ATM Traffic Management Raj Jain Professor of Computer and Info. Sciences The Ohio State University Columbus, OH 43210-1277
[email protected] These slides are available at http://www.cis.ohio-state.edu/~jain/cis777-00/ Raj Jain
The Ohio State University
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Dime Sale
One Megabit memory, One Megabyte disk, One Mbps link, One MIP processor, 10 cents each..... Raj Jain
The Ohio State University
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Overview
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Why worry about congestion? Congestion schemes for ATM Explicit Rate-based Control ABR Traffic Management Raj Jain
The Ohio State University
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Future Year 1980
In 1990, the memory will be so cheap that you will not have to worry about paging, swapping, virtual memory, memory hierarchy, and.... The Ohio State University Raj Jain 4
Why Worry About Congestion? Q: Will the congestion problem be solved when: q Memory becomes cheap (infinite memory)? q Links become cheap (very high speed links)? q Processors become cheap? A: None of the above.
No buffer 19.2 kb/s
Old age 1 Mb/s
SS SS SS SS SS SS SS SS File transfer time = 5 mins Time = 7 hours Raj Jain
The Ohio State University
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A A BB
CC SS All links 1 Gb/s
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Conclusions: q Congestion is a dynamic problem. Static solutions are not sufficient q Bandwidth explosion ⇒ More unbalanced networks q Buffer shortage is a symptom not the cause. Raj Jain
The Ohio State University
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Economic Reasons q
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Network is a shared resource Because it is expensive and needed occasionally (Like airplanes, emergency rooms) Most costs are fixed. Cost for fiber, switches, laying fiber and maintaining them does not depend upon usage ⇒ Underutilization is expensive But overutilization leads to user dissatisfaction. Need a way to keep the network maximally utilized
Raj Jain
The Ohio State University
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Traffic Management on the Information Superhighway 1 CAC 3 UPC
2 Shaping
Scheduling 4 5 Selective
7 Traffic Monitoring and feedback The Ohio State University
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6 Frame Discard Raj Jain
Traffic Management Functions q q q q q
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Connection Admission Control (CAC): Can requested bandwidth and quality of service be supported? Traffic Shaping: Limit burst length. Space-out cells. Usage Parameter Control (UPC): Monitor and control traffic at the network entrance. Network Resource Management: Scheduling, Queueing, virtual path resource reservation Selective cell discard: Cell Loss Priority (CLP) = 1 cells may be dropped Cells of non-compliant connections may be dropped Frame Discarding Feedback Control
Raj Jain
The Ohio State University
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Service Categories
Guaranteed
Standby
Joy Riders The Ohio State University
Confirmed 10
Raj Jain
Service Categories q
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ABR (Available bit rate): Source follows network feedback. Max throughput with minimum loss. UBR (Unspecified bit rate): User sends whenever it wants. No feedback. No guarantee. Cells may be dropped during congestion. CBR (Constant bit rate): User declares required rate. Throughput, delay and delay variation guaranteed. VBR (Variable bit rate): Declare avg and max rate. m rt-VBR (Real-time): Conferencing. Max delay guaranteed. m nrt-VBR (non-real time): Stored video.
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The Ohio State University
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Traffic Contract Parameters T q q q
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Peak Cell Rate (PCR): 1/T Cell Transfer Delay (CTD): First bit in to last bit out Cell Delay Variation (CDV): ~ Max CTD - Min CTD m Peak-to-peak CDV Cell Delay Variation Tolerance (CDVT) τ = GCRA limit parameter wrt PCR ⇒ GCRA(T, τ)
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The Ohio State University
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q
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Sustained Cell Rate (SCR): Average over a long period Burst Tolerance (BT) τs : GCRA limit parameter wrt SCR GCRA(1/Ts, τs) Maximum Burst Size: MBS = 1+BT/(1/SCR-1/PCR) BT ∈ [(MBS-1)(1/SCR-1/PCR), MBS(1/SCR1/PCR)] Cell Loss Ratio (CLR): Cells lost /Totals cells sent Minimum cell rate (MCR)
Raj Jain
The Ohio State University
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Peak-to-Peak CDV
Probability Density
Fixed Peak-to-peak CDV Cell Transfer Delay Delay Max CTD (1-α) quantile Raj Jain
The Ohio State University
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Service Categories Attribute PCR, CDVT4,5 SCR,MBS, CDVT4,5 MCR4 Peak-to-peak CDV Max CTD CLR4 Feedback
CBR Specified N/A
rt-VBR Specified Specified
nrt-VBR Specified Specified
UBR Specified2 N/A
ABR Specified3 N/A
N/A Specified
N/A Specified
N/A Unspecified
N/A Unspecified
Specified Unspecified
Specified Specified Unspecified
Specified Specified Unspecified
Unspecified Specified Unspecified
Unspecified Unspecified Unspecified
Unspecified Specified1 Specified6
1Network
specific 2Not subject to CAC/UPC 3PCR ⇒ Max ACR 4Explicitly/implicitly specified for PVC/SVC
5Not
signaled. Different values may apply at different interfaces along the path. 6Follow ABR rules Raj Jain
The Ohio State University
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ABR: Binary Rate Scheme EFCI Source Source
Destination Destination RM
DECbit scheme in many standards since 1986. q Forward explicit congestion notification (FECN) in Frame relay q Explicit forward congestion indicator (EFCI) set to 0 at source. Congested switches set EFCI to 1 q Every nth cell, destination sends an resource management (RM) cell to the source The Ohio State University Raj Jain q
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Go Go 30 30 km km East East 35 35 km km South South
Go Go left left Raj Jain
The Ohio State University
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ABR: The Explicit Rate Scheme Current CurrentCell CellRate Rate Explicit ExplicitRate Rate q q q q q
Sources send one RM cell every n cells The RM cells contain “Explicit rate” Destination returns the RM cell to the source The switches adjust the rate down Source adjusts to the specified rate Raj Jain
The Ohio State University
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Segment-by-Segment Control SS
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EFCI EFCI RM RM Virtual source/virtual destinations follow all notification/control rules Can be hop-by-hop SS
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EFCI EFCI EFCI EFCI RM RM RM RM Virtual dest/sources maintain per-VC queues. Raj Jain
The Ohio State University
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Guaranteed Frame Rate (GFR) UBR with minimum cell rate (MCR) ⇒ UBR+ q Frame based service m Complete frames are accepted or discarded in the switch m Traffic shaping is frame based. All cells of the frame have the same cell loss priority (CLP) m All frames below MCR are given CLP =0 service. All frames above MCR are given best effort (CLP =1) service. The Ohio State University Raj Jain q
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Congestion: Summary q
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Traffic Management is key to success of ATM Several different methods: CAC, Shaping, UPC, Scheduling, ... Service categories:CBR, VBR, ABR, UBR ER switches provide much better performance than EFCI. Raj Jain
The Ohio State University
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References Read Chapters 21-23 of McDysan’s book q R.Jain, "Congestion Control and Traffic Management in ATM Networks: Recent Advances and A Survey", Computer Networks and ISDN Systems, November 1996, http://www.cis.ohiostate.edu/~jain/ q K. Siu and R. Jain, "A Brief Overview of ATM: Protocol Layers, LAN Emulation, and Traffic Management," Computer Communications Review (ACM SIGCOMM), April 1995, http://www.cis.ohio-state.edu/~jain/
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The Ohio State University
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References (Cont) q
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User-Network Interface Specifications, V4.0, ftp://ftp.atmforum.com/pub/approved-specs/af-sig0061.000.ps “ATM Forum Traffic Management Specification, Version 4.0,” ftp://ftp.atmforum.com/pub/approvedspecs/af-tm-0056.000.ps Stallings’ ISDN and BISDN book, Chapter 16
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The Ohio State University
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