1. Rivier College CS575: Advanced LANs ATM Traffic Management
ATM Technology: Traffic Management

2. ATM Technology: Traffic Management
Overview
Why Traffic Management?
Network Congestion
Effects of Network Congestion
Traffic Parameters
ATM Service Categories
Quality of Service (QoS)
Quality of Service (QoS) Parameters
Traffic Contract
Traffic Control Functions
Usage Parameter Control
Packet Discarding
Jain
CS575
ATM Technology: Traffic Management

3. Why Traffic Management?
The ATM technology is intended to support a wide variety of services and applications such as voice, video, and data
ATM promises to support all these different requirements with a common network
Within such a network all connections may impact on each other
ATM must manage traffic fairly and provide effective allocation of network resources for these different applications
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
CS575
ATM Technology: Traffic Management

4. ATM Technology: Traffic Management
Network Congestion
Network congestion is a state when the network cannot meet the negotiated network performance objectives for established connections or for new connection requests
Network congestion can be caused by:
Unpredictable statistical fluctuation of traffic flows
Fault conditions within the network
ATM layer traffic control is a set of actions taken by the network to avoid network congestion
Traffic control takes measures to adapt to unpredictable fluctuations in traffic flows and other problems within the network.
ATM layer congestion control refers to the set of actions taken by the network to minimize the intensity, spread, and duration of congestion
CS575
ATM Technology: Traffic Management

5. Effects of Network Congestion
Source: Stallings: Data and Computer Communications p316
CS575
ATM Technology: Traffic Management

6. ATM Technology: Traffic Management
Traffic Parameters
Traffic parameters describe traffic characteristics of a connection
For a given connection, traffic parameters are grouped into a source traffic descriptor
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
Calculated over the duration of the connection
CS575
ATM Technology: Traffic Management

7. 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
CS575
ATM Technology: Traffic Management

8. ATM Service Categories
ATM carries a wide range of heterogeneous traffic mix
To ensure network resources are fairly allocated for each traffic type, ATM services are divided into different service categories
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
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
CS575
ATM Technology: Traffic Management

9. ATM Service Categories (continued)
All service categories apply to both VCCs and VPCs.
ATM services are characterized by the traffic parameters:
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
CS575
ATM Technology: Traffic Management

10. 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
Real-Time Variable Bit Rate (rt-VBR)
Intended for real-time applications
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
CS575
ATM Technology: Traffic Management

11. ATM Service Architecture (continued)
Application Example: native ATM voice with compression, interactive (real-time) compressed video (videoconferencing), and other types of multimedia communications
CS575
ATM Technology: Traffic Management

12. ATM Service Categories (continued)
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
CS575
ATM Technology: Traffic Management

13. ATM Service Categories (continued)
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: , LAN traffic, and TCP/IP traffic
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)
CS575
ATM Technology: Traffic Management

14. ATM Service Categories (continued)
The cell rate provided by the network can change throughout the connection
The user gets what’s available
CS575
ATM Technology: Traffic Management

15. 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
CS575
ATM Technology: Traffic Management

16. 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
CS575
ATM Technology: Traffic Management

17. Quality of Service (QoS)
QoS is a set of user-perceivable performance parameters that characterize the traffic over an ATM connection
Defined on an end-to-end basis
User requests a QoS class for an ATM connection
The requested QoS class is a part of the traffic contract
The network commits to meet the requested QoS as long as the user complies with the traffic contract
ATM Forum QoS Classes
CS575
ATM Technology: Traffic Management

18. Quality of Service (QoS) Parameters
QoS parameters describe the level of service for each connection
ATM Forum specified six QoS parameters
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)
CS575
ATM Technology: Traffic Management

19. Quality of Service (QoS) Parameters (continued)
Cell Transfer Delay Probability Density Model
Source: ATM Forum Traffic Management Specification Version 4.0
CS575
ATM Technology: Traffic Management

20. Quality of Service (QoS) Parameters (continued)
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
CS575
ATM Technology: Traffic Management

21. Quality of Service (QoS) Parameters (continued)
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
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
CS575
ATM Technology: Traffic Management

22. Quality of Service (QoS) Parameters (concluded)
Higher values of cell loss is dominated by the effects of queuing strategy and buffer sizes
Delay, delay variation, and cell loss are impacted by buffer size and buffering strategy
The error rate is determined by fiber transmission characteristics
CS575
ATM Technology: Traffic Management

23. ATM Technology: Traffic Management
Traffic Contract
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
Cell Delay Variation Tolerance (CDVT)
Conformance Definition
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
The upper bound on the cell clumping measure is CDVT
Traffic Contract
CS575
ATM Technology: Traffic Management

24. 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
CS575
ATM Technology: Traffic Management

25. Traffic Contract (concluded)
A separate traffic contract for each Virtual Path Connection (VPC) or Virtual Channel Connection (VCC)
Negotiated at connection time
Signaling message for SVC
Circuit provision for PVC
CS575
ATM Technology: Traffic Management

26. Traffic Control Functions
Connection Admission Control (CAC)
Usage Parameter Control (UPC)
Selective cell discarding
Traffic Shaping
Explicit Forward Congestion Indication (EFCI)
Cell Loss Priority Control
Network Resource Management (NRM)
Frame discard
ABR Flow Control
Others
CS575
ATM Technology: Traffic Management

27. Connection Admission Control
Responsible for determining whether a connection request is admitted or denied
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
CS575
ATM Technology: Traffic Management

28. Usage Parameter Control
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
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?
CS575
ATM Technology: Traffic Management

29. Usage Parameter Control (continued)
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
CS575
ATM Technology: Traffic Management

30. Usage Parameter Control (continued)
Discard
cell No
Yes
Conform to
SCR/BT
CLP = 0
Yes
Yes
Valid
VPI/VCI
Conform to
PCR/CDV
CLP = 0+1 In
CLP
Out 1
Conform to
SCR/BT
CLP = 1 No No
Yes No
Discard
cell
Discard
cell
Discard
cell
CS575
ATM Technology: Traffic Management

31. Usage Parameter Control (continued) Generic Cell Rate Algorithm
Used to define conformance with respect to the traffic contract
For each cell arrival, GCRA determines whether the cell conforms to the traffic contract of the connection
The UPC function may implement GCRA to enforce conformance
Equivalent representations of the GCRA
Continuous-State Leaky Bucket Algorithm
Virtual Scheduling algorithm
CS575
ATM Technology: Traffic Management

32. 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
Non
Conforming
Cell
Yes
X’ = 0
TAT > ta(k) + L ?
Non
Conforming
Cell
Yes
X’ > L ? No No
TAT = TAT + I
Conforming Cell
X = X’ + I
LCT = ta(k)
Conforming Cell
Virtual Scheduling Algorithm
Continuous-State Leaky Bucket Algorithm
CS575
ATM Technology: Traffic Management

33. Selective Cell Discard and EFCI
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
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
CS575
ATM Technology: Traffic Management

34. ATM Technology: Traffic Management
Traffic Shaping
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
Traffic shaping examples:
Peak cell rate reduction
Burst length limiting
Spacing cells in time to reduce CDV
Cell scheduling policy
CS575
ATM Technology: Traffic Management

35. ATM Technology: Traffic Management
Resource Management
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
CS575
ATM Technology: Traffic Management

36. ATM Technology: Traffic Management
Packet Discarding
The ATM Adaptation Layer (AAL) segments higher layer packets into small fixed-size cells for transporting over the ATM network
A cell discarded by a switch causes the loss of the entire packet and eventually requires end-to-end error recovery through packet retransmission
A small congestion problem could potentially escalate to a more serious one
To prevent congestion escalation, Early Packet Discard (EPD) and Partial Packet Discard (PPD) can be used to discard cells on a packet basis
EPD and PPD are applied for ABR and UBR traffic of AAL-5 connections
EPD
When congestion occurs and buffers are filling, EPD discards all cells associated with a new packet arriving at a queue
CS575
ATM Technology: Traffic Management

37. 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
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
CS575
ATM Technology: Traffic Management

38. Packet Discarding (concluded)
Source: Newbridge White Paper
CS575
ATM Technology: Traffic Management

39. ATM Technology: Traffic Management
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.
CS575
ATM Technology: Traffic Management