OCS Exposed - More Bandwidth the Better

March 18, 2008
Here's a few more words on optimizing bandwidth for performance and QoS. This chart represents approximate bandwidth requirements using various voice compression CODECs and data network topologies. In planning any VoIP/SIP network, bandwidth capacity planning is critical. The most common mistake is to simply do "busy hour" study of the number of voice calls and compare against a Poisson/Erlang chart and divide by the bandwidth. Worse yet, in some cases planners simply think they can add voice calls to an existing data network.   Data networks are often configured for asymmetrical communications - higher downstream than upstream data speeds. Traditional telephony is called "narrowband" because it supports (or passes audio signals only in the range of 300-3500 Hz (passband), a bandwidth of just 3.2 kHz. G.722.1 (also known as Siren) provides 7 kHz of audio bandwidth (50-7000 Hz) known as wideband audio.
 
An indepth knowledge of network issues is important as OCS supports UDP, TCP and G.711 which may have an impact on other VoIP/SIP connections. In addition, cable modem and DSL-Digital Subscriber Line services provide are asymmetrical (higher data rates for downstream than upstream) data services. The asymmetrical nature of these network services is often why VoIP does not perform well when used with symmetrical voice services. In addition, data services are generally "bursty" (large volumes at one time like when everyone checks their email in the morning or at lunch) or "chunky" (large database, video or multimedia downloads). The most important point is that in planning for SIP implementations allocate 80-100 KBPS per call for G.711 and around 30 KBPS per call for G.729. That is, while G.711 provides for 64KBPS of voice it needs more bandwidth because of the packetizing (overhead) for an internet protocol network. Here’s an easy rule of thumb, for G.711 take the total number of simultaneous (concurrent) calls times 100 KBPS and that is the bandwidth the customer needs for peak times. This means careful initial and long-term planning is critical to achieving and maintaining MOS 4.0 or higher in VoIP/SIP networks. A new concept called "wide" digits recommends that packet intervals should be at least 100ms or more.
 
If you want to know more, this information is also part of OCS-101 and SIP Essentials 2.0c available in the onsite and online courses. The online version is $299 for SIP 2.0c and for $499 as part of OCS-101 Office Communications Server online version per person or less with discounts. For more information go to http://www.techtionary.com or please call Tom Cross at 303-594-1694 or cross@gocross.com Discounts are also available to members of the SIP Forum. 


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