Data Communications TDC 362 / TDC 460. Circuit Switching and Packet Switching. 8.1 Circuit Switching. Space-Division Switch Time-Division Switch TDM Bus Combinations. Figure 8.1 Circuit-switched network. Figure 8.2 A circuit switch. Blocking or Non-blocking. Blocking
Data CommunicationsTDC 362 / TDC 460
Circuit Switching and
8.1 Circuit Switching
Figure 8.1Circuit-switched network
Figure 8.2A circuit switch
Figure 8.4Crossbar switch
Figure 8.5Multistage switch
Figure 8.6Switching path
Figure 8.7Time-division multiplexing, without and with a Time-slot interchange
Figure 8.8Time-slot interchange
Figure 8.9TDM bus
Figure 8.10TST (Time-space-time) switch
Figure 18.2Virtual Circuit Identifier (VCI)
VCI is known only between two switches. (It is not a global
Figure 18.4Switch and table
Figure 18.5Source-to-destination data transfer
S(witched)VC vs. P(ermanent)VC setup
A virtual circuit can be either switched or permanent.
If permanent, an outgoing VCI is given to the source,
and an incoming VCI is given to the destination.
The source always uses this VCI to send frames to
this particular destination.
The destination knows that the frame is coming from
that particular source if the frame carries the
corresponding incoming VCI.
If a duplex connection is needed, two virtual circuits
S(witched)VC vs. P(ermanent)VC setup
A PVC has several drawbacks:
1. Always connected, so always paying
2. Connection is between two parties only. If
you need a connection to another point, you
need another PVC.
Don’t like these disadvantages? Use an SVC.
Figure 18.6SVC setup request
1 - Setup frame sent from A to Switch I.
Note how the Outgoing VCI is not yet known.
Figure 18.7SVC setup acknowledgment
As the acknowledgment frame goes back, the VCI number
is placed into the Outgoing VCI entry in each table.
(Details in TDC 365/463)
In unicast routing, the router forwards the received packet through only one of its ports.
Three basic unicast routing protocols: RIP, OSPF, BGP
Figure 21.3Autonomous systems
R1, R2, R3 and R4 use an interior and exterior routing
protocol. The other routers use only an interior protocol.
RIP and OSPF are interior, BGP is exterior.
RIP (Routing Information Protocol) is an interior routing
Protocol based on distance vector routing which uses the
Each router shares its routing knowledge with its neighbors,
every 30 seconds.
This shared information is used to update a router’s routing
table. An entry in the routing table consists of the destination
network address, the shortest distance to reach the
destination in hop count, and the next router to which the
packet should be delivered. (see next slide)
Table 21.1 A distance vector routing table
RIP Updating Algorithm
Receive: a response RIP message
1. Add one hop to the hop count for each advertised destination.
2. Repeat the following steps for each advertised destination:
1. If (destination not in the routing table)
1. Add the advertised information to the table.
1. If (next-hop field is the same)
1. Replace entry in the table with the advertised one.
1. If (advertised hop count smaller than one in the table)
1. Replace entry in the routing table.
Figure 21.4Example of updating a routing table
OSPF (Open Shortest Path First) protocol is another interior
routing protocol for autonomous systems.
Special routers called autonomous system boundary routers
are responsible for dissipating information about other
autonomous systems into the current system.
To handle routing efficiently and in a timely manner, OSPF
divides an autonomous system into areas.
Figure 21.7Areas in an autonomous system
In OSPF, each router sends the state of its neighborhood to
every other router in the area. It does this by flooding.
The state of its neighborhood is only shared when there is
new information. This generates much less traffic than does
distance vector routing (RIP).
OSPF keeps information on its links (the connection between
two routers). There are 4 types of links: point-to-point,
transient, stub, and virtual.
To share information about their neighbors, each entity
distributes link state advertisements (LSAs).
There are 5 different types of LSAs: router link, network link,
summary link to network, summary link to AS boundary
router, and external link.
Every router in an area receives the router link LSAs and
network link LSAs from every other router and forms a
link state database.
Dijkstra’s least cost algorithm is applied to this link state
database to create the routing table. The routing table shows
the cost of reaching each network in the area.
RIP and OSPF have shortcomings.
RIP (distance vector routing) is not always optimal because
The smallest hop count is not always the optimal route. Plus,
bad news moves slowly.
OSPF (link state routing) has the shortcoming of a possibly
huge routing table. To use link state routing for the whole
internet would require each router to have a huge database.
What about BGP (Border Gateway Protocol)? It is an inter-
autonomous system routing protocol and is based on a routing
method called path vector routing.