Packet Switching
Messages are split into smaller pieces called packets
• These packets are numbered and addressed and sent
through the network one at a time
• Pipelining
Message Switching
Each message is addressed to a destination
• When the entire message is received at a router, the next
step in its journey is selected; if this selected channel is
busy, the message waits in a queue until the channel becomes
free
• Thus, the message “hops” from node to node through a
network while allocating only one channel at a time
• Analogy: Postal service
Circuit Switching
Provides service by setting up the total path of
connected lines from the origin to the destination
• Example: Telephone network
Header Overhead
Circuit < Message < Packet
Transmission Delay Short Bursty Messages:
Packet < Message < Circuit
Long Continuous Messages:
Circuit < Message < Packet
Packet Switching Pros
great for “bursty” data – sometimes has data to send, but at other times not
• resource sharing
• simpler, no call setup
Packet Switching Cons
excessive congestion possible: packet delay and loss due to buffer overflow • protocols needed for reliable data transfer, congestion control
Packet length
size of a packet (units = bits or bytes)
Channel speed or bandwidth:
How fast the channel can transmit bits (units = bits/second or Bytes/second or packets/second)
Packet transmission time:
amount of time to transmit an entire packet (units = seconds)
Propagation delay
Delay imposed by the properties of the link. Depends on the link’s distance (units = seconds)
Total transfer time
propagation delay + packet transmission time
1 kilobit (Kbit)
1000 bits
