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The ATM Layer

The ATM Layer. ATM Network Concepts. OSI no longer a complete, or good, match to ATM layers ATM is connections oriented virtual paths and virtual circuits ATM has general requirements for signaling, network performance, traffic control, OAM. Router-based Networking. Frame Relay Network.

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The ATM Layer

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  1. The ATM Layer

  2. ATM Network Concepts • OSI no longer a complete, or good, match to ATM layers • ATM is connections oriented • virtual paths and virtual circuits • ATM has general requirements for signaling, network performance, traffic control, OAM

  3. Router-based Networking

  4. Frame Relay Network

  5. ATM Network

  6. Traversing the Network

  7. ATM Standards

  8. ATM Layer Tasks • Uses short fixed-length cells (53 bytes - 48 bytes payload) • Multiplex logical channels within a physical channel • Fixed length cells  very-high-speed switching hardware • Transmission/Switching/Reception • Multiplexes and switches cells • Translates incoming VPI/VCI on a link to VPI/VCI for output link - cell address translation

  9. ATM Layer Tasks • At endpoints, generates and interprets cell headers • Cell header generation/removal at source/destination • Sequential delivery • Provides a generic flow control mechanism on UNI side only, not on NNI interfaces • Problems with traffic management - Congestion Control/Buffer management

  10. ATM Layer Functions • OUT • Attach a cell header to a cell payload and fill in the path and channel identifiers • Use traffic parameters and quality of service levels to create an appropriate multiplexed cell stream • IN • Deliver incoming cell payload to the AAL layer

  11. ATM Cell Format

  12. Anatomy of an ATM Cell 8 7 6 5 4 3 2 1 • GFC (UNI) or VPI (NNI) VPI • VPI VCI • VCI • VCI PTI CLP • HEC Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Header 48 Bytes Payload VPI: Virtual Path Identifier CLP: Cell Loss Priority VCI: Virtual Channel Identifier HEC: Header Error Check PTI: Payload Type Indicator GFC: Generic Flow Control

  13. ATM Cell Format

  14. ATM UNI Cell Format

  15. Generic Flow Control - GFC • Only used on the UNI • The intended use is to monitor local flow control • Default value = 0000 • Used to alleviate short-term overload conditions

  16. VPI/VCI • Each cell contains a 24/28-bit connection identifier • First 8/12 bits: Virtual Path Identifier • Last 16 bits: Virtual Channel Identifier • VP service allows new VC's w/o orders to carriers • Why VPI and VCI? • VPI/VCI=0/0  Idle cell; 0/n  Signaling

  17. VCs and VPs • Many VCs may make up a VP. • The transmission path may comprise several VPs.

  18. Preassigned VPI/VCI Values ILMI enables adjacent devices to exchange configuration information 0/18 Reserved for private network-to-network interface PNNI messages; used to discover network topology

  19. Payload Type Field Coding • The main purpose of this 3 bit field is to identify the type of information being carried in the cell • U bit - Left most bit indicates whether the cell contains user data (0) or OAM data (1) • C bit - Centre bit indicated whether the cell has experienced congestion - 1 indicates that the cell has experienced congestion. This bit is also called Explicit Forward Congestion Indication (EFCI) flag

  20. Payload Type Field Coding • The rightmost bit can be used for a higher layer protocol mechanism. It is some times called the ATM-user-to ATM-user flag.

  21. Payload Type Field Coding • 000 User data cell, no congestion, AAU = 0 • 001 User data cell, no congestion, AAU = 1 • 010 User data cell, congestion, AAU = 0 • 011 User data cell, congestion, AAU = 1 • 100 Segment OAM F5 cell • 101 End-to-end OAM F5 cell • 110 Resource management cell • 111 Reserved • ATM-user-to-ATM-user (AAU) bit available for user-to-user indication • OAM cells may be inserted in any VC  In-band signaling

  22. Operation Administration andMaintenance (OA&M) • For supervision, testing, and performance monitoring • Loopbacks for maintenance • ITU TS standard uses CMIP • Organized into 5 hierarchical levels

  23. OAM Flows

  24. OAM Flows

  25. Cell Loss Priority - CLP • Similar to Frame Relay Discard Eligibility bit • CLP = 1 means low priority cell compared to those with CLP = 0, cell may be discarded first if there is congestion • Who sets the CLP bit? • Both the endpoint device originating the cell can and • A switch will set CLP to 1 if a customer’s end-point device is transmitting cells with CLP = 0 at too high a rate

  26. Header Error Control - HEC • Uses CRC - P(X) = 1 + x + x 2 + x 8 • Calculated on first four octets of the Cell header - Protects cell header only • Can also correct single bit error in the header - Optional Correction mode: Correct one bit errors if no earlier errors • HEC is in fact used for Cell Synchronization - Used for cell delineation in SONET • Discard cells with bad HEC • HEC Recalculated on each hop

  27. HEC: Correct one bit errors if no earlier errors

  28. Other Points About HEC • After the CRC is calculated the result is XORed with 01010101 • Error correction does not work for some physical media because of the way data is encoded onto the media • HEC is actually a physical layer function and not performed at the ATM layer

  29. Cell Rate Decoupling • Most of the physical media over which ATM runs provide a constant flow of traffic such as SONET, T1/E1, etc. • To fill up the the transport frame when there is not ATM user data, the ATM layer inserts “unassigned cells”. An ATM layer receiver can recognize an unassigned cell by by its header and discards it. • This process is called Cell Rate Decoupling

  30. Cell Rate Decoupling • The physical layer can also insert extra cells to maintain a steady flow. In that case the extra cells are called “idle cells”

  31. Usage Parameter Control • Usage Parameter Control UPC is the process of examining incoming traffic to check if it conforms to the traffic contract, and then taking configured actions. • The UPC function is applied to traffic entering the network by the entry switch at the User-Network Interface. • All cells originating from the user are monitored by the network to ensure that agreed parameters are not violated.

  32. ATM Network to Network Interface • NNI cell header format differs slightly • There are more bits for the VPI because more of them are needed within the network - the VPI field is expanded from 8 bits to 12 bits

  33. Switching Tables • Cells are forwarded by a switch by looking up VPI and VCI in a Switching Table • ATM switches translate VPI/VCI values • VPI/VCI value unique only per interface—eg: locally significant and may be re-used elsewhere in network

  34. VCs and VPs • Many VCs may make up a VP. • The transmission path may comprise several VPs.

  35. ATM Virtual Channels and Paths • Virtual channel (VC) describes unidirectional transport of ATM cells associated with a common unique identifier value (VCI). • Virtual path (VP) describes unidirectional transport of cells belonging to VCs that are associated with a common identifier value (VPI).

  36. ATM VCI and VPI Usage

  37. VP Switching

  38. VP and VC Switching

  39. VC Switching

  40. ATM Switching • Connections (routes) set up by software • Routing (path through multiple-switch network) and resource allocation is performed once per connection by switch control CPU • Cells are switched by hardware • Hardware (table lookup + switching fabric) switches each incoming cell to appropriate output port • Once a connection is established, cells are not touched by software • More aggregate bandwidth • Negligible additional latency (10-50 microseconds per switch hop vs. 10000 microseconds per router hop)

  41. Cell Buffering • A switch sometimes must temporarily store incoming cells for a channel in a queue before transmitting them. For example: • The outgoing link to be used is too busy. Better than discarding the cell. This keeps the cell loss rate at an acceptable level • A short burst of traffic has arrived at a high bit rate. The switch queues the cells so that they can be transmitted onto the output line at a steady, controlled rate. This is called Traffic Shaping

  42. Cell Discard Strategies • Switch has limited resources i.e. buffers • Discarding cells at random can be wasteful • When a cell is discarded the remaining cells belonging to the same packet are of no use, so the switch better discard all remaining cells - This is called Partial Packet Discard PPD • A smarter switch uses Early Packet Discard - Discarding cells from the beginning of a packet till the end - EPD

  43. Why Traffic Management? • Lose one cell and the rest are useless • Need to re-transmit 32+ cells for one cell lost • Congestion collapseis the result Cell Loss—Data’s Critical Enemy Ethernet (1500 Bytes) = 32 Cells FDDI (4470 Bytes) = 96 Cells IP over ATM–1577 (9180 Bytes) = 192 Cells TCP/IP Packet X

  44. Traffic Control Techniques • Connection management—Acceptance • Traffic management—Policing • Traffic smoothing—Shaping

  45. Contract Traffic Control Techniques - Connection Management Contract ATM Network Contract • Traffic Parameters • Peak cell rate • Sustainable cell rate • Burst tolerance • Etc. • Quality of Service • Delay • Cell loss

  46. Traffic Control Techniques Connection ManagementConnection Admission Control (CAC) I want a VC: X Mbps Y Delay Z Cell Loss CAC Can I Support this Reliably without Jeopardizing Other Contracts Guaranteed QoS Request No or Yes, Agree to aTraffic Contract Contract ATM Network 64

  47. ATM Network Traffic Control Techniques Traffic ManagementUsage Parameter Control (UPC) aka Policing You are Not in Conformance with the Contract. What Should the Penalty Be?? Contract ?DECISION? REBEL APPLICATION • PASS • MARK CLP BIT • DROP 65

  48. Traffic Control Techniques • CLP Control—When congested dropmarked cells • Public UNI—Generic Cell Rate Algorithm (GCRA) Traffic Management UPC Marked 0 0 0 0 1 0 ?DECISION? D r o p • PASS • MARK CLP BIT • DROP

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