VoIP report

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VoIP

VoIP system physically consist of these:

  1. PBX – call center, usually a soft on PC
  2. Gateways to PSTN(Public Switched Telephone Network
  3. Telephones

Call setup mechanism

  1. the thelephone connects to PBX
  2. the PBX replyes the destination IP
  3. the two telephones sends UDP packets, the voice. The signaling is comunicated with BPX. It means, when one side finises sending to PBX, the PBX send to second phone, that the line was dropped.

Used protocols, its family and used bandwith

  1. Call setup techniques(norms)
    1. SIP – the most polular now, because of NAT support
    2. H.323 – was the most popular in “local loop”
  2. Voice data – the main bandwith factor
    Codec Information Bandwidth Calculations
    Codec & Bit Rate (Kbps) Codec Sample Size (Bytes) Codec Sample Interval (ms) Mean Opinion Score (MOS) Voice Payload Size (Bytes) Voice Payload Size (ms) Packets Per Second (PPS) Bandwidth MP or FRF.12 (Kbps) Bandwidth w/cRTP MP or FRF.12 (Kbps) Bandwidth Ethernet (Kbps)
    G.711 (64 Kbps) 80 Bytes 10 ms 4.1 160 Bytes 20 ms 50 82.8 Kbps 67.6 Kbps 87.2 Kbps
    G.729 (8 Kbps) 10 Bytes 10 ms 3.92 20 Bytes 20 ms 50 26.8 Kbps 11.6 Kbps 31.2 Kbps
    G.723.1 (6.3 Kbps) 24 Bytes 30 ms 3.9 24 Bytes 30 ms 34 18.9 Kbps 8.8 Kbps 21.9 Kbps
    G.723.1 (5.3 Kbps) 20 Bytes 30 ms 3.8 20 Bytes 30 ms 34 17.9 Kbps 7.7 Kbps 20.8 Kbps
    G.726 (32 Kbps) 20 Bytes 5 ms 3.85 80 Bytes 20 ms 50 50.8 Kbps 35.6 Kbps 55.2 Kbps
    G.726 (24 Kbps) 15 Bytes 5 ms 60 Bytes 20 ms 50 42.8 Kbps 27.6 Kbps 47.2 Kbps
    G.728 (16 Kbps) 10 Bytes 5 ms 3.61 60 Bytes 30 ms 34 28.5 Kbps 18.4 Kbps 31.5 Kbps

About QoS
Packet marking:

  1. Simple QoS. We should it configure it on client computer, and we should believe that they dont use it for another.
  2. Separate comps to one, and phones to another VLAN(802.1q). It means, that our switches should support 802.1q, and our softphones too.
  3. Or we should identify voice data somehow on switches.

Prioritizing:

  1. Marked packets to priority queue.
  2. Others to bandwidth queue.

There is a possibility to set up max. numbers of calls at one time on PBX to not overload the backbone or SW with calls.

Sources: