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Scheduling calls with known holding times
Reinette Grobler* Prof. M. VeeraraghavanUniversity of Pretoria Polytechnic [email protected] [email protected]
*Supported by Polytechnic University
Acknowledgement: David Rouse, Lucent Technologies
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Outline
• Problem statement• Motivation for solving this problem• Proposed algorithms: F and timeslots• Simulation comparison• Conclusions and future work
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Problem statement and motivation
• Problem statement– Define call scheduling algorithms for calls with
known holding times• Motivation
– There are applications that could generate calls with known holding times
– Improves network utilization and call blocking probabilities by allowing for call queueing
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Applications vs. data transfers
• “Communications applications” consist of data transfers• Data transfers can be classified as shown below
• Define a call as a “data transfer” rather than an “application session”
Interactive/Live streaming Recording
Stored streaming File transfers
Consuming endLive Stored
Live
Stored
Sending end
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Data transfers (“calls”) with known holding times
• For calls to have known holding times, two characteristics must be met:– Sending end of the data transfer is stored– Network uses preventive congestion control
• Examples:– Transferring a file on a circuit or CBR ATM connection
• Can compute holding time using knowledge of file size, data rate of circuit, and propagation delay
• Demonstrates need for the “preventive congestion control clause”– File transfer on a TCP/IP network does not have a known holding time
– Video-on-demand transfer on an VBR ATM connection
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Loss, delay, utilization
• Learning from the “packet world”– Packet switches use buffers to achieve high line
utilization and tradeoff packet delays with packet loss
– Apply this concept to calls • appears to be only possible if calls have known
holding times
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Call blocking vs. call queueing• Telephone networks, ATM networks and MPLS networks only allow call
blocking– If only call blocking is allowed (i.e., no delayed starts), then need to overprovision to keep call
loss low• Why not allow for delayed starts?
– If call holding times are unknown and calls are queued at each switch in sequence, then utilization could really suffer and call blocking could even increase with finite buffers
SETUP
The call waits (queues) until resources becomeavailable on link 1, reserves and holds bandwidth forthis call until the call is setup all through
SETUP
While call is being queued for link 2 resources, link 1resources are idle
Host Switch Switch Hostlink 1 link 2
referred to as kTwait scheme
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Knowledge of holding times
• Allows switches to immediately determine an agreed upon delayed start time for a call c
• Allow other calls sharing segments of the end-to-end path of c to use the network resources before c starts
• Results in high utilization and lower call loss
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Call scheduling schemes• Each switch maintains a time variant available capacity
function ai(t) for each outgoing interface I reflecting the scheduled start times of all admitted connections
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Call scheduling schemes (cont.)• F scheme:
– Ingress switch selects an earliest possible start time (epst) and reserves resources for a time period F (from epst), where F is much larger than the holding time, and sends this time period in the SETUP message
– Intermediate switches search for largest time period inside the received period during which it can accommodate the connection
• timeslots scheme:– The ingress switch selects a set of time ranges during which it has the
resources available for the new call, and sends these in SETUP message– An intermediate switch attempts to admit the call during each of the time
ranges or any part of each range greater than or equal to the holding time, the new time ranges are passed in a SETUP message
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Example: F and timeslots schemes
– F scheme (large time period: F=4): Swith1 - (10pm,2am), Switch2 – (10pm,12am)
SETUP SETUP
Switch1 Switch2
• Connection request for a call starting immediately with holding time of 1 hour
– timeslots scheme (number of time ranges = 3): Switch1 - ([3pm,5pm],[8pm,9pm],[10pm,], Swicth3 - ([4pm,5pm], [10pm,12am], [1am,2am])
Switch1 Switch2Host Host
SETUP
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Simulation
Switch1 Switch2 Switch4Source DestSwitch3
dest3dest2dest1
src1 src2 src3
Parameter (scheme) ValuesF (Large period) 20, 50 and 100 secondsn ( number of timeslots)
2, 3 and 4
•kTwait has no parameters.
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Results: Blocked calls
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Results: Start time delay• Indicates time
from connection request to start of data transmission of study traffic
• High F values increase delay
• Large queueing delays cause kTwait to provide later start times
• timeslots scheme performs best
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Results: Utilization• timeslots scheme
allows for close to optimal utilization
• kTwait unable to handle load of more than 70%
• Increasing F decreases utilization
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Conclusions and future work
• Use known holding times to schedule connections to improve network resource utilization and call queueing delays
• Required extensions:– Switch programming time– Propagation delay– Time synchronization
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