This blog is intended to help you understand the Fax services over IP Network, Fax/Modem Pass-through, Relay and Store-forward.
Like Voice and video, Fax and modem communication is supported over an IP network. Fax and modems are deployed with certain mechanisms such as pass-through, relay and store-forward
Fax over IP network
There are 3 methods for carrying fax machine to fax machine communications across packet network.
1. Fax pass-through
2. Fax relay
3. Fax store and forward
1. Fax pass-through : Modulated Fax information from the PSTN is passed in-band end-to-end over a voice speech path in a IP network. There are 2 pass-through techniques.
a. The configured voice codec is used for fax transmission. This technique works only when the configured codec is G711 with no VAD and no echo cancellation (EC) or when the configured codec is a clear channel codec or G.726/32. Low bit-rate codecs cannot be used for fax transmission.
b. Gateway dynamically changes the codec from the codec configured for voice to G.711 with no VAD and no EC for the duration of the fax session. This method is reffered to as "codec up speed" or "fax pass-through with up speed".
Fax pass-through call flow
2. Fax relay: Fax from the PSTN is demodulated at the sending gateway. The demodulated content is enveloped into packets, sent over the IP network and re-modulated into fax at the receiving end.
- Cisco Fax Relay is a Cisco-proprietary method and is the default fax transmission mechanism on Cisco voice gateways.
- T.38 fax relay is based on the ITU-T T.38 standard. T.38 is a real-time fax transmission method, which means that having two fax machines communicating with each using T.38 fax relay is like having a direct phone line between them.Both fax relay mechanisms are supported by the H.323, SCCP, MGCP, and SIP protocols.T.38 fax relay can be configured globally under voice service voip or per dial-peer basis using the fax protocol t.38 command.
3. Fax store and forward: Breaks the fax process into distinct sending and receiving processes and allows fax messages to be stored between these processes. Store and forward fax is based on ITU-T T.37 standard and it also enables fax transmission to be received from or delivered to computers rather than fax machines.
Cisco IOS supports 2 types of fax relay. T.38 fax relay and Cisco Fax relay (Cisco Proprietary)
Modem service over IP Network
Similar to fax transmission, modem transmission can be accomplished in 2 ways
1. Modem pass-through
2. Modem relay
1. Modem pass-through over VOIP provides the transport of modem signals through a packet network by using PCM-encoded packets. It is based on same logic as fax pass-through - an analog voice stream is encoded into G.711 passed through the network and decoded back to analog signals at the far end. Modem pass through does not support the switch from G.clear and G.711.VAD and echo cancellation needs to be disabled.
Modem pass-through can be configured globally in voice service voip or per dial-peer basis
using the modem passthrough command.
2. Modem relay supports modem connections across traditional TDM networks. Similar to fax relay, modem relay demodulates a modem signal at one voice gateway and passes it as packet data to the receiving voice gateway, where the signal is re-modulated and sent to the receiving modem. Cisco voice gateways, upon detection of the modem answer tone, switch into modem pass-through mode. If the Call Menu (CM) signal is detected, the gateways switch into modem relay mode. Modem relay supports V.34 modulation and V.42 error correction. Signaling support includes SIP, MGCP, SCCP, and H.323. Modem relay can be configured globally in voice service voip or per dial-peer basis using the modem relay command.
Configuration
Fax pass-through
voice service voip
fax protocol pass-through g711alaw
Modem pass-through
voice service voip
modem passthrough nse codec g711ulaw
Modem relay
modem relay nse codec g711ulaw
modem relay latency 250
modem relay sse redundancy interval 16
modem relay sse redundancy packet 2
modem relay sse t1 2100
modem relay sse retries 5
modem relay sprt retries 10
modem relay sprt v14 receive playback hold-time 32
modem relay sprt v14 transmit hold-time 12
modem relay sprt v14 transmit maximum hold-count 22
Mgcp configuration
ccm-manager mgcp
no ccm-manager fax protocol cisco
ccm-manager config
!
mgcp
mgcp call-agent 2.11.33.88 service-type mgcp version 0.1
mgcp dtmf-relay voip codec all mode nte-gw
mgcp rtp payload-type nte 101
mgcp package-capability mdste-package
mgcp package-capability srtp-package
MGCP Modem Pass-through
mgcp modem passthrough voip mode nse
Modem Relay
mgcp modem relay voip sprt retries 30
mgcp modem relay voip sprt v14 receive playback hold-time 250
mgcp modem relay voip sprt v14 transmit hold-time 30
mgcp modem relay voip sprt v14 transmit maximum hold-count 128
mgcp sdp simple
mgcp profile default
Related Information
