ATM service typesSlide 2Constant Bit Rate (CBR)Slide 4Variable Bit Rate (VBR)The Issues of VBRAvailable Bit Rate (ABR)Slide 8Impact of Cell loss in ATM networksFlow controlSlide 11Slide 12Slide 13Slide 14Slide 15ATM service typesCBR (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)-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.ATM service classesConstant Bit Rate (CBR)Has defined rate (cells per second)Peak Cell Rate (PCR) equals Sustainable Cell Rate (SCR): no burstinessCells are carried at high priorityEmulates fixed-bandwidth channelCDV accumulation problemBut it is a very important service:DiffServ defined virtual leased line serviceThe Issue of the CBRVariable Bit Rate (VBR)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 priorityITU divides by use:Class B: VBR audio and videoClass C: Connection-oriented data (e.g., Frame Relay or X.25 over ATM)Class D: Connectionless data (e.g., IP over ATM)The Issues of VBRAdvantages:Statistically multiplexed, more efficient use of bandwidthDisadvantages:non-zero cell losscurrently no parameter to specify the source type,it is difficult to engineer the traffic so that the QoSis guaranteed while multiplexing is achievedAvailable Bit Rate (ABR)Use unused bandwidth by CBR and VBRUser can exceed PCR and SCR: well-suited for bursty data trafficFlow control required to make efficient useThe Issues of ABR• The share of available bandwidth for each ABR connectionis dynamic and may be down to a specified minimum cellrate.• The ABR service is appropriate only for application whichadapt their rates to the time-varying available bandwidth and tolerate unpredictable cell delays. ABR guaranteed a minimum cell loss for those applicationImpact of Cell loss in ATM networks1% 2% 3% 4% 5%0102030405060708090100Probability of Packet Getting through (%)1% 2% 3% 4% 5%Cell Loss Rate1500 byte packetsWhat we can do for the issue ?Flow control-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)-HybridThe 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 ?•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 segmentsThe issues of the EFCI ABR• Advantages: How about If 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 ?The Issues For The ER ABRThe Issues For the ABR Service • Policing issue• Policing complexity issue for EFCI ABRCBR rtVBRnrtVBRABR UBRPCRyes yes yes yes n/aCDVTyes yes yes yes n/aSCR&MBSn/a yes yes n/a n/aMCRn/a n/a n/a yes n/aTraffic parameter
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