[VoiceOps] GLBX soft switch

Alex Balashov abalashov at evaristesys.com
Wed Aug 5 02:33:01 EDT 2009

Ric Messier wrote:

> Good question. I did mean IP PBX SIP connect trunking. I'd normally think of
> carrier to carrier as transport or something similar rather than trunking.

SIP trunking simply refers to a static arrangement of IP endpoints for 
the purpose of setting up multiple sessions between them in order to 
pass traffic.  It can either be access (for a customer) or traffic of an 
intra-industrial nature, such as private SIP peering between carriers or 

Also, the endpoints of a trunk need to be network elements related to 
service delivery rather than end-user handsets, ATAs and similar devices 
designed for end-user access.  Client-side equipment designed for 
further switching - most notably, a PBX - qualifies under that 
definition, insofar as a PBX is a micro-switch of sorts, but anything 
that would be typically attached to that PBX as an extension does not.

Like many terms imported into VoIP from the TDM world by marketing 
departments, the term "trunking" is neither precise nor astute.  The 
essence of its original meaning, which draws on key pieces of 
electromechanical, digital and analog telephony history and heritage, is 
lost when marshaled in a VoIP context.  "Trunks" make most sense when 
applied to physically distinctive bundles of circuitry intended to move 
traffic in between distribution nodes in an intensely hierarchical 
topology; most classically, the distribution nodes are central 
offices/telephone exchanges containing some sort of switch, but more 
recently, premise-based "miniature" switches like PBXs, which have the 
distinction of "internal" lines (to handsets) versus "outside lines" 
used for PSTN access, too.

"SIP trunking" is really no better than "virtual PRI" in this respect. 
It's a transplanted figment of marketing imagination that otherwise 
lacks conceptual integrity from an engineering perspective - in my 
opinion, at least.  Although, I suppose the notion of a VoIP "line" 
really takes the cake.  Even a SIP "channel" is really pushing it;  SIP 
does not establish "channels," it establishes sessions.  Aside from 
certain logical configuration parameters (network and transport-layer 
reachability information) on equipment, there is no kind of intrinsic 
"container" in which those sessions participate, so there's nothing to 
"channelise" as with a T1 electrical or ISDN link-layer synchronous 
framing structure.

-- Alex

Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
Tel    : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
Mobile : (+1) (678) 237-1775

More information about the VoiceOps mailing list