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Atm traffic management

  1. 1. 12/20/12 16:27 1 Rivier College CS575: Advanced LANs ATM Traffic Management CS575 ATM Technology: Traffic Management 1
  2. 2. 12/20/12 16:27 2 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 CS575 ATM Technology: Traffic Management 2
  3. 3. 12/20/12 16:27 3 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 CS575 ATM Technology: Traffic Management 3
  4. 4. 12/20/12 16:27 4 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 CS575 ATM Technology: Traffic Management 4
  5. 5. 12/20/12 16:27 5 Effects of Network Congestion Source: Stallings: Data and Computer Communications p316 CS575 ATM Technology: Traffic Management 5
  6. 6. 12/20/12 16:27 6 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 CS575 ATM Technology: Traffic Management 6
  7. 7. 12/20/12 16:27 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 7
  8. 8. 12/20/12 16:27 8 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 CS575 ATM Technology: Traffic Management 8
  9. 9. 12/20/12 16:27 9 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 CS575 ATM Technology: Traffic Management 9
  10. 10. 12/20/12 16:27 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 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 CS575 ATM Technology: Traffic Management 10
  11. 11. 12/20/12 16:27 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 11
  12. 12. 12/20/12 16:27 12 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 CS575 ATM Technology: Traffic Management 12
  13. 13. 12/20/12 16:27 13 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) CS575 ATM Technology: Traffic Management 13
  14. 14. 12/20/12 16:27 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 14
  15. 15. 12/20/12 16:27 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 15
  16. 16. 12/20/12 16:27 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 16
  17. 17. 12/20/12 16:27 17 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 CS575 ATM Technology: Traffic Management 17
  18. 18. 12/20/12 16:27 18 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) CS575 ATM Technology: Traffic Management 18
  19. 19. 12/20/12 16:27 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 19
  20. 20. 12/20/12 16:27 20 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 CS575 ATM Technology: Traffic Management 20
  21. 21. 12/20/12 16:27 21 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 CS575 ATM Technology: Traffic Management 21
  22. 22. 12/20/12 16:27 22 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 CS575 ATM Technology: Traffic Management 22
  23. 23. 12/20/12 16:27 23 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 CS575 ATM Technology: Traffic Management 23
  24. 24. 12/20/12 16:27 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 24
  25. 25. 12/20/12 16:27 25 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 CS575 ATM Technology: Traffic Management 25
  26. 26. 12/20/12 16:27 26 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 CS575 ATM Technology: Traffic Management 26
  27. 27. 12/20/12 16:27 27 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 CS575 ATM Technology: Traffic Management 27
  28. 28. 12/20/12 16:27 28 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? CS575 ATM Technology: Traffic Management 28
  29. 29. 12/20/12 16:27 29 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 CS575 ATM Technology: Traffic Management 29
  30. 30. 12/20/12 16:27 30 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 CS575 ATM Technology: Traffic Management 30
  31. 31. 12/20/12 16:27 31 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 CS575 ATM Technology: Traffic Management 31
  32. 32. 12/20/12 16:27 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 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 CS575 ATM Technology: Traffic Management 32
  33. 33. 12/20/12 16:27 33 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 CS575 ATM Technology: Traffic Management 33
  34. 34. 12/20/12 16:27 34 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 CS575 ATM Technology: Traffic Management 34
  35. 35. 12/20/12 16:27 35 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 CS575 ATM Technology: Traffic Management 35
  36. 36. 12/20/12 16:27 36 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 CS575 ATM Technology: Traffic Management 36
  37. 37. 12/20/12 16:27 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 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 CS575 ATM Technology: Traffic Management 37
  38. 38. 12/20/12 16:27 38 Packet Discarding (concluded) Source: Newbridge White Paper CS575 ATM Technology: Traffic Management 38
  39. 39. 12/20/12 16:27 39 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 39