The E1 signal format carries data at a rate of 2.048 million bits per second and can carry 32 channels of 64 Kbps each. E1 carries at a somewhat higher data rate than T-1 (which carries 1.544 million bits per second) because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use.
T1 -- the T-carrier systemintroduced by the Bell System in the U.S. in the 1960s, was the first successful system that supported digitized voice transmission. The original transmission rate (1.544 Mbps) in the T1 line is in common use today in Internet service provider (ISP) connections to the Internet. Another level, the T3 line, providing 44.736 Mbps, is also commonly used by Internet service providers.
So in summary, an E1 can carry 32 voice channels and a T1 can carry 24 voice channels. From a VoIP perspective we do not have the concept of voice channels -- VoIP calls are calculated on how much bandwidth they use and this depends on the voice codecs applied. The PSTN equivalent is G.711 and this uses 64 Kbps, but with header and signaling overheads, it's at 100 Kbps per call. However for cost effectiveness, VoIP uses compression techniques to maximize available bandwidths For example, the g729a ITU-T standard for voice coding compresses 8 KHz linear audio signals and encodes them for transmission at 8 Kbps. With signaling overheads, this equates to 19 Kbps per call. g729a is the most common VoIP codec used by business customers over WANs. Therefore, an E1 with 2.048 Mbps bandwidth can handle a maximum of 107 simultaneous calls -- although it would never be loaded to threshold because the loading would be high for the terminal equipment processors and this could lead to packet discard and voice quality issues.
ISDN -- ISDN 2 has two 64Kbps voice channels. VoIP is not applied on this access technology. VoIP is applied to DSL technologies and the number of calls is dependent on upstream bandwidth purchased. So a typical ADSL of 256 Kbps upstream bandwidth could in theory carry 13 simultaneous calls. Again, service providers would never load to this level. Internet telephony providers tend to use g723 codecs at 6.3 Kbps. Most ADSL service providers do not offer quality of service on ADSL and therefore voice quality is not guaranteed. Voice quality will be compromised should speech be attempted when sending or receiving large files that breach the bandwidths available.
ISDN30 -- 30 voice channels. It is often used as a PSTN connection to IP PBXs via voice gateways -- such as, Cisco AVVID.
This was first published in May 2006