Packet Switching Versus Circuit Switching: Statistical Multiplexing

Packet Switching Versus Circuit Switching: Statistical Multiplexing

Having explained circuit switching and packet switching, let us compare the two. Opponents of packet switching have frequently argued that packet  switching is not appropriate for real-time services (for example, telephone calls and video conference calls) because of its changeable and unpredictable end-to-end delays (due primarily to variable and unpredictable queuing delays). Supporters of packet switching argue that (1) it offers better sharing of bandwidth than circuit switching and (2) it is simpler, more efficient, and less costly to implement than circuit switching. An interesting discussion of packet switching versus circuit switching is [Molinero-Fernandez 2002]. Generally speaking, people who do not like to hassle with restaurant reservations prefer packet switching to circuit switching.

Why is packet switching more efficient? Let's look at a simple example. Assume users share a 1 Mbps link. Also assume that each user  alternates between periods of activity, when a user produces data at a constant rate of 100 kbps, and periods of inactivity, when a user produces no data. Assume further that a user is active only 10 percent of the time (and is idly drinking coffee during the remaining 90 percent of the time). With circuit switching, 100 kbps must be reserved for each user at all times. For instance, with circuit-switched TDM, if a one-second frame is divided into 10 time slots of 100 ms each, then each user would be assigned one time slot per frame.

In this way, the circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simultaneous users. With packet switching, the possibility that a specific user is active is 0.1 (that is, 10 percent). If there are 35 users, the possibility that there are 11 or more simultaneously active users is approximately 0.0004. When there are 10 or fewer simultaneously active users (which happens with possibility 0.9996), the collective arrival rate of data is less than or equal to 1 Mbps, the output rate of the link. Therefore, when there are 10 or fewer active users, userís packets flow through the link essentially without delay, as is the case with circuit switching. When there are more than 10 simultaneously active users, then the collective arrival rate of packets exceeds the output capacity of the link, and the output queue will begin to grow. (It continues to grow until the aggregate input rate falls back below 1 Mbps, at which point the queue will begin to reduce in length.) Because the possibility of having more than 10 simultaneously active users is very small in this example, packet switching gives essentially the same performance as circuit switching, but does so while allowing for more than three times the number of users.

Let's now look at a second simple example. Assume there are 10 users and that one user suddenly produces one thousand 1,000-bit packets, while other users remain inactive and do not produce packets. Under TDM circuit switching with 10 slots per frame and each slot consisting of 1,000 bits, the active user can only use its one time slot per frame to transmit data, while the remaining nine times slots in each frame remain idle. It will be 10 seconds before all of the active user's one million bits of data has been transmitted. In the case of packet switching, the active user can constantly send its packets at the full link rate of 1 Mbps, since there are no other users producing packets that need to be multiplexed with the active user's packets. In this case, all of the active user's data will be transmitted within 1 second.

The above examples demonstrate two ways in which the performance of packet switching can be superior to that of circuit switching. They also highlight the fundamental difference between the two forms of sharing a link's transmission rate among multiple data streams. Circuit switching pre-allocates use of the transmission link regardless of demand, with allocated but unneeded link time going unused. Packet switching on the other hand assigns link use on demand. Link transmission capacity will be shared on a packet-by-packet basis only among those users who have packets that need to be transmitted over the link. Such on-demand (rather than pre-allocated) sharing of resources is sometimes referred to as the statistical multiplexing of resources.

Although packet switching and circuit switching are both common in today's telecommunication networks, the trend has indeed been in the direction of packet switching. Even many of today's circuit-switched telephone networks are gradually migrating toward packet switching. Particularly, telephone networks frequently use packet switching for the expensive overseas portion of a telephone call.



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packet switching, circuit switching, transmission rate

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