lec21 - ECE 333: Introduction to Communication Networks...

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1 ECE 333: Introduction to Communication Networks Fall 2002 Lecture 21: Switching and Multiplexing Static Multiplexing and Circuit Switching Statistical Multiplexing and Packet Switching 2 Switching and Multiplexing In the last several lectures our focus has been on medium access control techniques for broadcast networks. These networks are commonly used in a LAN environment. In this lecture, we begin to examine point-to-point networks, that is networks that consist of point-to-point links connecting routers and hosts in a mesh topology. This type of network is commonly used in a WAN environment. First, let us consider why there is this difference between the WAN and LAN environement. One reason, as we have seen is that with most MAC techniques, performance goes down as the geographical size and the number of users increases. One solution to this is to use bridges to interconnect LANs. This still has limited use as size of network grows. Usually at most on the order of 10 LANs are interconnected this way. Another reason is differences in economics and traffic characteristics between WANs and LANs. These considerations favor using a point-to-point technology in a WAN.
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3 Recall, from Lecture 1, that the topology of a WAN can be divided into hosts or users and the subnet. The subnet consists of nodes , which are also called switches or routers. Host Subnet Host Host Host Host 4 Routers/switches Routers or switches are multi-input/multi-output devices. Each input/output is called a port and is connected to a transmission link. Switches receive data on input link and transmit the data on an output link. The details of how this is accomplished is referred to as switching. The transmission links in subnet usually carry traffic from several users. How this traffic is combined onto a link is referred to as Multiplexing. The reason for this is again primarily economic. In particular, transmission lines exhibit an economy of scale , what this means is that the cost per unit bandwidth for installing a transmission line is decreasing with the bandwidth. In this lecture we discuss switching and multiplexing in greater detail. Router or Switch
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5 Switching and Multiplexing Conceptually multiplexing is very similar to medium access control. The main difference is with medium access control, the users accessing the channel are spatially separate, so coordination and contention arise. With multiplexing the channel sharing is done at one location, namely in a switch, so contention and coordination are not as difficult (but, as we will see, these issues still can arise.) As with medium access control, multiplexing techniques can be divided into static and dynamic approaches (usually called statistical multiplexing). With static approaches each user is allocated a sub-channel, i.e. a fixed portion of the channel, while with dynamic approaches the allocation varies according to demand. Recall, in Lecture 1 we identified 2 types of switching - circuit switching and
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This note was uploaded on 10/12/2009 for the course ECE ECE 333 taught by Professor Randallberry during the Fall '02 term at North-West Uni..

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lec21 - ECE 333: Introduction to Communication Networks...

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