1.3 • THE NETWORK CORE 31
all times. For example, with circuit-switched TDM, if a one-second frame is divided
into 10 time slots of 100 ms each, then each user would be allocated one time slot
per frame.
Thus, the circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simul-
taneous users. With packet switching, the probability that a specific user is active is
0.1 (that is, 10 percent). If there are 35 users, the probability that there are 11 or
more simultaneously active users is approximately 0.0004. (Homework Problem P8
outlines how this probability is obtained.) When there are 10 or fewer simultane-
ously active users (which happens with probability 0.9996), the aggregate arrival
rate of data is less than or equal to 1 Mbps, the output rate of the link. Thus, when
there are 10 or fewer active users, users’ 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 aggregate 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 diminish in length.) Because the probability of having more than
10 simultaneously active users is minuscule in this example, packet switching pro-
vides essentially the same performance as circuit switching, but does so while
allowing for more than three times the number of users.
Let’s now consider a second simple example. Suppose there are 10 users and that
one user suddenly generates one thousand 1,000-bit packets, while other users
remain quiescent and do not generate 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 time 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 con-
tinuously send its packets at the full link rate of 1 Mbps, since there are no other users
generating 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 illustrate two ways in which the performance of packet
switching can be superior to that of circuit switching. They also highlight the crucial
difference between the two forms of sharing a link’s transmission rate among multi-
ple data streams. Circuit switching pre-allocates use of the transmission link regard-
less of demand, with allocated but unneeded link time going unused. Packet
switching on the other hand allocates link use on demand. Link transmission capac-
ity will be shared on a packet-by-packet basis only among those users who have
packets that need to be transmitted over the link.
Although packet switching and circuit switching are both prevalent in today’s
telecommunication networks, the trend has certainly been in the direction of packet
switching. Even many of today’s circuit-switched telephone networks are slowly
migrating toward packet switching. In particular, telephone networks often use
packet switching for the expensive overseas portion of a telephone call.
all times. For example, with circuit-switched TDM, if a one-second frame is divided
into 10 time slots of 100 ms each, then each user would be allocated one time slot
per frame.
Thus, the circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simul-
taneous users. With packet switching, the probability that a specific user is active is
0.1 (that is, 10 percent). If there are 35 users, the probability that there are 11 or
more simultaneously active users is approximately 0.0004. (Homework Problem P8
outlines how this probability is obtained.) When there are 10 or fewer simultane-
ously active users (which happens with probability 0.9996), the aggregate arrival
rate of data is less than or equal to 1 Mbps, the output rate of the link. Thus, when
there are 10 or fewer active users, users’ 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 aggregate 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 diminish in length.) Because the probability of having more than
10 simultaneously active users is minuscule in this example, packet switching pro-
vides essentially the same performance as circuit switching, but does so while
allowing for more than three times the number of users.
Let’s now consider a second simple example. Suppose there are 10 users and that
one user suddenly generates one thousand 1,000-bit packets, while other users
remain quiescent and do not generate 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 time 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 con-
tinuously send its packets at the full link rate of 1 Mbps, since there are no other users
generating 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 illustrate two ways in which the performance of packet
switching can be superior to that of circuit switching. They also highlight the crucial
difference between the two forms of sharing a link’s transmission rate among multi-
ple data streams. Circuit switching pre-allocates use of the transmission link regard-
less of demand, with allocated but unneeded link time going unused. Packet
switching on the other hand allocates link use on demand. Link transmission capac-
ity will be shared on a packet-by-packet basis only among those users who have
packets that need to be transmitted over the link.
Although packet switching and circuit switching are both prevalent in today’s
telecommunication networks, the trend has certainly been in the direction of packet
switching. Even many of today’s circuit-switched telephone networks are slowly
migrating toward packet switching. In particular, telephone networks often use
packet switching for the expensive overseas portion of a telephone call.
