1.3 • THE NETWORK CORE 25
Queuing Delays and Packet Loss
Each packet switch has multiple links attached to it. For each attached link, the
packet switch has an output buffer (also called an output queue), which stores
packets that the router is about to send into that link. The output buffers play a key
role in packet switching. If an arriving packet needs to be transmitted onto a link but
finds the link busy with the transmission of another packet, the arriving packet must
wait in the output buffer. Thus, in addition to the store-and-forward delays, packets
suffer output buffer queuing delays. These delays are variable and depend on the
level of congestion in the network. Since the amount of buffer space is finite, an
arriving packet may find that the buffer is completely full with other packets wait-
ing for transmission. In this case, packet loss will occur—either the arriving packet
or one of the already-queued packets will be dropped.
Figure 1.12 illustrates a simple packet-switched network. As in Figure 1.11,
packets are represented by three-dimensional slabs. The width of a slab represents
the number of bits in the packet. In this figure, all packets have the same width and
hence the same length. Suppose Hosts A and B are sending packets to Host E. Hosts
A and B first send their packets along 10 Mbps Ethernet links to the first router. The
router then directs these packets to the 1.5 Mbps link. If, during a short interval of
time, the arrival rate of packets to the router (when converted to bits per second)
exceeds 1.5 Mbps, congestion will occur at the router as packets queue in the link’s
output buffer before being transmitted onto the link. For example, if Host A and B
each send a burst of five packets back-to-back at the same time, then most of these
packets will spend some time waiting in the queue. The situation is, in fact, entirely
analogous to many common-day situations—for example, when we wait in line for
a bank teller or wait in front of a tollbooth. We’ll examine this queuing delay in
more detail in Section 1.4.
10 Mbps Ethernet
C
A
1.5 Mbps
Queue of
packets waiting
for output link
B
Key: D E
Packets
Figure 1.12 Packet switching
Queuing Delays and Packet Loss
Each packet switch has multiple links attached to it. For each attached link, the
packet switch has an output buffer (also called an output queue), which stores
packets that the router is about to send into that link. The output buffers play a key
role in packet switching. If an arriving packet needs to be transmitted onto a link but
finds the link busy with the transmission of another packet, the arriving packet must
wait in the output buffer. Thus, in addition to the store-and-forward delays, packets
suffer output buffer queuing delays. These delays are variable and depend on the
level of congestion in the network. Since the amount of buffer space is finite, an
arriving packet may find that the buffer is completely full with other packets wait-
ing for transmission. In this case, packet loss will occur—either the arriving packet
or one of the already-queued packets will be dropped.
Figure 1.12 illustrates a simple packet-switched network. As in Figure 1.11,
packets are represented by three-dimensional slabs. The width of a slab represents
the number of bits in the packet. In this figure, all packets have the same width and
hence the same length. Suppose Hosts A and B are sending packets to Host E. Hosts
A and B first send their packets along 10 Mbps Ethernet links to the first router. The
router then directs these packets to the 1.5 Mbps link. If, during a short interval of
time, the arrival rate of packets to the router (when converted to bits per second)
exceeds 1.5 Mbps, congestion will occur at the router as packets queue in the link’s
output buffer before being transmitted onto the link. For example, if Host A and B
each send a burst of five packets back-to-back at the same time, then most of these
packets will spend some time waiting in the queue. The situation is, in fact, entirely
analogous to many common-day situations—for example, when we wait in line for
a bank teller or wait in front of a tollbooth. We’ll examine this queuing delay in
more detail in Section 1.4.
10 Mbps Ethernet
C
A
1.5 Mbps
Queue of
packets waiting
for output link
B
Key: D E
Packets
Figure 1.12 Packet switching
