Call Admission and Redirection in IP Telephony A Performance Study

1. Call Admission and Redirection in IP TelephonyA Performance Study Matthew Caesar, Dipak Ghosal, Randy Katz {mccaesar, randy}@cs.berkeley.edu ghosal@cs.ucdavis.edu

2. Approach • Goal: High quality, economically efficient telephony over the Internet. • Low blocking probability • Provide preferential treatment, high QoS • Questions: • How to perform call admission control? • How best to route calls through converged network? • Solutions: • Congestion sensitive call admission control • ITG selection • Techniques • Awareness of ITG congestion • Path quality between important points in network

3. 5 3 6 4 1 2 System Architecture ITG ITG LS LS ITG LS ITG LS ITG LS ITG LS Admin. Domain (AD) Example Call Setup Internet Example Advertisement Gateway (ITG) ITG IP Terminal Example Call Session Location Server (LS) LS

4. Method Scope of study • All calls are net-to-phone • ADs cooperate to provide service. • Use IETF’s TRIP architecture to support interoperability. • Disregard degradation in access network. • Prices determined at start of call. • ITGs offer equal PSTN reachability. Experimental Setup • Modified ns-2 • Run for 1.5 simulated hrs. • User Model • Bid uniformly distributed • Self-similar cross-traffic • Metrics: • Blocking Probability • Average call QoS • Used Mean Opinion Score (MOS) based on RTP loss rate • Economic efficiency • Ratio of price paid to QoS achieved

5. 0 a0l m 1 a1l 2m 2 . . . m-1 am-1l mm m Congestion-sensitive Call Admission Control • Goal: prevent system overload and generate revenue • Price of call • function of number of voice ports in use • rises when highly utilized • Used M/M/m/m (m-server loss system) to calculate price • responsive server • loss system • discouraged arrivals • Found price-congestion function that maximized revenue with respect to a

6. Congestion Pricing Analysis Analysis • Exponential function generates most revenue • Stepwise linear function performs almost as well • Maximum system price charged early Simulation • Flat pricing unnecessarily blocks many callers • Congestion pricing changes system price dynamically with load

7. Redirection • Problem: finding the “best” ITG • Approach: tradeoffs between quality and load • Method: LS maintains • Average measured path quality • Number voice ports in use • Redirection Schemes • Random Redirection (RR) (baseline) • QoS Sensitive Redirection (QR) • Use RTCP RRs to monitor path congestion • Route over best paths to maximize call QoS • Congestion Sensitive Redirection (CR) • Use TRIP advertisements to estimate ITG utilization • Route to least utilized ITG to improve load balance

8. Hybrid Redirection (CQR) • Choosing nearby ITG improves call quality, but can unbalance load. • Algorithm: • Compute Rdm =  *Mi+(1-)*Qi • Miis utilization, Qi is loss rate • Select randomly from k ITGs with lowest Rdm • Tradeoffs: • Use  to trade off call quality and load balance • Use k to vary flash crowd protection • Price Sensitive CQR (PCQR) • Decrease  for higher bids

9.  Call Redirection • Congestion sensitivity decreases blocking probability • Small k few blocked calls • Congestion Sensitive Redirection (CR) improves balance over Random Redirection (RR) • QoS sensitivity minimizes effects of cross traffic • Small amount of sensitivity vastly improves call quality

10. Summary • Admission Control Schemes: • Congestion sensitive pricing decreases unnecessary call blocking, increases revenue, and improves economic efficiency • Derived exponential price-congestion function that maximizes revenue • Redirection Schemes: • Hybrid scheme achieves “best of both worlds” • Low call blocking and high call QoS • Price sensitivity improves economic efficiency • Future Work: • Improve user model • Study flash-crowd effects • ITG Placement