Transcript Intro Circuit and Packet Switching

Data Communications
TDC 362 / TDC 460
Circuit Switching and
Packet Switching
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8.1 Circuit Switching
Space-Division Switch
Time-Division Switch
TDM Bus
Combinations
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Figure 8.1
Circuit-switched network
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Figure 8.2
A circuit switch
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Blocking or Non-blocking
Blocking
A network is unable to connect stations because all
paths are in use
A blocking network allows this
Used on voice systems
Short duration calls
Non-blocking
Permits all stations to connect (in pairs) at once
Used for some data connections
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Figure 8.4
Crossbar switch
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Figure 8.5
Multistage switch
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Figure 8.6
Switching path
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Three Stage Switch
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Figure 8.7 Time-division multiplexing, without and with a
Time-slot interchange
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Figure 8.8
Time-slot interchange
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Figure 8.9
TDM bus
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Figure 8.10
TST (Time-space-time) switch
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Circuit-Switched Routing
Many connections will need paths through more
than one switch
Need to find a route
Efficiency
Resilience
Public telephone switches are a tree structure
Static routing uses the same approach all the time
Dynamic routing allows for changes in routing
depending on traffic
Uses a peer structure for nodes
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Alternate Routing
Possible routes between end offices predefined
Originating switch selects appropriate route
Routes listed in preference order
Different sets of routes may be used at
different times
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Alternate Routing Diagram
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Control Signaling Functions
Audible communication with subscriber
Transmission of dialed number
Call can not be completed indication
Call ended indication
Signal to ring phone
Billing info
Equipment and trunk status info
Diagnostic info
Control of specialist equipment
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Control Signals
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Location of Signaling
Subscriber to network
Depends on subscriber device and switch
Within network
Management of subscriber calls and network
ore complex
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In Channel Signaling
Use same channel for signaling and call
Requires no additional transmission facilities
Inband
Uses same frequencies as voice signal
Can go anywhere a voice signal can
Impossible to set up a call on a faulty speech path
Out-of-band
Voice signals do not use full 4kHz bandwidth
Narrow signal band within 4kHz used for control
Can be sent whether or not voice signals are present
Need extra electronics
Slower signal rate (narrow bandwidth)
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Drawbacks of In Channel
Signaling
Limited transfer rate
Delay between entering address (dialing) and
connection
Overcome by use of common channel signaling
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Common Channel Signaling
Control signals carried over paths independent of
voice channel
One control signal channel can carry signals for a
number of subscriber channels
Common control channel for these subscriber lines
Associated Mode
Common channel closely tracks interswitch trunks
Disassociated Mode
Additional nodes (signal transfer points)
Effectively two separate networks
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Common vs. In Channel
Signaling
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Signaling
Modes
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Signaling System Number 7
SS7
Most widely used common channel signaling
scheme
Internationally standardized and general
purpose
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SS7
SS7 network and protocol used for:
Basic call setup, management, tear down
Wireless services such as PCS, roaming,
authentication
Toll free and toll (900) wireline services
Enhanced features such as call forwarding, caller ID,
3-way calling
Efficient and secure worldwide telecommunications
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SS7
SS7 messages are exchanged between central
offices and specialized databases via signal
transfer points (packet switches).
Control plane
Responsible for establishing and managing
connections
Information plane
Once a connection is set up, info is transferred in the
information plane
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SS7 Signaling Network
Elements
Service switching point (SSP)
SSPs enable central offices to communicate with SS7
databases (the user entry point into SS7)
Signal transfer point (STP)
A signaling point (packet switch) capable of routing
control messages
Service control point (SCP)
SCPs contain databases with call routing instructions
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SS7
SCP
SCP
STP
SSP
Central
Office
STP
SSP
Central
Office
SSP
Central
Office
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SS7 Characteristics
SSPs are telephone switches that send signaling
messages to other SSPs to setup, manage, and
release voice circuits
An SSP may also send a query message to a
centralized database (an SCP) to determine how
to route a call (e.g. a toll-free number)
Because the SS7 network is critical to call
processing, SCPs and STPs are deployed in
mated pair configurations in separate physical
locations
Links between signaling points are also in pairs
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Packet Switching Principles
Circuit switching designed for voice
Resources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed
Both ends must operate at the same rate
What if we don’t want a dedicated call, or the data rate
is bursty? You want packet switching!
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Basic Operation
Data transmitted in small packets
Typically 1000 bytes
Longer messages split into series of packets
Each packet contains a portion of user data plus some
control info (such as addressing info or packet type)
Packets are received, stored briefly (buffered) and
passed on to the next node
Store and forward (only ATM does not do this)
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Advantages
Line efficiency
Single node to node link can be shared by many
packets over time
Packets queued and transmitted as fast as possible
Data rate conversion
Each station connects to the local node at its own
speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy
Delivery may slow down
Priorities can be used
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Two Basic Forms of Packet
Switching
Packets handled in two ways
Datagram
Virtual circuit
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Datagram
Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may get lost or delayed
Up to receiver to re-order packets and recover
from missing packets
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Virtual Circuit
Preplanned route established before any packets
sent
Call request and call accept packets establish
connection (handshake)
Each packet contains a virtual circuit identifier
instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path
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Figure 18.2 Virtual Circuit Identifier (VCI)
VCI is known only between two switches. (It is not a global
address.)
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Figure 18.4
Switch and table
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Figure 18.5
Source-to-destination data transfer
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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
are established.
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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.
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Figure 18.6
SVC setup request
1 - Setup frame sent from A to Switch I.
Note how the Outgoing VCI is not yet known.
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Figure 18.7
SVC setup acknowledgment
As the acknowledgment frame goes back, the VCI number
is placed into the Outgoing VCI entry in each table.
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Virtual Circuits vs Datagram
Virtual circuits
Network can provide sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node looses all circuits through that node
Datagram
No call setup phase
Better if few packets
More flexible
Routing can be used to avoid congested parts of the network
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Packet Size
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Event Timing
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Routing
Complex, crucial aspect of packet switched
networks
Characteristics required
Correctness
Simplicity
Robustness
Stability
Fairness
Optimality
Efficiency
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Performance Criteria
Used for selection of route
Minimum hop
Least cost
Dijkstra’s algorithm most common
Finds the least cost path from one starting node to all
other nodes
Algorithm can be repeated for each starting node
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Dijkstra’s Least Cost Example
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Dijkstra’s Least Cost Example
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Decision Time and Place
Time
Packet or virtual circuit basis
Place
Distributed
Made by each node
Centralized - dead
Source - dead
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Basic Routing Strategies
Adaptive versus Fixed (dead?)
Distributed versus Centralized (dead?)
Flooding
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Centralized
and
Distributed
Routing
Tables
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Flooding
No network info required
Packet sent by node to every neighbor
Incoming packets retransmitted on every link except
incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be
discarded
Nodes can remember packets already forwarded to keep
network load in bounds
Can include a hop count in packets
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Flooding
Example
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Properties of Flooding
All possible routes are tried
Very robust
At least one packet will have taken minimum hop
count route
Can be used to set up virtual circuit
All nodes are visited
Useful to distribute information (e.g. routing)
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Adaptive Routing
Used by almost all packet switching networks
Routing decisions change as conditions on the network
change
Failure
Congestion
Requires info about network
Decisions more complex
Tradeoff between quality of network info and overhead
Reacting too quickly can cause oscillation
Reacts too slow to be relevant
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Adaptive Routing - Advantages
Improved performance
Aid congestion control
Complex system
May not realize theoretical benefits
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Where does routing info come
from?
Local (isolated)
Route to outgoing link with shortest queue
Can include bias for each destination
Rarely used - do not make use of easily available info
Adjacent (neighbor) nodes only
All nodes in network
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Chapter 21
Unicast
Routing Overview:
Routing Protocols
(Details in TDC 365/463)
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Figure 21.1
Unicasting
In unicast routing, the router forwards
the received packet through only one of
its ports.
Three basic unicast routing protocols:
RIP, OSPF, BGP
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Figure 21.3
Autonomous 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.
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RIP
RIP (Routing Information Protocol) is an interior routing
Protocol based on distance vector routing which uses the
Bellman-Ford algorithm.
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)
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Table 21.1 A distance vector routing table
Destination
Hop
Count
Next
Router
163.5.0.0
7
172.6.23.4
197.5.13.0
5
176.3.6.17
189.45.0.0
4
200.5.1.6
115.0.0.0
6
131.4.7.19
Other information
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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.
2. Else
1. If (next-hop field is the same)
1. Replace entry in the table with the advertised one.
2. Else
1. If (advertised hop count smaller than one in the table)
1. Replace entry in the routing table.
3. Return.
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Figure 21.4 Example of updating a routing table
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OSPF
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.
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Figure 21.7 Areas in an autonomous system
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OSPF
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).
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OSPF
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.
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BGP
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 interautonomous system routing protocol and is based on a routing
method called path vector routing.
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