1 / 39

Chapter 13

Chapter 13. ATM Traffic & Congestion Control. Introduction . ATM congestion problem overview ITU-T and ATM Forum framework for control of delay-sensitive traffic ATM traffic control mechanisms ATM congestion control Congestion control schemes for bursty traffic (ABR and GFR).

barclay
Download Presentation

Chapter 13

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 13 ATM Traffic & Congestion Control

  2. Introduction • ATM congestion problem overview • ITU-T and ATM Forum framework for control of delay-sensitive traffic • ATM traffic control mechanisms • ATM congestion control • Congestion control schemes for bursty traffic (ABR and GFR) Chapter 13: ATM Traffic & Congestion Control

  3. Why Typical Tools Are Inadequate for ATM • Majority of ATM traffic sources are not amenable to flow control • Long-haul ATM ttrans << tprop … slow feedback • Broad range of application types, flow control may indiscriminately penalize some • bandwidth requirements • traffic patterns • service requirements • Very high-speed switching increases volatility re: control mechanisms Chapter 13: ATM Traffic & Congestion Control

  4. Latency/Speed Effects • Issue: rapid insertion rate of ATM cells vs. relatively long round-trip delays • small size (53 bytes) of ATM cell • small switching delays • high bandwidth links in ATM networks • Simplified Example: • 150 Mbps data rate • tinsert = ttrans = = 2.8 x 10-6 seconds • U.S. coast-to-coast roundtrip dprop = 48 msec • Then the number of cells inserted (N) during RTT = = 1.7 x 104 cells = 7.2 million bits 53 x 8 bits 150 x 106 bps 48 x 10-3 seconds 2.8 x 10-6 seconds Chapter 13: ATM Traffic & Congestion Control

  5. Cell Delay Variation • General requirement: delay should be short • ATM designed to minimize delay • For some applications, rate of delivery of cells to destination must be constant (ATM’s CBR service level) • Contributors to cell delay variation • network queuing and processing • variation at UNI (user network interface) due to cell processing Chapter 13: ATM Traffic & Congestion Control

  6. General Procedure: Time Reassembly of CBR Cells  = 1/R = inverse of insertion rate V(0) = est. tolerable delay variation cell arrives late: discarded Slope = R cells/sec = 1/  cell insertion rate D(i) = end-to-end delay for the ith cell V(i) = V(i-1) – [ti – (ti-1 + )] Chapter 13: ATM Traffic & Congestion Control

  7. Per I-371 Origins of Cell Delay Variation at UNI Interleaving prior to delivery to physical layer Further delays possible at the physical layer Chapter 13: ATM Traffic & Congestion Control

  8. ATM Service Categories • Constant Bit Rate (CBR) • fixed data rate required at guaranteed capacity • Real-Time Variable Bit-Rate (rt-VBR) • tightly constrained delay and delay variation • sustained rate & guaranteed fast burst rate • Non-Real-Time Variable Bit-Rate (nrt-VBR) • no delay variation bound • Unspecified Bit Rate (UBR) • best-effort service • Available Bit Rate (ABR) • guaranteed minimum capacity, with bursts • Guaranteed Frame Rate (GFR) • like UBR/ABR, expressed in terms of frame rate Chapter 13: ATM Traffic & Congestion Control

  9. ATM Attributes How we describe an ATM traffic flow • Traffic descriptors • QoS parameters • Congestion (for ABR) • Other (for UBR) Chapter 13: ATM Traffic & Congestion Control

  10. Traffic Parameters • Connection Traffic Descriptor • Source Traffic Descriptor: PCR, SCR, MBS, MCR, MFS • Cell Delay Variation Tolerance (): upper bound on amount of cell delay that is introduced by the network interface and the UNI (due to interleaving, physical layer overhead, multiplexing, etc.) • Conformance Definition: unambiguous specification of conforming cells of a connection at the UNI (see GCRA, later) Chapter 13: ATM Traffic & Congestion Control

  11. Traffic Parameters • Source Traffic Descriptor • Peak Cell Rate (PCR): upper bound on traffic submitted by source (PCR = 1/T, where T = minimum cell spacing • Sustainable Cell Rate (SCR): upper bound on average rate of traffic submitted by source (over a larger T) • Maximum Burst Size (MBS): maximum number of cells sent continuously at PCR • Minimum Cell Rate (MCR): used with ABR and GFR… minimum cell rate requested, access to unused capacity up to PCR (elastic capacity = PCR-MCR?) • Maximum Frame Size (MFS): maximum size of a frame in cells available for GFR service Chapter 13: ATM Traffic & Congestion Control

  12. QoS Parameters (as defined by the ATM Forum) • Peak-to-peak cell delay variation (CDV): acceptable delay variation at destination • Maximum Cell Transfer Delay (maxCTD): maximum time between transmission of first bit of a cell at the source UNI to receipt of its last bit at the destination UNI • Cell Loss Ratio: ratio of lost cells to total transmitted cells on a connection Chapter 13: ATM Traffic & Congestion Control

  13. Cell Transfer Delay Probability Density (real-time services) Variable component of delay, due to buffering and cell scheduling. Chapter 13: ATM Traffic & Congestion Control

  14. Congestion Control and Other Attributes • Congestion Control • defined only for ABR service category • uses network feedback controls • ABR flow control mechanism (more later) • Other Attributes (introduced July 2000) • Behavior class selector (BCS): • for IP differentiated services (DiffServ) • provides for different levels of service among UBR connections • implementation dependent, no guidance in specs • Minimum desired cell rate (MDCR): • UBR application minimum capacity objective Chapter 13: ATM Traffic & Congestion Control

  15. Service Category-Attribute Relationship Chapter 13: ATM Traffic & Congestion Control

  16. Traffic & Congestion Control Function Classification affect more than one connection, effective over long timeframe determine if/how network can accommodate connection at a given QoS network responds within the round-trip lifetime of a cell react immediately to a cell as it is transmitted Chapter 13: ATM Traffic & Congestion Control

  17. Chapter 13 – Section 2 ATM Traffic Control (continued)

  18. Resource Management Using Virtual Paths • Multiple VCCs with different QoS requirements in same VPC • Cases to consider: • User-to-User application: VPC between pair of UNIs, QoS is user’s responsibility • User-to-network application: VPC between UNI and network node, QoS is network’s responsibility • Network-to-network application, QoS is network’s responsibility Chapter 13: ATM Traffic & Congestion Control

  19. Example VCC/VPC Configuration • How to allocate capacity to each VPC? • Aggregate peak demand • Statistical multiplexing Chapter 13: ATM Traffic & Congestion Control

  20. Connection Admission Control (CAC) • Network accepts the connection only if it can commit resources that satisfy a given connection request • Service category (CBR, VBR, …) • Connection traffic descriptor (PCR, CDVT, conformance definition,….) • QoS (peak-to-peak CDV, CLR,…) • Cell loss priority (CLP bit 0 or 0+1) • If accepted, a “traffic contract” is awarded to the user Chapter 13: ATM Traffic & Congestion Control

  21. Usage Parameter Control • Purpose: after connection is established, protect the network resources from overload/abuse by violating connections • Monitors connection for conformance to the traffic contract • detect violation of assigned parameters • take appropriate action Chapter 13: ATM Traffic & Congestion Control

  22. Usage Parameter Control Function Location Chapter 13: ATM Traffic & Congestion Control

  23. UPC Traffic Management • Peak Cell Rate Algorithm • Regulates the peak cell rate and the associated CDVT of a connection • Sustainable Cell Rate Algorithm • Regulates the sustainable cell rate and associated burst tolerance of a connection • Traffic Shaping • Smoothes out traffic to reduce “clumping” • Reduce delays, ensure fair resource allocation Chapter 13: ATM Traffic & Congestion Control

  24. Generic Cell Rate Algorithm (GCRA): Virtual Scheduling GCRA (I, L): I = Increment L = Limit ta(k) = Time of arrival of a cell TAT = Theoretical arrival time At time of arrival ta(1) of the first cell of connection, TAT = ta(1) Late arrival OK Early arrival TEST Early arrival beyond limit NOT OK • Algorithm takes two arguments, I and L • With PCR of R, I = T = 1/R • CDVT limit,  = L • Then peak cell rate algorithm is expressed as: GCRA(T, ) I.e., this cell arrived too late. Early arrival within limit OK Reset TAT Chapter 13: ATM Traffic & Congestion Control

  25. Leaky Bucket Algorithm Chapter 13: ATM Traffic & Congestion Control

  26. GCRA: Continuous-State Leaky Bucket • Equivalent to virtual scheduling • Max Bucket capacity is T+ • Counter X is incremented by T for each compliant cell • Bucket decrements (drains at 1 unit/unit time GCRA (I, L): I = Increment L = Limit ta(k) = Time of arrival of a cell X =Value of leaky bucket counter X = Auxiliary variable LCT = Last compliance time At the time of arrival ta(1) of the first cell, X = 0 and LCT = ta(1) Chapter 13: ATM Traffic & Congestion Control

  27. Cell Arrival at UNI (T=4.5) Ideal ( = 0.5) Possible ( = 1.5) Possible ( = 3.5) Note:  = T -  N = 1 + ( / T-) Possible ( = 7) Note:  > T -  N = 1 + ( / T-) Chapter 13: ATM Traffic & Congestion Control

  28. 1 1 SCR PCR Sustainable Cell Rate Algorithm • Uses GCRA (Ts, s), where: Ts = 1/Rsis the interarrival time at the sustainable cell rate, Rs, and s is the burst tolerance, or the time scale during which cell rate fluctuations are allowed • s is derived from the burstiness of the traffic stream: Burst Tolerance = s = (MBS-1) - Chapter 13: ATM Traffic & Congestion Control

  29. 1 1 SCR PCR Sustainable Cell Rate Algorithm • Note that, if the traffic stream is constrained by both GCRA (T, ) and GCRA (Ts, s) , then Maximum Burst Size (MBS) is: • s is derived from the burstiness of the traffic stream: Burst Tolerance = s = (MBS-1) - s Ts-T MBS = 1 + Chapter 13: ATM Traffic & Congestion Control

  30. UPC Function – Possible Actions based on CLP bit Forward cell or discard it? (see p. 377) P? = Compliance test for parameter P Chapter 13: ATM Traffic & Congestion Control

  31. Token Bucket for Traffic Shaping Tokens are generated and fill the bucket at the constant rate, . To pass, a token is removed from the bucket. If bucket is empty, cell is queued to wait for next token. Departures rate is “smoothed” to . Chapter 13: ATM Traffic & Congestion Control

  32. ABR Traffic Management • CBR, rt-VBR, nrt-VBR: traffic contract with open-loop control • UBR: best effort sharing of unused capacity • ABR: share unused (available) capacity using closed-loop control of source • Allowed Cell Rate (ACR): current max. cell transmission rate • Minimum Cell Rate (MCR): network guaranteed minimum cell rate • Peak Cell Rate (PCR): max. value for ACR • Initial Cell Rate (ICR): initial value of ACR Chapter 13: ATM Traffic & Congestion Control

  33. ABR Traffic Management • ACR is dynamically adjusted based on feedback to the source in the form of Resource Management (RM) cells • RM cells contain three fields: • Congestion Indication (CI) bit • No Increase (NI) bit • Explicit Cell Rate (ER) field Chapter 13: ATM Traffic & Congestion Control

  34. ABR Source Reaction Rules RIF – Fixed rate increase factor (default 1/16) RDF – Fixed rate decrease factor (default 1/16) Chapter 13: ATM Traffic & Congestion Control

  35. Variations in Allowed Cell Rate Chapter 13: ATM Traffic & Congestion Control

  36. Flow of Data and RM Cells – ABR Connection Nrm parameter usually set to 32 FRM cell flow  BRM Cell flow Chapter 13: ATM Traffic & Congestion Control

  37. ABR RM Cell Format Chapter 13: ATM Traffic & Congestion Control

  38. Initial Values of Cell Fields Chapter 13: ATM Traffic & Congestion Control

  39. ABR Parameters Chapter 13: ATM Traffic & Congestion Control

More Related