ATM Traffic Management

1
ATM Traffic Management
2
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

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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

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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

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Effects of Network Congestion

• Source Stallings Data and Computer
  Communications p316

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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
• 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

• 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

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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

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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
• 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)

• 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)

• 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
• 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 whats 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 networks 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

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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

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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)

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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)

• 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)

• Maximum Cell Transfer Delay (maxCTD)
• CTD is the elapsed time between a cells 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

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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

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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
• 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 networks 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)

• 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

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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

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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 networks
  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

• 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?

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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

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Usage Parameter Control (continued)
No
Yes
Conform to SCR/BT CLP 0
0
Yes
Yes
Conform to PCR/CDV CLP 01
Valid VPI/VCI
In
Out
CLP
1
Conform to SCR/BT CLP 1
No
No
Yes
No
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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

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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 lt ta(k) ?
Yes
X lt 0 ?
No
TAT ta (k)
No
Yes
TAT gt ta(k) L ?
Yes
X gt L ?
No
No
TAT TAT I Conforming Cell
X X I LCT ta(k) Conforming Cell
Virtual Scheduling Algorithm
Continuous-State Leaky Bucket Algorithm
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Selective Cell Discard and EFCI

• 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
• 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

• 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

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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

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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

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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
• 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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Additional Links

• http//www2.rad.com/networks/1999/atm/home.htm
• www-ee.uta.edu/online/wang/Atm-traf.pdf
• www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq23
  559.pdf