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G.711 Codec (PCMU and PCMA)

Updated 2026-10-23 · By the SMS Conecta team (SMSConecta Inc.)

Short answer: G.711 is the ITU-T voice codec that encodes audio at 64 kbit/s with 8 kHz sampling, offered in two flavours: µ-law (PCMU), used predominantly across North America and Japan, and A-law (PCMA), the default pretty much everywhere else. It is the baseline codec that virtually every SIP device can fall back on.

Since G.711 applies next to no compression, it delivers clean narrowband voice with minimal processing overhead, at the cost of chewing through more bandwidth than compressed alternatives.

How it works

Each audio sample gets logarithmically compressed down to 8 bits. Over RTP, PCMU runs on static payload type 0 while PCMA uses type 8 (RFC 3551). Counting packet headers, a call generally needs somewhere around 80 to 90 kbit/s in each direction.

G.711 µ-law compared with A-law

VariantRTP nameStatic payload typeSMS Conecta
µ-lawPCMU0Supported
A-lawPCMA8Supported

How it applies to SMS Conecta

SMS Conecta supports both G.711 µ-law (PCMU) and A-law (PCMA). While setting up Zoiper, MicroSIP, Asterisk or another client against sip.smsconecta.com, switch on at least one of these codecs so a call can successfully negotiate.

Codecs: PCMU (G.711 u-law), PCMA (G.711 A-law)
Server: sip.smsconecta.com
Username: <SIP_USERNAME>

Related terms

  • SIP
  • DTMF
  • NAT and STUN
  • Softphone

Frequently asked questions

PCMU or PCMA, which one should I pick?

Both work fine with SMS Conecta; turning on both and letting the call negotiate is the simplest approach.

Why is there still no audio even with G.711 on?

One-way or missing audio is almost always a NAT or firewall problem rather than a codec issue.

Are Opus or G.729 supported?

Only G.711 PCMU and PCMA are listed as supported codecs.

Related pages

Sources