Vertices are transmitters, receivers, and intermediaries
Links/edges are "direct links" (either using a guided or unguided
medium)
Higher Levels of Abstraction (Sometimes Called Overlay Networks):
Vertices can be networks, cities, countries, etc...
Links/edges can be routes/paths that use several different media
The Need for Intermediaries
Complete Network:
Every transmitter/receiver pair is connected
with a direct link
Other Networks:
Data from some transmitters to some receivers
must go through an intermediary
A Note About Edge Weights
In General:
The edge weights will almost always be positive (which
is important for some of the algorithms that we will consider)
In Some Situations:
We may sometimes add "dummy" edges that have a weight of 0
Circuit Switching
Defined:
Data from a transmitter to a receiver travels along
a dedicated path (usually through one or more
switches)
The Process:
Establish the path (reserve channel capacity)
Transfer data
Disconnect (release channel capacity)
Packet Switching
Defined:
Data from a transmitter to a receiver
are wrapped in a packet (that contains the receivers address)
that is forwarded by intermediaries
until it reaches the destination
Approaches:
Datagram Approach - Each packet is routed
independently
Virtual Circuit Approach - All packets from
transmitter to receiver follow the same path
Packet Switching (cont.)
Store-and-Forward:
Most packet switches do not begin to transmit the packet
until they have received the entire packet (i.e., they
store the packet, however briefly, before forwarding it)
This leads to a store-and-forward delay of
a few microseconds or milliseconds
Queuing Delay:
Most packet switches have an output buffer for
each outbound link that holds packets in case that
link is not available (e.g., is being used to transmit
another, perhaps unrelated, packet)
Typically a few microseconds or milliseconds
Processing Delay:
The time required to examine/process the packet's header
and perform error checking/correcting
Typically a few microseconds
Examples
Circuit Switching:
Leased line networks
Integrated Service Digital Network (ISDN)
Packet Switching:
X.25 (Tymnet)
Telenet
ARPAnet
Frame Relay
Asynchronous Transfer Mode (ATM)
Internet Protocol (IP)
Comparison
Advantages of Circuit Switching:
Almost no transmission delay
Very little variation in delay (i.e., low jitter)
No overhead (e.g., header bits)
after the circuit is created
Advantages of Packet Switching:
More efficient use of bandwidth (i.e., less idle time)
Packets can be prioritized
It is always possible to communicate (though, perhaps,
with increased delivery delay)
More robust (i.e., more paths between any two nodes)?
Network Topologies
Bus (Two Representations):
Ring (Two Representations):
Star:
Hub and Spoke:
Routing
The Issue:
There might be multiple paths that can be used
to transmit information from a sender to a receiver
Approaches:
Pass it on and hope it gets there
Use a path/route (e.g., determine the "best" path/route when
needed; use a look-up table)