Figure 4.1 Depiction of the use of FDM across a channel
These Notes are divided into hyperlinked section
Introduction
Why Multiplex Signals?
Frequency
Division Multiplexing
Time Division Multiplexing
Synchronous
Time Division Multiplexing
Statistical
Time Division Multiplexing
TDM in T1 and E1
TDM Hierarchies
Multiplexing Bursty
Traffic
Conclusion
Further Reading
Web Resources
This lecture will:-
· Discuss the need and uses of multiplexing
· Compare FDM with TDM
· Compare subdivisions of TDM that have created
Synchronous and Statistical Time Division Multiplexing
· See how TDM is used in T1 and E1 channels
· View TDM Hierarchies
When a large number of separate signals need to be carried across a single transmission medium, some form of sharing is needed to carry the signals over the medium. The result it that many low bandwidth signals can share the high bandwidth of the transmission medium.
There are two main types of multiplexing in use:
· Frequency Division Multiplexing
· Time Division Multiplexing
Frequency Division Multiplexing
In FDM, each signal is given a share of the bandwidth
of the channel. The share is a range of frequencies that are allocated
to each signal. This means that several signals may be carried across the
transmission medium simultaneously. This is the case with DWDM. This is
also the technology used in Modems.
Figure 4.1 Depiction of
the use of FDM across a channel
The signals that are carried by the channel are analogue and as such susceptible to picking up interference. This cannot be removed from the signal. Each channel in the scheme needs a dedicated Modem at each end of the channel to place the signal on a carrier and remove it at the far end.
This technique may be thought of as analogous to many lanes of vehicles travelling at the same speed on a motorway with the changing of lanes being forbidden. To separate the vehicles there must be a gap between lanes to prevent the vehicles coming into contact with other vehicles in adjacent lanes.
With FDM, the data arriving can be either digital or analogue.
Each channel must have a modem (modulator/ demodulator) pair at the ends
of the multiplexed stream. The input signals are modulated onto separate
carrier frequencies.
Figure 4.2 Using FDM to multiplex 3 signals onto one carrier
Each channel has an individual bandwidth centred about its own carrier frequency and the range of frequencies that each channel occupies is arranged so as not to interfere with adjacent channels, the channels being separated by guard bands. Each signals is modulated onto a different carrier frequency.
Before the advent of fibre optic cabling, this was the technology that carried signals over coaxial cabling. This technique is simple but inefficient in terms of bandwidth. It is used for low traffic flows.
Examples of FDM include radio and television broadcasts, wireless LANs and DSLs.
TDM can only be used when the data rate of the transmission medium exceeds the sum of the data rates of the signals to be carried. TDM allows users that are sharing the line to send data during a short time slice determined by the demand (i.e. number of users requesting the channel) for transmission. Each user has total use of the transmission line for a short time period. The data are arranged into frames.
TDM can be further subdivided into Synchronous Time-Division Multiplexing (Syn-TDM) and Statistical Time Division Multiplexing (Stat-MUX).
Synchronous Time-Division Multiplexing
In this technique, signals are carried on the high capacity transmission line by interleaving portions of each signal in time. The interleaving can be at the bit level or alternatively at the bytes level. In the diagram below, data from individual channels are put sequentially into a time frame (slot) that corresponds to each of the channels that require to be carried across the link.
Figure 4.3 Synchronous Time Division Multiplexing
From the above diagram it can be seen that the multiplexed link is able to carry 6 separate channels of data. Incoming data from each source is buffered, the buffers being equal to one time slot in length. The buffers are scanned sequentially to form a composite data stream. This scan takes less time than the time required to fill the buffer. Each channel in TDM is allotted a fixed time slice. TDM can only operate in synchronous mode, thus naming this form of multiplexing. The multiplexer combines data from the 6 input lines and transmits them one after the other over the higher capacity data link. The data carrying capacity (bits per second) of the multiplexed link must be at least equal to the sum of the 6 individual inputs.
The demultiplexer accepts the multiplexed data stream, separates the data according to the channel, then delivers them to the appropriate output lines. Because the signaling used on the multiplexed link is digital, the signals occupy the entire spectrum of frequencies that the link can carry.
An advantage of TDM is that each incoming line always has a channel available for transmission across the link so the line will never be busy if the device using that channel wishes to transmit data.
A disadvantage is that each channel gets a share of the transmission line regardless of whether it has data to be transmitted or not and thus empty slots will be transmitted when input(s) are sending no data.
Statistical Time division Multiplexing
An alternative to TDM is statistical TDM which uses the same technique as discussed above, except the mathematical sum of the data capacities of the inputs exceeds the data capacity of the multiplexed stream.
This is possible because, on average, the input lines are not all transmitting at the same time. The MUX dynamically allocates time slices on demand. As in Syn-TDM, the channels are scanned sequentially and the MUX stops at any channel that has a message to be transmitted. The time for each channel is fixed and can be bit, byte or block. This method saves transmitting empty slots. As the position of each time slot varies according to demand, addressing information needs to be sent along with the data to ensure proper delivery.
Figure 4.4 Statistical Time division Multiplexing
Because there is a need to identify the sender of the signal within a slot, bandwidth is taken up with source identification information. This is the overhead that Stat-MUX must carry.
A form of Stat-MUX is used to carry signals across fibre optic cables. This was used in PDH and is employed in SONET/ SDH today.
A T1 channel is used in North America for carrying voice or data signals. It has a bit rate of 1.544 Mbit/ sec which is equivalent to 24 X 64 kbit/ sec voice channels.
An E1 channel is the ITU-T specification followed in Europe for carrying voice or data signals. It has a bit rate of 2.048 Mbit/ sec which is equivalent to 30 X 64 kbit/ sec voice channels.
In both T1 and E1, frames are generated every 125msec, in other words 8000 frames per second.
The frames are then further subdivided into bytes and
each tributary signal is assigned a byte within the frame. By allowing
the frame to be divided into many byte slots allows many signals to share
a channel.
Figure 4.5 The Operation of Time Division Multiplexing
This technique is used for multiplexing digital signals onto a digital transmission line. Regardless of the nature of the signals upon entry to the transmission system (digital or analogue) they must be transmitted to their respective destinations. Analogue speech signals are sampled, quantised and encoded into digital form.
In practice TDM frames last 125 ms giving a total of 8000 frames per second. Each frame in the multiplexed stream has a framing bit or byte (depending upon the implementation scheme, USA or Europe) to mark the start of each frame. This is to enable the receiving end to decode the multiplexed stream. The number of slots per frame varies with location and data rate.
There are two major systems in use, those following the North American System and those following the European system. In Europe and USA, one byte is inserted per slot and each slot carries one channel of 64 kbit/sec. The major difference is that the North American System carries 24 slots per frame and European scheme carries 32 slots per frame.
For an American T1 channel capable of carrying 1.544 Mbit/second, one 125 ms frame has 24 time slots plus one framing bit. Each T1 time slot carries one 64 kbit/sec voice grade channel.
The European standard allows 32 time slots per 125 ms
frame using slots 0 and 16 for the framing byte and signalling byte respectively.
This leaves 30 free slots for data channels. The total capacity of an E1
channel is 2.048 Mbit/sec although the useful capacity is 1.92 Mbit/sec,
the rest being taken up as overhead.
Figure 4.6 North American Example of TDM on T1 Channel
Figure 4.7 European Example of TDM channel on E1 Channel
Over the years there have been developed several TDM hierarchies, the most common of which are depicted below in table 4.1.
Table 4.1 Comparison between US and European Digital Hierarchies
The hierarchies depicted above create a system where for instance six T3 channels can be multiplexed onto one T4 channel and four E4 channels can be multiplexed onto one E5 channel. If we wish to obtain a voice channel from an E5 channel it must first be taken down to E4 then E2 before we can recover the signal.
Provided that the transmitting and receiving multiplexers remain in synchronisation it is a relatively simple job to recover the relevant input channels at the receiving end.
The nature of traffic flowing onto networks is unpredictable
and has been termed bursty. This means that the data may arrive in a certain
period of time and then nothing for another period of time. The transport
schemes that are used today within the Internet are capable of coping with
bursty traffic.
Figure 4.8 Statistical Multiplexing of bursty traffic
Buffering will be required within the SMUX to cope with the bursty nature of the traffic arriving on the input channels. Asynchronous Multiplexing is an extension of statistical multiplexing where the tributary signals are not related in time. They may have differing data rates too. This is a key innovation and is used in ATM. Substantial buffering is needed in the multiplexer. This will be covered later in the course.
We have seen that the reason for multiplexing is to allow many signals to share a single transmission medium.
There are two main types of multiplexing, Frequency Division Multiplexing and Time Division Multiplexing. FDM is used to send many channels simultaneously whereas TDM allots time slices to transmissions.
TDM is further subdivided into Synchronous TDM and Statistical TDM. Synchronous TDM has a fixed number of input channels that are scanned sequentially for transmissions, but is wasteful of bandwidth if a channel has nothing to send, a slot still being allocated to it.
Statistical TDM is used to allow many channels to share a transmission medium, allocating slots dynamically, on-demand to a channel that needs to send data. It has a drawback that bandwidth is wasted on overhead.
TDM is used in T1 and E1 channels to carry many signals over a medium. There exists a hierarchy of channels to allow many T1 or E1 channels to be carried over a higher capacity link.
Statistical TDM solves the problems caused by the bursty nature of today’s Internet traffic.
Tannenbaum, Computer Networks:
(c) M Clements 2001