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ATM service types

CBR (Constant Bit Rate) rt-VBR (Real-time Variable Bit Rate) nrt-VBR (Non real-time Variable Bit Rate) ABR (Available Bit Rate) UBR (Unspecified Bit Rate). ATM service types. ATM service classes. Four Specified QoS classes are defined: Class 1: (CBR) Suitable for Circuit Emulation.

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ATM service types

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  1. CBR (Constant Bit Rate) rt-VBR (Real-time Variable Bit Rate) nrt-VBR (Non real-time Variable Bit Rate) ABR (Available Bit Rate) UBR (Unspecified Bit Rate) ATM service types

  2. ATM service classes • Four Specified QoS classes are defined: • Class 1: (CBR) Suitable for Circuit Emulation. • Class 2: (VBR) packetized video/audio -- eg. motion JPEG • Class 3: (nrt VBR) CONS -- eg. frame relay, X.25 • Class 4: (ABR) CLNS -- eg. IP • Class 0: (UBR) Best Effort Service.

  3. Has defined rate (cells per second) Peak Cell Rate (PCR) equals Sustainable Cell Rate (SCR): no burstiness Cells are carried at high priority Emulates fixed-bandwidth channel Constant Bit Rate (CBR)

  4. CDV accumulation problem But it is a very important service: DiffServ defined virtual leased line service The Issue of the CBR

  5. Has Peak Cell Rate (PCR) that is higher than the Sustainable Cell Rate (SCR) Conforming cells get high priority; excessive cells are given lower cell loss priority ITU divides by use: Class B: VBR audio and video Class C: Connection-oriented data (e.g., Frame Relay or X.25 over ATM) Class D: Connectionless data (e.g., IP over ATM) Variable Bit Rate (VBR)

  6. Advantages: Statistically multiplexed, more efficient use of bandwidth Disadvantages: non-zero cell loss currently no parameter to specify the source type, it is difficult to engineer the traffic so that the QoS is guaranteed while multiplexing is achieved The Issues of VBR

  7. Use unused bandwidth by CBR and VBR User can exceed PCR and SCR: well-suited for bursty data traffic Flow control required to make efficient use Available Bit Rate (ABR)

  8. The Issues of ABR • The share of available bandwidth for each ABR connection • is dynamic and may be down to a specified minimum cell • rate. • The ABR service is appropriate only for application which • adapt their rates to the time-varying available bandwidth • and tolerate unpredictable cell delays. ABR guaranteed a • minimum cell loss for those application

  9. Impact of Cell loss in ATM networks What we can do for the issue ?

  10. Rate based (ATM Forum) Explicit forward congestion indication (EFCI) Explicit rate (ER) Both are end to end types of flow control Credit based - hop by hop Generic flow control (end-station only) Hybrid Flow control

  11. The issues of the EFCI ABR • When ATM switch activates EFCI in a connection. • What is the problem ? • Can only indicate the existence of congestion • somewhere along the connection,but not the location • or severity. • How about some cell carry these type of info ?

  12. The issues of the EFCI ABR • Actually it is a delayed report for network status. • How to improve it ? What are disadvantages ? • Ideas: (1) backward • (2) Congestion flag in backward RM cells • (3) Breaks the feedback loop into separate segments

  13. The Issues For The ER ABR • Advantages: • How aboutIf the RM cell is lost due to congestion ? • Source rate transitions is explicit and immediately • to proper rate. • Congestion relieved faster • How about RM cell direction ? • When the RM cells should be sent back by the dst ?

  14. The Issues For the ABR Service • Policing issue • Policing complexity issue for EFCI ABR

  15. Traffic parameter mapping

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