Sizing SIP Trunks Gary Audin #700

What You Will Learn Learn about traffic patterns How to calculate the number of inbound sessions for a specific Grade of Service using Erlang B Determine SIP trunk bandwidth Review design considerations

Speaker Background Communications and security consultant for 34 years, VN Speaker at Enterprise Connect, ITExpo, IAUG, and 100s of user conferences Article and blog sites: – – – – –

Why Be Concerned Over design wastes money Over design requires more bandwidth Under design blocks calls Under design frustrates customers Under designed bandwidth can affect call quality

Trunk Traffic

Aggregate Traffic

Traffic Distribution

If You Have T1/PRI Connections T1 = 24 Channels N.A. PRI = 23 Channels Int’l PRI = 30 Channels It is very likely the connections have been overprovisioned.

Inbound Grade of Service The SIP trunk designer has to consult with the business unit that wants to support the inbound voice calls to determine the Grade of Service (GoS) the business unit wants to implement. Most designers should start with a 99.0% probability (GoS) that a call will go through, meaning only 1% of the callers receive busy signal during the peak voice call traffic time, the busy hour. For an IVR, the GoS should be 99.9% or 0.001% blocked Erlang B method is used to determine the minimum number of inbound lines/channels.

Applying Erlang B The first question is how much traffic must be carried, the Erlang load? Next, how long is the call, measured in minutes? A business decision has to be made. How often is a call blocked – Grade of Service (GoS)?

Erlang B Calculator Components Busy Hour Traffic (BHT): the number of hours of call traffic during the busiest hour of operation also called the Erlang load. During the busy hour, voice callers will experience the desired Grade of Service. Blocking (busy signal GoS): the percentage, for example 1% or GoS of 0.01%, of calls that are blocked (busy signal) because not enough channels are available. Channels: the number of channels in a SIP trunk group. One channel can carry one voice call at a time.

Erlang Load The Erlang load (BHT) = CAR X H/60 minutes. Where: – Call Arrival Rate (CAR) is the number of calls during the busy hour. – The average call length or Holding (H) time is measured in minutes. – One Erlang is equivalent to one channel busy for one hour.

Inbound Line Requirement 2000 calls during the busy hour 5 minutes per call BHT = 2000 X 5/60 = Erlangs Grade of Service (GoS) = 99% or 0.01% blocked Go to Enter as BHT and GoS as 0.01 Minimum number of inbound lines required is 187

Erlang B Results: GoS 99 & 99.9%

Bandwidth VoIP PacketPacket SizeOver Ethernet Over PPP w/RTP Over PPP w/cRTP IPv4 w/G.711 & G.722 at 64 kbps 20 ms/160 bytes/packet 87 kbps82 kbps68 kbps IPv6 w/G.711 & G.722 at 64kbps 20 ms/160 bytes/packet 95 kbps90 kbps68 kbps IPv6 + Header Extension w/G.711 & G.722 at 64kbps 20 ms/160 bytes/packet 98 kbps94 kbps69 kbps IPv4 w/G.729 at 8 kbps 20 ms/20 bytes/packet 31 kbps26 kbps12 kbps IPv6 w/G.729 at 8 kbps 20 ms/20 bytes/packet 39 kbps34 kbps12 kbps IPv6 + Header Extension w/G.729 at 8 kbps 20 ms/20 bytes/packet 42 kbps38 kbps13 kbps

WAN Bandwidth by Hour

Bandwidth Tool

Designing for Outbound Calls Assign one outbound line per agent If predictive dialer operating then add 10% more outbound lines For employees that are not part of the call center, determine the call load for outbound calls and apply Erlang B calculations for this traffic.

SIP Licenses A SIP license per SIP trunk path is required for the IP PBX and Session Border Controller. -Some IP PBXs and SBCs come with 5 to 50 licenses. -Licenses can be added increments of 4, 5, 10, etc. -SIP licenses are in addition to the SIP trunk provider charges.

Design Considerations Use G.711 for call centers and provision up to 20% more channels and bandwidth than calculated. Do not implement silence suppression unless there are more than 24 channels and music on hold is off and no Fax transmissions. Test the SIP trunk operation with the SBC connected.

Recommendations Compare to ITSP’s billing proposal Base on total IP Trunking Usage Base on the number of concurrent calls Likely, only calls that terminate to the PSTN Not on-network calls Calculate total concurrent calls destined for the IP trunk(s) to estimate bandwidth Even if they’re on-network calls Both incoming & outgoing calls consume bandwidth

Traffic Engineering Resources “Traffic Engineering Techniques in Telecommunications” by Richard Parkinson, ineering.pdf provides a detailed discussion on this subject. ineering.pdf “SIP Trunk Bandwidth: How Much?” trunk-bandwidth-how-much?pgno=1

Contact Information Gary Audin Will be at Telecom Reseller booth