1.3 • THE NETWORK CORE 29
Specifically, the link dedicates a frequency band to each connection for the
duration of the connection. In telephone networks, this frequency band typically
has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). The width
of the band is called, not surprisingly, the bandwidth. FM radio stations also use
FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each
station being allocated a specific frequency band.
For a TDM link, time is divided into frames of fixed duration, and each frame
is divided into a fixed number of time slots. When the network establishes a connec-
tion across a link, the network dedicates one time slot in every frame to this connec-
tion. These slots are dedicated for the sole use of that connection, with one time slot
available for use (in every frame) to transmit the connection’s data.
Figure 1.14 illustrates FDM and TDM for a specific network link supporting up
to four circuits. For FDM, the frequency domain is segmented into four bands, each
of bandwidth 4 kHz. For TDM, the time domain is segmented into frames, with four
time slots in each frame; each circuit is assigned the same dedicated slot in the
revolving TDM frames. For TDM, the transmission rate of a circuit is equal to the
frame rate multiplied by the number of bits in a slot. For example, if the link trans-
mits 8,000 frames per second and each slot consists of 8 bits, then the transmission
rate of a circuit is 64 kbps.
Proponents of packet switching have always argued that circuit switching is
wasteful because the dedicated circuits are idle during silent periods. For example,
FDM
4KHz
Link Frequency
4KHz
TDM
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Slot Frame
Time
Key:
All slots labeled “2” are dedicated
2
to a specific sender-receiver pair.
Figure 1.14 With FDM, each circuit continuously gets a fraction of the
bandwidth. With TDM, each circuit gets all of the bandwidth
periodically during brief intervals of time (that is, during slots)
Specifically, the link dedicates a frequency band to each connection for the
duration of the connection. In telephone networks, this frequency band typically
has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). The width
of the band is called, not surprisingly, the bandwidth. FM radio stations also use
FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each
station being allocated a specific frequency band.
For a TDM link, time is divided into frames of fixed duration, and each frame
is divided into a fixed number of time slots. When the network establishes a connec-
tion across a link, the network dedicates one time slot in every frame to this connec-
tion. These slots are dedicated for the sole use of that connection, with one time slot
available for use (in every frame) to transmit the connection’s data.
Figure 1.14 illustrates FDM and TDM for a specific network link supporting up
to four circuits. For FDM, the frequency domain is segmented into four bands, each
of bandwidth 4 kHz. For TDM, the time domain is segmented into frames, with four
time slots in each frame; each circuit is assigned the same dedicated slot in the
revolving TDM frames. For TDM, the transmission rate of a circuit is equal to the
frame rate multiplied by the number of bits in a slot. For example, if the link trans-
mits 8,000 frames per second and each slot consists of 8 bits, then the transmission
rate of a circuit is 64 kbps.
Proponents of packet switching have always argued that circuit switching is
wasteful because the dedicated circuits are idle during silent periods. For example,
FDM
4KHz
Link Frequency
4KHz
TDM
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Slot Frame
Time
Key:
All slots labeled “2” are dedicated
2
to a specific sender-receiver pair.
Figure 1.14 With FDM, each circuit continuously gets a fraction of the
bandwidth. With TDM, each circuit gets all of the bandwidth
periodically during brief intervals of time (that is, during slots)
