- Recall that:
- In circuit switching,
all packets from the same
session
follows the same path
from source to destination
Important consequence:
- This property allows us to
make resourse reservations
at the nodes along the path
- Reserving network resources can provide performance guarantee to the connection (= user) !!!
- In circuit switching,
all packets from the same
session
follows the same path
from source to destination
- Analogy:
- If I
know
that route I will
take to get
to a destination (e.g.: Las Vegas)
on Interstate Highways:
- I can reserve a hotel room in advanced on the route to guarantee that I have a place to stay for the night !!!
- If I
know
that route I will
take to get
to a destination (e.g.: Las Vegas)
on Interstate Highways:
- Phases in the communication using circuit switching:
- Setup phase:
- Before transmitting any data, a
transmission path (circuit)
with a given
transmission capacity
(e.g., 64 Kbps capacity)
is set up
- Each router on the circuit will reserve the given transmission capacity
This action is called a connection setup and the established path (= route) is called a circuit
The source can send data at the requested transmission rate, and it cannot exceed the requested rate
- Before transmitting any data, a
transmission path (circuit)
with a given
transmission capacity
(e.g., 64 Kbps capacity)
is set up
- Transmission phase:
- The actual message (data) exchange occurs in this phase
- Setup phase:
- Reserving transmission capacity:
- Transmission time
is divided into
periods
Each period is divided in a number of slots:
- When a source makes a
connection request,
one (or more)
time slots
will be reserved
for packets from that source:
This is done in every node along the path from source to the destination
Analogy:
- Using a
conveyer belt analogy,
we can visualize the result
of these reservations:
Operation:
- Packets from the source will
transverse towards
the destination
virtually without any delay
- When a packet arrives
at an intermediate node, it only
need to wait
at most ONE period of time
before it will be
transmitted in the reserved slot
to the next node
- Example:
- Suppose a
message is
received by
a node:
-
In order for the node to
transmit the
message
to the next node,
the node must
wait for
the slot reserved for this
message:
Note:
- The waiting time is very short because the duration of 1 period is in the order of micro seconds !!!
- As soon as the
reserved slot arrives,
the message can be
transmitted to the
next node:
- Suppose a
message is
received by
a node:
- Transmission time
is divided into
periods