1 5/1/2015 12:58 CS575ATM Technology: Traffic Management1 Rivier College CS575: Advanced LANs ATM Traffic Management

CS575 ATM Technology: Traffic Management

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Rivier CollegeCS575: Advanced LANs

ATM Traffic Management

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2ATM Technology: 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

0 Traffic Control Functions

0 Usage Parameter Control

0 Packet Discarding

Jain


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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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- 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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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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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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- The cell rate provided by the network can change throughout the connection

- The user gets what’s available


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- 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-rateand


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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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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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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

=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

Traffic Contract


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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)

Yes

Yes

In

No

ValidVPI/VCI

CLP

Conform to SCR/BT CLP = 0

Conform to SCR/BT CLP = 1

Discard cell

OutYes

No

No

Yes

0

1

No

Conform to PCR/CDV CLP = 0+1

Discard cell Discard

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

Yes

Yes

No

X’ < 0 ?

TAT > ta(k) + L ?

TAT < ta(k) ?

No

Yes

No

NonConforming Cell

NonConforming Cell

Yes

X’ = X - (t a(k) - LCT)

X’ = 0

X’ > L ?

X = X’ + I LCT = ta(k)Conforming Cell

No TAT = ta (k)

TAT = TAT + IConforming Cell

Arrival of a cell k at time ta(k)

Virtual Scheduling Algorithm Continuous-State Leaky Bucket

Algorithm

TAT: Theoretical Arrival Timeta(k): Time of arrival of a cell


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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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W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall,

2000, Chapter 11

References

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.

Documents

1 5/1/2015 12:58 CS575ATM Technology: Traffic Management1 Rivier College CS575: Advanced LANs ATM Traffic Management