to
SCR/BT
No No CLP = 1 Yes
No
Discard Discard
cell cell Discard
cell
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Usage Parameter Control (continued)
Generic Cell Rate Algorithm
0 Used to define conformance with respect to the traffic contract
0 For each cell arrival, GCRA determines whether the cell conforms to the
traffic contract of the connection
0 The UPC function may implement GCRA to enforce conformance
0 Equivalent representations of the GCRA
- Continuous-State Leaky Bucket Algorithm
- Virtual Scheduling algorithm
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Usage Parameter Control (concluded)
Equivalent Versions of GCRA
Arrival of a cell k at time ta(k)
TAT: Theoretical Arrival Time
ta(k): Time of arrival of a cell
X’ = X - (t a(k) - LCT)
Yes
TAT < ta(k)
?
Yes
X’ < 0
No TAT = ta (k) ?
No
Yes X’ = 0
Non
Conforming TAT > ta(k) + L
Cell Yes
? Non
Conforming X’ > L
Cell ?
No No
TAT = TAT + I X = X’ + I
Conforming Cell LCT = ta(k)
Conforming Cell
Virtual Scheduling Algorithm
Continuous-State Leaky Bucket Algorithm
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Selective Cell Discard and EFCI
0 Selective Cell discard
- A congested network may selectively discard cells which meet either or
both the following conditions:
= Cells which belong to a non-compliant ATM connection
= Cells which have CLP = 1
- This is to protect the CLP = 0 flow as much as possible
0 Explicit Forward congestion Indication (EFCI)
- A network element in an impending congested state or a congested state
may set an EFCI in the cell header
- This indication may be examined by the destination end-system
- The end-system may adaptively lower the cell rate of the connection
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Traffic Shaping
0 A mechanism that alters the traffic characteristics of a cell stream on a
connection to achieve better network efficiency or to ensure conformance to
the traffic parameters in the traffic contract
0 Traffic shaping examples:
- Peak cell rate reduction
- Burst length limiting
- Spacing cells in time to reduce CDV
- Cell scheduling policy
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Resource Management
0 Resource Management
- Two critical resources
= Buffer space
= Trunk bandwidth
- One way of simplifying the management of the trunk bandwidth is
through the use of virtual paths
= If every node in a network is interconnected by a VPC, then only
the total available entry-to-exit VPC bandwidth need be considered
in CAC decisions
= A VPC is easier to manage as a larger aggregate than multiple,
individual VCCs
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Packet Discarding
0 The ATM Adaptation Layer (AAL) segments higher layer packets into small
fixed-size cells for transporting over the ATM network
0 A cell discarded by a switch causes the loss of the entire packet and
eventually requires end-to-end error recovery through packet
retransmission
0 A small congestion problem could potentially escalate to a more serious one
0 To prevent congestion escalation, Early Packet Discard (EPD) and Partial
Packet Discard (PPD) can be used to discard cells on a packet basis
0 EPD and PPD are applied for ABR and UBR traffic of AAL-5 connections
0 EPD
- When congestion occurs and buffers are filling, EPD discards all cells
associated with a new packet arriving at a queue
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Packet Discarding (continued)
- The remaining buffer space can then be used for cells belonging to
packets that already have entered the queue
- EPD maximizes the chances for already queued packets to leave the
queue successfully
0 PPD
- If EPD does not remove congestion and cells arriving at a queue have to
be discarded because of buffer overflow PPD is applied
- PPD discards all subsequent cells associated with the same packet
rather than just a few cells within the packet during buffer overflow
- PPD minimizes the number of packets becoming invalid in the queue
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Packet Discarding (concluded)
Source: Newbridge White Paper
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References
W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall,
2000, Chapter 11
W. Stalling, Data and Computer Communications, 6th edition, Prentice Hall,
2002, Chapters 11-12
A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier College,
2001.
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