Computer networks, put simply, are digital telecommunications networks with a series of connected nodes that share resources with each other.
Devices use data links (either cable, optic cable or wireless) to exchange data with each other. Nodes, then, could refer to phones, servers, personal computers and networking hardware, and any two devices that are exchanging information can be considered a network.
Computer networks can be traced back as far as the late 50s, when the Department of Defense set up the Semi-Automatic Ground Environment (SAGE) for radar installations.
By the mid 60s, computer systems were being set up for land-line telephone networks, and ARPANET came into being in 1969, as a packet-switching network for advanced research at the national laboratory and university level.
From there came research on hierarchical routing and the primitive Ethernet system of the early 70s. By 1995, transmission speed for Ethernet jumped from 10 Mbit/s to 100 Mbit/s.
These, of course, were all Stone Age computer networks by today’s standards.
As higher speeds and new technologies such as fiber optic cable came to fruition, time-division multiplexing became a necessity.
What Is Time Division Multiplexing?
The concept of time-division multiplexing goes all the way back to the 1870s, when telegraph companies found a need to handle multiple transmissions concurrently over a single telegraph transmission line.
By WWII, microwave transmission and multiplexing was used for secured military communications in Europe. This technology then evolved to microwave relay stations for telephone communications and long-distance calling.
Later, relay setups were needed for the audio portion of television signals. These setups needed to be able to handle greater capacity since television’s audio signal required much more bandwidth than radio.
In 1962, Bell Labs developed technology that could combine 24 digitized phone calls over a single line, with a channel bank that could slice the signal into 8,000 separate frames of 24 bytes each. Each byte then represented a single telephone call, encoded with a constant bit rate signal of 6 kbit/s.
Time-division multiplexing can be thought of as two or more signals or bit streams running concurrently as sub-channels of a single communications channel, but “taking turns” being carried on the channel. The time domain is broken down into several time slots of specific length, one for each sub-channel, with a data block of sub-channel 1 being allotted time slot 1, sub-channel 2 during time slot 2, and so on.
Frames In Time Division Multiplexing
In packet switching data systems, a frame can be thought of as the “container” for a single packet. In time-division multiplexing systems, the frame is the recurring structure and includes frame synchronization features. These features are a code of bits or symbols that denote the beginning and end of the data being streamed.
This frame is a “data block” with a specific number of time slots, corresponding to time-division multiplexing channels.
An example might be the SONET/SDH and ISDN circuit switched B-channel. The frame includes a synchronization channel and possibly an error correction channel along with the synchronization. The information goes through, with error correction and synchronization. Then the cycle starts over with a new frame starting with the second sample or data block.
Applications Of Time Division Multiplexing
Plesiochronous digital hierarchy
PDH allows data streams to run simultaneously at the same rate, but with some offset. The data rate is controlled by a clock, with variance of as much as 50 ppm or 2048kbit/s. In other words, the data streams may run at slightly different rates from each other.
PDH is an older system that’s designed for four-wire copper cable or fiber cable in a digital telephone network. The “gap” of frames running slightly out-of-sync with each other can be “stuffed” to include a synchronization word, allowing the decoder to identify the start of each frame. Others include control bits which identify whether the stuff-able bit contains data or not.
Synchronous digital hierarchy
Also known as synchronous optical networking (SONET), SDH/SONET has superseded PDH. One of the biggest differences in the two technologies is the tight synchronization across entire SDH networks, governed by atomic clocks. Both SONET and SDH can encapsulate earlier digital transmission protocols, such as PDH, or can support either Asynchronous Transfer Mode or packet-over-SONET/SDH networking. In other words, SDH and SONET are generic, versatile all-purpose transport systems for either voice or data.
SDH can operate at 155.520 Mbits/s, with an additional basic unit of transmission that operates at 51.84 Mbit/s. SDH is also part of 10 Gigabit Ethernet technology, which uses a lightweight SDH/SONET frame to encapsulate Ethernet data, making it compatible with equipment designed for SDH/SONET signals. 10 Gigabit Ethernet isn’t necessarily interoperable with SDH/SONET equipment at the bitstream level. This makes it different from WDM system transponders, which can usually support thin-SONET -framed 10 Gigabit Ethernet.
Basic Rate Interface
Having been the standard for voice-grade telephone service, BRI is an integrated services digital network (ISDN) configuration. It consists of two data (bearer) channels at 64 kbit/s each and one control channel at 16 kbit/s. The data channel is used for data or voice, while the control channel can be used for control/signaling, data, or X.25 packet networking. The two data channels can be aggregated by channel bonding for an overall data rate at 128 kbit/s.
Resource Interchange File Format (RIFF)
Developed by Microsoft, RIFF is a container format for familiar audio file container systems such as AVI, ANI or WAV. It consists of “chunks,” each of which devotes four bytes to an ASCII identifier, four bytes to an unsigned, little-endian 32-bit integer that denotes the length of the chunk, a variable-sized field for the chunk data, and a pad byte (if the chunk’s length is odd). RIFF is an audio standard that weaves left and right stereo signals on a per-sample basis.
Multiplexed Digital Transmission
In a legacy system such as the public switched telephone network, it’s necessary to carry multiple calls over the same transmission medium to make the most of the cable’s bandwidth. TDM allows the telephone infrastructure’s switching mechanisms to create discrete channels within that transmission stream.
A standard voice signal has a bit rate of 64 kbit/s, and each voice slot in a TDM frame can be considered a channel. The TDM circuit, however, runs at a bandwidth that’s much higher, allowing the bandwidth to be divided into time slots for each signal, which is then transmitted in multiplex through the medium. European TDM systems contain 30 digital voice channels, and American systems contain 24 channels. Both systems feature extra bits or bit time slots devoted to synchronization and signaling.
Anything more than 24 or 30 digital voice channels is considered “higher order multiplexing.” Higher order multiplexing involves multiplexing the standard TDM channels.
A European 120-channel TDM frame is a multiplex of four standard 30-channel TDM frames. Four TDM frames from the immediate lower order can be combined.
Statistical Time-Division Multiplexing
One of the newer advances in time-division multiplexing, STDM enhances routing by transmitting the address of the terminal and the data itself simultaneously. It allows for bandwidth to be split over one line, making it a great system for college or enterprise-level systems.
With a 10-Mbit line in a network, STDM can provide 178 terminals with a 56k connection each, or can grant the bandwidth on an as-needed basis. STDM assigns a slot when a terminal actually needs data to be received or sent, rather than parceling out time slots for each terminal. It’s used for circuit mode setups, with constant bandwidth per channel, and a scheduling algorithm reserves time slots in each frame on a dynamic, real-time basis, based on traffic demand.
Dynamic TDMA is integral to:
- Dynamic synchronous transfer mode
- IEEE 802.16a
You may not give much thought to scalable time division multiplexing, but it’s part and parcel of every phone call and every piece of data that you receive through the internet or through your phone.
It’s come a long way since the days of microwave relay stations for long-distance phone calls, but the principle remains the same over the decades.