being transmitted on a freely chosen set of timeslots or on a different physical channel. When using E1 frames for data communication, some systems use those timeslots slightly differently, either TS0: Framing, TS1-TS31: Data traffic — This is named Channelized E1, and is used where the framing is required, it allows any of the 32 timeslots to be identified and extracted. TS0-TS31: Data traffic — Often referred to as Clear Channel E1 or Unchannelized, it is used where no framing is required, timeslot extraction is not required and the full bandwidth (2 Mb/s) is required. Hierarchy levels The PDH based on the E0 signal rate is designed so that each higher level can multiplex a set of lower level signals. Framed E1 is designed to carry 30 or 31 E0 data channels plus 1 or 2 special channels, all other levels are designed to carry 4 signals from the level below. Because of the necessity for overhead bits, and justification bits to account for rate differences between sections of the network, each subsequent level has a capacity greater than would be expected from simply multiplying the lower level signal rate (so for example E2 is 8.448 Mbit/s and not 8.192 Mbit/s as one might expect when multiplying the E1 rate by 4). Note, because bit interleaving is used, it is very difficult to demultiplex low level tributaries directly, requiring equipment to individually demultiplex every single level down to the one that is required. See also D 0 (DS0) Digital Signal 1 (DS1, T1) HDB3 encoding scheme List of device bandwidths Multiplexing Plesiochronous Digital Hierarchy STM-1 T-carrier Time-division multiplexing Nonblocking minimal spanning switch - discussion of practical telephone switches. Clos network - the mathematics of telephone switches. References External links Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services eBook Telecommunications standards Multiplexing Telecommunication protocols