Transcript Data transfer and signaling networks

TELEPHONE NETWORK
Telephone networks use circuit switching. The
telephone network had its beginnings in the late
1800s. The entire network, which is referred to as the
plain old telephone system (POTS), was originally an
analog system using analog signals to transmit voice.
Public Circuit Switched Network
Telecomm Components
• Subscriber
– Devices attached to network
• Local Loop
– Subscriber loop
– Connection to network
• Exchange
– Switching centers
• Trunks
– Branches between exchanges
– Multiplexed
A telephone system
Circuit Establishment
LATAs
Local Access transport areas
• US was divided into more than 200 LATAs in
1984.
• Inside the LATA services are called Intra LATA
services.
Intra-LATA services are provided by
local exchange carriers.
Since 1996, there are two
types of LEC s: incumbent local
exchange carriers and competitive
local exchange carriers.
Switching offices in a LATA
Inter LATA Services
• The services between LATAs are handled
by Interexchange carriers (IXCs).
• These carriers sometimes called long
distance companies.
• Point of presence is the switching office to
interact between IXC to LATA.
Point of presences (POPs)
Note
The tasks of data transfer and signaling
are separated in modern telephone
networks: data transfer is done by one
network, signaling by another.
Routing
• Many connections will need paths through more
than one switch
• Need to find a route
– Efficiency
– Resilience
Types of Routing:
- Static routing uses the same approach all the
time
- Dynamic routing allows for changes in routing
depending on traffic
Right through 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
Alternate Routing Diagram
Own exchange routing
Own exchange routing or distributed
routing allows alternative routes to be
chosen at the intermediate nodes
Computer controlled routing
Separate computers are used for signaling
& some algorithms are decided to chose
the appropriate path.
Location of Signalling
Subscriber loop signalling:
Subscriber to network, Depends on subscriber
device and switch
Intraexchange signalling:
In the exchange or switching system
Interexchange signalling:
Signalling between the exchanges
Control Signaling Functions
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Audible communication with subscriber
Transmission of dialed number
Indication that call can not be completed
Call ended indication
Signal to ring phone
Billing info
Equipment and trunk status info
Control of specialist equipment
Control Signal Sequence
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Both phones on hook
Subscriber lifts receiver (off hook)
End office switch signaled
Switch responds with dial tone
Caller dials number
If target not busy, send ringer signal to target subscriber
Feedback to caller
– Ringing tone, engaged tone, unobtainable
Target accepts call by lifting receiver
Switch terminates ringing signal and ringing tone
Switch establishes connection
Connection release when Source subscriber hangs up
Type of signaling
• In channel signaling
• Common channel signaling
In Channel Signaling
• Use same channel for signaling and call
– Requires no additional transmission facilities
• In band
– 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
– Uses the frequencies above the voice band but below
the upper limit
– Can be sent whether or not voice signals are present
– Need extra hardware
– Narrow bandwidth is available for signalling
Drawbacks of In Channel
Signaling
• Limited transfer rate
• Signalling capability is less
• Delay between entering address (dialing)
and connection
• Overcome by use of common channel
signaling
Common Channel Signaling
• Control signals carried over paths independent
of voice channel
• One control signal channel (Common control
channel) can carry signals for a number of
subscriber channels
• Associated Mode
– Common channel closely tracks interswitch trunks
• Disassociated Mode
– Additional nodes (signal transfer points)
– Effectively two separate networks
Quasiassociated Mode
Common v. In Channel
Signaling
Common
Channel
Signaling
Modes
Signalling standards by CCITT
• Signalling number 1-7 have been adopted
• The signalling systems SS1 to SS5 deal
with inchannel signalling
• SS6 & SS7 deals with common channel
signalling
Signaling System Number 7 (SS7)
• Common channel signaling scheme
• ISDN
• Call control, remote control, management
and maintenance
• Reliable means of transfer of info in
sequence
• Will operate over analog and below 64k
• Can work with digital SPC exchanges
(Speed less than 64kbps)
• Point to point terrestrial and satellite links
SS7
Signaling Network Elements
• Signaling point (SP)
– Any point in the network capable of handling
SS7 control message
• Signal transfer point (STP)
– A signaling point capable of routing control
messages
• Control plane
– Responsible for establishing and managing
connections
• Information plane
– Once a connection is set up, info is
transferred in the information plane
Data transfer and signaling networks
Performance of Signaling Network
• STP capacities
– Number of signaling links that can be handled
– Message transfer time
– Throughput capacity
• Network performance
– Number of SPs
– Signaling delays
• Availability and reliability
– Ability of network to provide services in the
face of STP failures
Layers in SS7
Layers in SS7
• Physical layer : MTP Level1 : Uses various transmission
media specifications
• Data link Layer : MTP Level2: Source destination
address, CRC, Packetizing
• Network layer: MTP Level 3: Routing from source to
destination
• Transport Layer
SCCP (Signaling connection control point): Special
services
TUP (Telephone user port): Responsible for setting up
voice calls
TCAP (Transaction capabilities application port) : Transfer
of calls or services
ISDN (Integrated services digital network) : Support
services to ISDN network
Softswitch Architecture
• General purpose computer running software to make it a
smart phone switch
• Lower costs
• Greater functionality
– Packetizing of digitized voice data
– Allowing voice over IP
• Most complex part of telephone network switch is
software controlling call process
– Call routing
– Call processing logic
• Physical switching done by media gateway
• Call processing done by media gateway controller
Softswitch