Communication Networks MAP-TELE 2011/12 José Ruela Network basic mechanisms Multiplexing Multiplexing • Multiplexing is a technique that allows a single resource to be shared among multiple users – sharing can be static or dynamic • Multiplexing in a physical link means that multiple signals, which represent different flows of information, are simultaneously transmitted over the medium – a separate channel is provided to each flow • Different methods are possible to realize such “physical” multiplexing, either on point-to-point or multipoint (broadcast) links • On point-to-point links, the basic multiplexing techniques divide the physical link into channels along one of three dimensions – frequency, time or wavelength, but a combination of multiplexing techniques is possible –FDM –Frequency Division Multiplexing – TDM – Time Division Multiplexing –WDM –Wavelength Division Multiplexing (optical domain equivalent of FDM) • Multiplexing (channelization) in multipoint links is one solution (among others) to the multiple access problem (to be studied separately) Multiplexing L1 R1 L2 R2 Mux 1 Mux 2 L3 R3 FDM – Frequency Division Multiplexing • FDM is an analogue technique that divides the bandwidth of a link (in Hz) into channels of smaller bandwidth – the channels occupy different (non overlapping) frequency bands – Example: multiplexing 12 analogue voice channels (4 kHz nominal bandwidth) into a link with bandwidth equal to 48 kHz • Signals generated by different transmitters can be analogue or digital and modulate carriers on different frequencies (thus occupying a different band or frequency slot) – For digital signals it is possible to relate the channel capacity (in bit/s) with its bandwidth (in Hz) • FDM creates circuits of fixed bandwidth and thus has a number of limitations (especially for data traffic) – The bandwidth is shared among a maximum fixed number of flows, which has to be decided in advance, and thus it is not practical to resize the bandwidth quantum (let alone change it in a dynamic way) – The bandwidth of an idle channel cannot be reused by active flows – The bandwidth of a channel allocated to a flow cannot be reused when the flow is temporarily idle (case of variable bit rate flows) FDM – Frequency Division Multiplexing • Transmission in all channels is simultaneous, on separate frequency bands (for digital signals, the bit rate of a channel is a fraction of the total bit rate that may be achieved on the link) B (Hz) CH 4 CH 3 CH 2 CH 1 t FDM – Frequency Division Multiplexing FDM – Frequency Division Multiplexing WDM – Wavelength Division Multiplexing • WDM is conceptually similar to FDM, but the multiplexing and demultiplexing functions are realized on optical signals transmitted through optical fibers on different wavelengths () TDM – Time Division Multiplexing • TDM is a digital technique that divides the capacity of a link (in bit/s) into multiple channels using temporal interleaving – the channels occupy different (non overlapping) time slots • The basic form of TDM is Synchronous Time Division Multiplexing (STM) – fixed size time slots allocated to each channel repeat periodically and time slots in the same cycle constitute a frame – Example: for a frame made up of 32 time slots with a period of 125 s and 8 bits transmitted per slot, the channel capacity is 64 kbit/s • A channel is identified by the position of the slot it occupies in the frame (position multiplexing), which requires a mechanism for identifying the start of a frame (frame synchronization) • Each flow is transmitted on the multiplexed link at the link rate and a conversion to the original rate occurs at the demultiplexer – the flow suffers a fixed delay (that corresponds to the time to access its slot) • Since STDM provides channels of fixed capacity, it has the same limitations as FDM • The flexibility of TDM allows exploiting alternatives that overcome the limitations of STDM STDM – Synchronous Time Division Multiplexing • Channel transmissions are not “truly” simultaneous – transmission on each channel occurs at the link rate during a fraction of the frame time (on its time slot), but temporal transparency (with some delay) is achieved by the joint operation of the multiplexer and demultiplexer C (bit/s) CH 1CH 2 CH 3 CH 4 CH 1CH 2 CH 3 CH 4 t Frame i Frame i + 1 STDM – Synchronous Time Division Multiplexing Limitations of STDM • In the first place, STDM provides fixed bit rate channels – This is adequate for constant bit rate but not for variable bit rate flows – Even for constant bit rate flows, the channel rate may not match the natural bit rates of the flows • For variable bit rate flows, the channel rate limits the maximum instantaneous bit rate of the flow – The channel is not fully used when the flow is transmitting with an average rate lower than the channel rate (there will be idle periods) – Handling bursts with an instantaneous peak rate higher than the channel rate requires buffering, which introduces variable delays (and even losses) • A channel may remain idle during some time periods, whenever the corresponding time slots are not allocated to any flow • Time slots not used for any of these reasons cannot be reused by active flows (but this would be desirable when the instantaneous bit rate of a flow exceeds the capacity of its channel) Limitations of STDM – examples Frame i Frame i+1 Frame i+2 CH1 CH2 CH3 CH4CH1 CH2 CH3 CH4 CH1 CH2 CH3 CH4 1 2 3 Idle slot • 1 – all channels are fully used • 2 – channel 3 is idle (all corresponding slots are unused) • 3 – some slots are unused due to variable bit rate traffic on some channels STDM – channel aggregation • It is possible to aggregate channels to form higher bit rate channels, by allocating multiple slots to a channel • In this way it is possible to create channels of different rates, but the bit rates of such channels form a discrete set (the possible values are multiples of the basic rate) – Since such channels are of fixed rates, they have the same problems as the basic channels • Creating such channels in a static (fixed) way is straightforward – a channel is allocated a number of slots in known positions on the frame (and this may be achieved by configuration) • The application of this concept to switching (multi-rate switching) is highly complex because it would be necessary to preserve the temporal order of the slots throughout the network • All these limitations may be overcome by dynamically allocating slots to channels, which is more efficient but more complex – this is the idea behind Asynchronous Time Division Multiplexing (ATDM) ATDM – Asynchronous Time Division Multiplexing • ATDM is based on a dynamic allocation of time slots to channels, which may be performed on different ways • Starting with an STDM frame structure and fixed size slots, it is necessary to identify the channel that is associated with each time slot, since the number and position of slots allocated to a channel are not defined in advance (in particular, they depend on the traffic submitted to each channel and slot availability) – One possible solution is to include on each frame a map of the slot usage, which associates a channel identifier with each slot – Another solution is to carry on each slot a channel identifier (label) together with the data (label multiplexing) • The need for a channel identification introduces overhead, which may be very high when the slots carry a small number of bits • An alternative solution is not using physical frames but allocating time slots of variable size to carry variable length data frames, which carry an identifier on their header (e.g., addresses or labels) – A delineation mechanism for data frames (framing) is necessary ATDM – Asynchronous Time Division Multiplexing • Channels created by ATDM are logical channels, since resources (time slots) are not previously reserved and allocated in a fixed way – Resources are allocated on demand to competing flows and thus statistical multiplexing can be exploited – Competition is arbitrated by means of scheduling mechanisms that may implement different strategies (based on the characteristics and requirements of the traffic flows, possibly of different classes) – As a result of dynamic allocation of resources, channel rates may have any value in a continuous range • A transmission link may be divided into multiple physical channels (using, for example STDM) or handled as a single physical channel – In either case, a physical channel may carry multiple logical channels by means of ATDM • ATDM is the basis for Packet Switching – a packet switch has multiple input and output ports (a multiplexer has a single output port) – Incoming and outgoing links carry ATDM flows – Flows are demultiplexed at each input port, independently switched to output ports (spatial switching), where they are again multiplexed ATDM – example ■■■ STDM vs. ATDM • Consider input links with Capacity Ci, which places an upper limit on the bit rate of a flow (its peak rate), and bursty flows with an average rate Ri • An important question is how to choose the capacity C of the transmission link in STDM and ATDM Ci Ci 1 1 2 M M 2 A C U U B X X N N STDM vs. ATDM • In STDM the capacity of the multiplexed link is C = Ci (ignoring overhead bits) – the multiplexed link is equivalent to N independent links, each with capacity Ci – The ratio Ri / Ci is critical (as far as resource utilization vs. delay) • In ATDM, statistical multiplexing may be exploited – The average rate of all multiplexed flows is R = Ri – The aggregate flow is expected to have a smoother pattern than the individual flows (this is the basis for statistical multiplexing) –Ci is usually chosen so that Ri is (much) lower than Ci (this means that the input link may handle bursts with low delay) – The capacity of the multiplexed link (C) must be higher than R, but does not need to be as high as Ci (R = Ri < C < Ci) – this means that traffic is concentrated – When the multiplexed link is moderately loaded, an active flow sending at its peak rate (Ci) may profit from the spare capacity on the multiplexed link (this would not be possible if Ci was chosen with a value slightly higher than Ri – the input link would be a bottleneck for the flow) STDM vs.