CHANNEL RATE VS. MULTIPLEX RATE

One of the major points of confusion, both inside and outside the Telecom industry is the misunderstanding and misuse of channel interface and multiplex rate terms. In publication after publication, the PDH hierarchy is explained by simply calling out the various multiplex levels or signal rates without delineating the difference between the rates that specify channel rates through the network, and the rates resulting from multiplexing.

Channel rates apply to channel interface, which is the point of network access and egress. Most of the PDH multiplex aggregate rates and channel rates covered by global and national standards. Basically, the standards are structured toward use in three regions: Europe and the rest of world (E), except North America (T) and Japan (J). Multiplex levels generally are designated with a DS, meaning digital signal (level) and channel interface and bandwidth with a T, E, or J.

Another factor that is often completely missed is the fact that telecom networks are dominated by voice traffic. The nature of voice traffic caused the network to evolve into circuit switched 64 Kbs channels. These channels are multiplexed into higher order bit streams. An OC192 transmission facility configured to handle only voice traffic contains 192-DS3 equivalents, each capable of 672 64 Kbs channels for a total of 129,024 unique, independent segments of bandwidth. An STM64 configured to handle only voice traffic contains 122,880 bandwidth segments. What about using some of this bandwidth to carry something of a different nature than a 64 Kbs voice? It is entirely possible, and that’s where network channel interface comes into play. From a network transmission facility perspective, a bit is a bit. The transmission network doesn’t know or care if the bit carries voice, data, or anything else, including errors, as long as it meets network clocking and time duration parameters. What this means is that the transmission network will accept access and egress or drop and insert segments of bandwidth that just happen to be sized and structured to fit in place of 24 (T1), 32 (E1), 480 (E3), 672 (T3) voice grade equivalent, or 64 Kbs channels.

Another point of confusion is unchannelized or clear channel rates. This type facility is used for non-voice channel applications such as Internet access, wholesale encrypted traffic (encryption applied to a T1 or group of individual channels rather than individually encrypting each channel), asynchronous transfer mode (ATM) network, physical convergence layer protocol (PLCP) access, compressed program content over E3 or DS3, and other applications that require contiguous bandwidth that cannot operate over 64 Kbs channelized facilities. IP routers are likely to have a PDH wide access network interface at T1/E1/J1 or E3/J3/DS3.