Tuesday 18 February 2014

Lync SIP trunks: internet or private?

A recurrent question during Lync technical workshops (and a key design decision in Lync Enterprise Voice deployments with PSTN SIP trunking) is identifying the most appropriate SIP trunk type. Internet or private?

SIP trunks can be deployed over the Internet or over a dedicated private WAN connection (MPLS typically); each, with supposedly clear pros and cons. In reality, there's a bit of confusion around this topic and a decision is not necessarily straightforward.

There is great deal of public literature around the subject if you want to dive deeper into each aspect. The purpose of of the article is only to provide a quick reference table for elements and criteria that should be considered and discussed to determine which circuit is most suitable.

FeatureInternet-based SIP trunkMPLS-based SIP trunk
ProsConsMitigating factorsProsConsMitigating factors
PROVISIONINGFast delivery times. Easy to set upSlower delivery times. More complex to set up. Likely to require a dedicated circuit to SIP trunk carrierIf core MPLS carrier is also a Telco might be able to provide SIP trunk on existing circuit
COSTRelatively cheap - lower TCOExpensive - higher TCO
NETWORK READINESSPratically 100% businesses already connected to internet and might potentially have capacity for SIP trunking. If not, in-place bandwidth upgrade is usually viable and requires no infrastructure upgradeHigher investment for small businesses that don't have MPLS circuits in place
BANDWIDTHRelatively abundant and cheapBandwidth is not guaranteed (best effort, no SLA). Asynchronous connections like ADSL provide limited upstream bandwidthEnsure accurate capacity planning is carried out to ensure bandwidth is adequate for expected number of PSTN sessions. Lync CAC may also helpBandwidth is guaranteed with SLASignificantly more expensive than Internet bandwidthAppropriate capacity planning and QoS adoption would make significantly more efficient use of bandwidth
AVAILABILITYUptime is not guaranteed (best effort, no SLA)Redundant internet connections to different carriers may reduce downtimeUptime is usually guaranteed with SLA and significantly higher that internet connections
NETWORK PERFORMANCEMore network hops, more subject to packet loss, latency and jitter. Unpredictable performance irrespective of bandwidthGeographically closer SIP trunk carriers might require fewer hops to their infrastructure. Network performance is more predictable especially if QoS is implemented. MPLS less subject to packet loss, latency and jitter
CALL-CARRYING CAPACITYUsually more abundant nominal bandwidth theorically allows for a greater call-carrying capacityCall-carrying capacity is influenced by many other factors and network conditions, which become more likely as sessions are added on the wire. Internet-based SIP trunking is generally advisable for low call volume requirementsCall-carrying capacity is more easily predictable and voice traffic can be shaped. MPLS-based SIP trunking advisable for higher call volumes
QoSQoS cannot be implemented end-to-end. Traffic cannot be prioritised by typeBuying a dedicated additional Internet line for SIP trunk would help segregating data/web and voice traffic, yet proper QoS is not achievableQoS can be implemented. Proper traffic prioritisation by type is viable
SECURITYVoice traffic flows through uncontrolled network and may potentially be intercepted. Use a SIP trunk provider that supports TLS (encrypts SIP signalling and SRTP (encrypts media), or deploy SIP trunk through VPN (less recommended: adds further network overhead and may impact QoE)Voice traffic flows through private and screened network. TLS and SRTP are still advisable for enhanced security
NATSIP protocol not NAT-friendly, several known issues when NAT is usedPrefer avoiding NAT when deploying SIP trunk. Alternatively, ensure firewalls are SIP-aware (SIP ALG)NAT less likely to be deployed in MPLS network, but if so, same considerations applyprefer avoiding NAT when deploying SIP trunk. Alternatively, ensure firewalls are SIP-aware (SIP ALG)

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