11.2. Telecommunication fundamentals
Figure 11.1 shows a switching network. Lines are the media links. Ovals are called network nodes. Media links simply carry data from one point to other. Nodes take the incoming data and route them to an output port.
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Figure-11.1: Switching network
If two different communication paths intersect through this network they have to share some resources. Two paths can share a media link or a network node. Next sections describe these sharing techniques.
11.2.1. Media sharing techniques
Media sharing occurs when two communication channels use the same media.
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Figure-11.2: Media sharing
This section presents how some communication channels can use the same media link without architecture considerations. There are three main techniques.
11.2.1.1. Time Division Multiple Access (TDMA)
This simple method consists on multiplexing data in time. Each user transmits a period of time equal to 1/(number of possible channels) in full bandwidth W. This sharing mode can be synchronous or asynchronous.
Figure 11.3 shows a synchronous TDMA system. Each channel uses a time slot each T periods. Selecting a time slot identifies one channel. Classical wired phone uses this technique.
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Figure-11.3: Synchronous TDMA diagram
In synchronous TDMA, if an established channel stops transferring data without freeing the assigned time slot, the unused bandwidth is lost and hence, other channels can not take advantage of this. This technique has evolved to asynchronous TDMA to avoid this problem.
Figure 11.4 shows an asynchronous TDMA system. Each channel uses a time slot when the user needs to transfer data and when a time slot is unused. The header of each time slot data stream identifies the channel identification. ATM networks use this technique.
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Figure-11.4: Asynchronous TDMA diagram
These two techniques are used to connect users. Providing broadcast channels in TDMA can not be done easily. Frequency Division Multiple Access technique avoids this problem. Next section presents this sharing mode.
11.2.1.2. Frequency Division Multiple Access (FDMA)
This sharing method consists on giving to each channel a piece of available bandwidth.
Each user transmits over a constant bandwidth equal to W/(number of possible channels). Filtering with a bandwidth equal to W’ = W/(number of possible channels) the whole W bandwidth spectrum selects one channel. TV and radio broadcasters use this media sharing technique. Figure 11.5 shows an FDMA spectrum diagram.
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Figure-11.5: FDMA diagram
Another method has been developed based on the frequency dimension. This method called Code Division Multiple Access uses an encoding-decoding system used, initially, for military communications. Today consumer market applications also use this technique. Next section presents this method.
11.2.1.3. Code Division Multiple Access (CDMA)
Each user transmits using the full bandwidth. Demodulating the whole W band using a given identification code selects one channel out of the others. Next mobile phones standard (IS-95 or W-CDMA) uses this media sharing technique. Figure 11.6 shows a CDMA spectrum diagram.
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Figure-11.6: CDMA diagram
These techniques can be merged together. For example, the Global System Mobile (GSM = Natel D) phone standard uses an FDMA-TDMA technique.
After this description, we will present in next section how a network node routes data from an input port to a given output port.
11.2.2. Node sharing technique
Node sharing occurs when two communication channels use the same network node. The question is how some communication channels can use the same node in a cell switching network, i.e. an ATM network.
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Figure-11.7: Shared node
Before answering this question, we have to define the specification of the switching function. Next section presents this concept.
11.2.2.1. Switching function
As shown in figure 11.8, a switch has N input ports and N output ports. Data come in the lines attached to the input ports. After identifying their destination, data are routed through the switch to the appropriate output port. After this stage, data can be sent to the communication line attach to the output port.
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Figure-11.8: Canonical switch
We can directly implement on hardware this canonical switch. However, this technological solution poses some throughput problems. In section 11.6.1.2.1 (the one describing the crossbar switch architectures) we will see why. In section 11.6.1.2.2 (the one describing the Batcher-Banyan network) we will see how the throughput problems can be solved.
Furthermore, the incoming data sequence can pose some routing problems. Next part of this section shows these critical scenarios.
11.2.2.2. Switching scenario
Figure 11.9 shows some switching scenarios. Scenario 1 shows two cells from two different input ports going through the switch to two different output ports. These two cells can be simultaneously routed. Scenario 2 shows two cells from the same input port going through the switch to two different output ports. Both cells are routed to their output destinations.
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Figure-11.9: Three switch scenarios
Scenario 3 shows two cells from two different input ports going trough the switch to the same output port. There are five possible strategies to solve this problem:
Section 11.6.1 considers why output buffering is better than input buffering.
