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ATM service types CBR (Constant Bit Rate)

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Presentation on theme: "ATM service types CBR (Constant Bit Rate)"— Presentation transcript:

1 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) 9 22

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. 10 23

3 Constant Bit Rate (CBR)
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 24

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

5 Variable 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 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) 25

6 The Issues of VBR 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

7 Available Bit Rate (ABR)
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 27

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 ? 5 29

10 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) Hybrid

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 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 ?

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

15 Traffic parameter mapping
13 62


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