For the last couple decades, TDM connections known as a T1, DS1, or E1 were used to provide connections, the backhaul, between the Base Transceiver Sub-system or Station (BTS) at the cell site and the Mobile Switching Centre (MSC) that provides the phone services and connected the mobile call to the PSTN or a public or private data network.
This TDM connection was supplied by the local or regional carrier as a service to the Wireless Service Provider (WSP), which during the rollout of 3G was usually just one or two T1s (1.5Mbps each). The WSP would then take their supplier’s backhaul connection and groom the bandwidth for the different applications.
A CSU/DSU was used to do this – a few DS0 channels to a 1G BTS, a few more to 2G BTS, one to a remote management device and others as terminal server connections. As bandwidth demands increased to where 6 or more DS1 or T1 connections were required, higher order TDM and SONET connections were demanded from the carrier or transport provider. DS3 (45Mbps) and OC-3 (155Mbps) services were leased and the WSP deployed deMUXing solutions such as M13 multiplexer (MUX) or SONET/SDH ADMs to break down the lower cost per bit DS3 and OC-3 services to the T1 interfaces the existing BTS and CSU/DSUs required to operate. These MUXes, ADMs and DCS were deployed by the WSP at both the both ends of the backhaul connection the cell site and the mobile telephone switching office (MTSO) and any hub or intermediate aggregation site sitting between.