You may wonder why a T3 or DS3 is often referred to as a 45 Mbps circuit. The fact is that a clear channel DS3 circuit is actually capable of speeds close to 50 Mbps. Most hardware vendors support transmission rates of 45 Mbps on their DS3 interfaces. If the DS3 service leased from the telco is channelized, the actual throughput of the circuit is a little less than 45 Mbps because of the signaling overhead of maintaining the 28 individual DS1 channels with the DS3. IF the DS3 is not channelized, the transmitting and receiving interfaces can utilize the full 45 Mbps. The European equivalents to the T1 and T3 are the E1 and the E3. An E1 is essentially a bundle of 32 channels and has a bandwidth capacity of 2.048 Mbps. An E3 consists of 16 E1 channels and has an available bandwidth of 34.368 Mbps (including overhead).
For more information on Cisco Router Configuration and Troubleshooting, Second Edition, visit http://www.newriders.com/cfm/prod_book.cfm?view=1&RecordID=644.