Scheduling calls with known holding times

1. Scheduling calls with known holding times Reinette Grobler* Prof. M. Veeraraghavan University of Pretoria Polytechnic University rgrobler@cs.up.ac.za mv@poly.edu *Supported by Polytechnic University Acknowledgement: David Rouse, Lucent Technologies

2. Outline • Problem statement • Motivation for solving this problem • Proposed algorithms: F and timeslots • Simulation comparison • Conclusions and future work

3. 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

4. Consuming end Stored Sendingend Live Live Stored Applications vs. data transfers • “Communications applications” consist of data transfers • Data transfers can be classified as shown below Interactive/ Live streaming Recording Stored streaming File transfers • Define a call as a “data transfer” rather than an “application session”

5. 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

6. 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

7. link 1 SETUP link 2 SETUP Host Switch Switch Host 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 referred to as kTwait scheme While call is being queued for link 2 resources, link 1 resources are idle The call waits (queues) until resources become available on link 1, reserves and holds bandwidth for this call until the call is setup all through

8. 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

9. 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

10. 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

11. SETUP SETUP SETUP Switch1 Switch2 Host Host Switch1 Switch2 Example: F and timeslots schemes • F scheme (large time period: F=4): Swith1 - (10pm,2am), Switch2 – (10pm,12am) • 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])

12. Simulation src1 src2 src3 Switch1 Switch2 Switch3 Switch4 Source Dest dest1 dest2 dest3 • kTwait has no parameters.

13. Results: Blocked calls

14. 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

15. Results: Utilization • timeslots scheme allows for close to optimal utilization • kTwait unable to handle load of more than 70% • Increasing F decreases utilization

16. 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