1 3/27/ :29 CS575ATM Technology: Traffic Management1 Rivier College CS575: Advanced LANs ATM Traffic Management

2 3/27/ :29 CS575ATM Technology: Traffic Management2 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

3 3/27/ :29 CS575ATM Technology: Traffic Management3 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

4 3/27/ :29 CS575ATM Technology: Traffic Management4 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

5 3/27/ :29 CS575ATM Technology: Traffic Management5 Effects of Network Congestion Source: Stallings: Data and Computer Communications p316

6 3/27/ :29 CS575ATM Technology: Traffic Management6 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

7 3/27/ :29 CS575ATM Technology: Traffic Management7 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

8 3/27/ :29 CS575ATM Technology: Traffic Management8 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

9 3/27/ :29 CS575ATM Technology: Traffic Management9 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

10 3/27/ :29 CS575ATM Technology: Traffic Management10 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

11 3/27/ :29 CS575ATM Technology: Traffic Management11 ATM Service Architecture (continued) -Application Example: native ATM voice with compression, interactive (real-time) compressed video (videoconferencing), and other types of multimedia communications

12 3/27/ :29 CS575ATM Technology: Traffic Management12 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

13 3/27/ :29 CS575ATM Technology: Traffic Management13 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: , 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)

14 3/27/ :29 CS575ATM Technology: Traffic Management14 ATM Service Categories (continued) -The cell rate provided by the network can change throughout the connection -The user gets what’s available

15 3/27/ :29 CS575ATM Technology: Traffic Management15 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

16 3/27/ :29 CS575ATM Technology: Traffic Management16 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

17 3/27/ :29 CS575ATM Technology: Traffic Management17 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

18 3/27/ :29 CS575ATM Technology: Traffic Management18 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)

19 3/27/ :29 CS575ATM Technology: Traffic Management19 Quality of Service (QoS) Parameters (continued) Cell Transfer Delay Probability Density Model Source: ATM Forum Traffic Management Specification Version 4.0

20 3/27/ :29 CS575ATM Technology: Traffic Management20 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

21 3/27/ :29 CS575ATM Technology: Traffic Management21 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

22 3/27/ :29 CS575ATM Technology: Traffic Management22 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

23 3/27/ :29 CS575ATM Technology: Traffic Management23 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 = Cell Delay Variation Tolerance (CDVT) =The upper bound on the cell clumping measure is CDVT Traffic Contract

24 3/27/ :29 CS575ATM Technology: Traffic Management24 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

25 3/27/ :29 CS575ATM Technology: Traffic Management25 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

26 3/27/ :29 CS575ATM Technology: Traffic Management26 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

27 3/27/ :29 CS575ATM Technology: Traffic Management27 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

28 3/27/ :29 CS575ATM Technology: Traffic Management28 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?

29 3/27/ :29 CS575ATM Technology: Traffic Management29 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

30 3/27/ :29 CS575ATM Technology: Traffic Management30 Usage Parameter Control (continued) Yes In No Valid VPI/VCI CLP Conform to SCR/BT CLP = 0 Conform to SCR/BT CLP = 1 Discard cell Out Yes No Yes 0 1 No Conform to PCR/CDV CLP = 0+1 Discard cell Discard cell Discard cell

31 3/27/ :29 CS575ATM Technology: Traffic Management31 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

32 3/27/ :29 CS575ATM Technology: Traffic Management32 Usage Parameter Control (concluded) Equivalent Versions of GCRA Yes NoNo X’ < 0 ? TAT > t a (k) + L ? TAT < t a (k) ? No Yes No Non Conforming Cell Non Conforming Cell Ye s X’ = X - (t a (k) - LCT) X’ = 0 X’ > L ? X = X’ + I LCT = t a (k) Conforming Cell No TAT = t a (k) TAT = TAT + I Conforming Cell Arrival of a cell k at time t a (k) Virtual Scheduling Algorithm Continuous-State Leaky Bucket Algorithm TAT: Theoretical Arrival Time t a (k): Time of arrival of a cell

33 3/27/ :29 CS575ATM Technology: Traffic Management33 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

34 3/27/ :29 CS575ATM Technology: Traffic Management34 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

35 3/27/ :29 CS575ATM Technology: Traffic Management35 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

36 3/27/ :29 CS575ATM Technology: Traffic Management36 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

37 3/27/ :29 CS575ATM Technology: Traffic Management37 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

38 3/27/ :29 CS575ATM Technology: Traffic Management38 Packet Discarding (concluded) Source: Newbridge White Paper

39 3/27/ :29 CS575ATM Technology: Traffic Management39 W. Stalling, Local and Metropolitan Area Networks, 6 th edition, Prentice Hall, 2000, Chapter 11 References W. Stalling, Data and Computer Communications, 6 th edition, Prentice Hall, 2002, Chapters A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier College, 2001.