The terms E1 refers to the standard ISDN primary rate lines used in Europe and most of the world apart from North America and Japan. The E1 ISDN primary rate line is a bundled trunk link of 30 bearer channels and two signal channels. The traffic carrying bearer channels are numbered 0 to 14 and 16 to 30 with channels 15 and 31 carrying signaling traffic. Each bearer channel is 64Kbps and the 30 channels provides for 2.048 Mbps bandwidth. E1 ISDN can be used for both voice traffic and be connected to a PBX to provide 30 incoming/outgoing external calls each call being assigned the standard PSTN 64Kbps TDM standard format. Alternatively an E1 can be provisioned as an un-channeled single 2Mbps data link for packet switching networks.
E1 trunk links are important in VoIP as they are required to connect the IP/PBX to the PSTN. The E1 trunk is often provisioned by the telephone company in partial amounts so that dependent on the size of the company and the required number of external PSTN lines required for calling or receiving phone calls from real non-VoIP telephones you can rent only the line you require. When used a data link it can be used to provision a direct IP private link to a virtual hosted IP/PBX which guarantees bandwidth and throughput. This can be necessary if the normal ADSL internet links are not reliable enough due to heavy congestion on the internet. E1 are private lines sometimes referred to as leased lines, in contrast ADSL broadband lines are shared lines with contention ratios varying between 10:1 and 30:1 for business and residential respectively. Therefore an E1 can make a big difference to the performance and quality of the VoIP system however they are more expensive than even business ADSL.