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Rivier College
CS575: Advanced LANs
ATM Traffic Management
CS575
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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
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QoS Class
QoS Parameters
Application
0
1
2
3
4
Unspecified
Specified
Specified
Specified
Specified
Best Effort, At Risk
Circuit Emulation, CBR
VBR Video/Audio
Connection-Oriented Data
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
0
Yes
In
Valid
VPI/VCI
Conform to
SCR/BT
CLP = 0
Yes
Conform to
PCR/CDV
CLP = 0+1
Out
CLP
1
No
No
Conform to
SCR/BT
CLP = 1
Yes
No
Discard
cell
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Discard
cell
ATM Technology: Traffic Management
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
No
X’ < 0
?
TAT = ta (k)
No
Non
Conforming
Cell
Yes
X’ = 0
TAT > ta(k) + L
?
Ye
s
X’ > L
?
N
o
No
TAT = TAT + I
Conforming Cell
Virtual Scheduling Algorithm
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Non
Conforming
Cell
X = X’ + I
LCT = ta(k)
Conforming Cell
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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