Call Admission and Redirection in IP Telephony A Performance Study Matthew Caesar, Dipak Ghosal, Randy Katz {mccaesar,

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

System Architecture ITG LS Example Call Setup Example Advertisement Gateway (ITG) IP Terminal Location Server (LS) Internet Admin. Domain (AD) Example Call Session ITG LS

Method Scope of study 1. All calls are net-to- phone 2. ADs cooperate to provide service. 3. Use IETF’s TRIP architecture to support interoperability. 4. Disregard degradation in access network. 5. Prices determined at start of call. 6. 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

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  m-1 m      m-1   mm

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

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

Hybrid Redirection (CQR) Choosing nearby ITG improves call quality, but can unbalance load. Algorithm: Compute Rdm =  *M i +(1-  )*Q i  M i is utilization, Q i 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

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 

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