Telephone network
T-110.300
Jouni Karvo, Raimo Kantola, Timo Kiravuo
Telephone network
•
•
•
•
•
•
•
•
•
•
•
•
Background
Topology
Reference connection
End systems
Access network
Transmission
Switching & Signaling
Layers
User-Network interaction
Liberalization
Business roles
Telephone numbers
Background
•
•
•
•
•
•
World's largest machine; extends to all countries
Huge economic and social importance
Specialized in voice transmission
Other applications have been created: fax, data etc.
Basic service: full-duplex voice transmission
Small end-to-end delays, small delay variation (more than
150ms delay disturbs discussion --note satellites!)
• Call admission control, and accepted calls will complete
• Grows all the time, now mostly growth of mobile networks
• Although most traffic in the telecom networks is now data,
most of the money comes from voice
Background cont.
• Telephones are addressed by telephone
numbers, that are unique
• There are special numbers or area codes that
need translation
• The network formed by the end systems
(telephones, faxes, modems etc.) and the
hierarchic switching and transmission systems is
called Public Switched Telephone Network
(PSTN) or informally as Plain Old Telephone
System (POTS).
Topology
• Basic structure is tree-like
• The core network more fully connected
• Direct links for some special busy routes
Reference connection
• ITU-T E.830
• The longest possible international telephone connection
model
• LE = Local Exchange, ISC = International Switching Center
• Delay of more than 20ms requires echo cancellation
End systems
• Telephones: Voice-electrical signal transducers
(microphone+earphone), dialer, ringer, switch
hook
• A single twisted pair is used for signal
transmission for both directions ⇒ echo
• Dialing+ringing: either pulses or tones
The Access Network
• a) distribution panel
• b) subscriber terminal
equipment
• c) radio link for emergency
traffic (112, 911 etc.)
• d) fiber link,
•
•
•
•
•
i) PCM multiplex
e) concentrator
f) telephone exchange
g) fiber link
h) subscriber lines
The Access Network
• The distribution panel connects operator's cables to
subscriber lines
– E.g. a thick cable of 30 pairs to individual lines to apartments
– Usually located in customer premises (building)
– The subscriber lines to the apartments are often the
responsibility of the building owner (Finland)
– The cable to the telephone network is the responsibility of the
telco operator
• The concentrator collects a group of subscriber lines
– I/O cards for subscriber lines.
– A/D and D/A circuitry for the analog lines
– Fiber link or twisted pairs carrying PDH/SDH towards the
telephone exchange
– May have a radio link for emergency calls (if the main link fails)
The Access Network
• Static concentrators have PCM streams flowing constantly
for each subscriber (even if there is no call going on).
• Dynamic concentrators generate a PCM stream only when
necessary, using PDH/SDH towards the telephone
exchanges.
• Concentrators do not switch. If a subscriber (A) wants to
make a call to subscriber (B) connected to the same
concentrator, the call goes through the nearest telephone
exchange.
• The only switching type action is routing emergency calls
through the radio link in case of failure in the fiber link.
Transmission
• The network elements are connected with transmission links
• Link metrics: capacity (usually measured in 64kbps
channels), and delay.
• For digital transmission, signal distortion is usually not a
problem, but ITU-T has defined signal quality assessment
methods, originally for analog transmission
• Delay can be a problem for digital transmission, too;
normally delays over 0.1-0.2s irritate people discussing
• Multiplexing techniques (PDH, SDH) are used to reduce the
number of individual lines in cables
• Link costs dominated by installation labor and rights of way
• Technologies: twisted pair, coaxial, terrestrial microwave,
satellite microwave, optical fiber
Switching & Signaling
• The way to connect a subscriber to another --- no need for a
fully connected topology
• Switch hardware carries the voice
• Switch controller controls the hardware
• Signaling is the means for switches to communicate control
information with each other
• In PSTN, control and data are separated in different
networks
• The control network is called signaling network
• The signaling network is a packet switched network, and
uses common channel signaling, i.e. signaling for all users
uses the same signaling channels.
Layers
• [7] IN services; answering machines
– Need to differentiate, new services, billing
• [3] switches, exchanges
– Dimensioning, routing
• [2] transmission
– Coverage, capacity, radio design
• [1] twisted pair, coaxial, fiber, radio
– Rights of way, re-use, natural monopoly
• Different parts of the system have internally
protocols of any layer
User-Network Interaction
• The users of the telephone network act randomly, and
generate load that varies through the day.
• The stochastic nature of load leads to the use of probability
theory for defining performance properties of the system.
• ITU-T has defined the so-called Grade-of-Service
parameters for telephone systems.
• As an example, the standard ITU-T E.543 defines the
following GoS parameters for telephone exchanges:
Parameter
Normal load
High load
Incoming response delay
(X>0.5s) <= 5%
(X>1s) <= 5%
Exchange call setup delay
(X>0.5s) <= 5%
(X>1s) <= 5%
Through connection delay
(X>0.5s) <= 5%
(X>1s) <= 5%
Internal loss probability
0.002
0.01
Liberalization
• Providing telecommunications services has traditionally
been a government or private monopoly
– In Finland, local telcos ("puhelinyhdistys'') private
– Government long distance monopoly (Tele)
• Liberalization opened the market in the 90's:
– Several long distance national and international carriers
– Several GSM networks
– Local network owners have to provide for other operators
– Local loop still often like a monopoly in practice
• Liberalization is supposed to lower prices, improve customer
service and introduce new telecom services
• Current regulation sets requirements for technology and
critical services
Business roles
• The model is an idealistic view, but starting to
happen
• Many GSM service operators do not own their
own network, but focus on customer management
and billing instead
Telephone numbers (ITU-T
E.164)
• A telephone number can point to a subscriber or to a service
• A telephone number of a subscriber is also an address for
routing of the call
– Area code, city areas
• Number portability breaks this connection
• Service numbers are "logical'' and require number
translation for use as routing addresses
– 112, 118 etc.
• The user must be able to deduce the expected cost of the
call from the telephone number. Thus, the number allocation
is tied to geography and network topology
Telephone Exchanges
T-110.300
Jouni Karvo, Timo Kiravuo
Telephone exchanges
• Subscriber lines are connected to the telephone
exchanges
– Originally each customer had a direct connection
(electrical pair) to the exchange
• Current exchanges are complex and expensive
– A need to reduce their number
• Physical cabling becomes an obstacle
– Concentrators are used to reduce the number of cables
• As the number of telephone exchanges is
decreasing, the hierarchy of telephone exchanges
is flattening
The structure of a telephone
exchange
• a) Control unit
• b) I/O
• c) Switching
fabric
The most important tasks for
telephone exchanges
• Connecting to subscriber terminal equipment and
other exchanges
• Telephone number analysis and translation
• Collecting accounting data for billing
• Logging data for the use of authorities
• Switching calls
I/O
• I/O circuitry connects the telephone exchange to
the outside world. The I/O is arranged in hotswappable I/O cards.
• There are several types of I/O cards (part of which
are also used in concentrators)
– subscriber line cards (which contains circuits for
connecting several subscribers)
– PDH/SDH/ATM cards
Subscriber line cards have
several tasks:
•
•
•
•
[B] Giving power to the terminal equipment (battery)
[O] Over voltage protection
[R] Ringing
[S] Supervising the status of the terminal equipment (e.g. if
the handle is on or off hook in an old style telephone)
• [C] Coder and Decoder (A/D or D/A converters, A-law/µ-law
(de)coding )
• [H] Hybrid - a transformer between two half-duplex and one
full-duplex connection
• [T] Testing the status of the subscriber line
• Memory rule; BORSCHT, the red soup.
Switching fabric
• The task of the switching fabric is to transmit the PCM
streams between two I/O ports.
• Both Time and Space Division multiplexing are used.
• One measure of the size of the exchange is the capacity of
its switching fabric.
• It is easy to find books describing design of switching fabrics
– in a sense it is one of the "easy'' but interesting parts of the
telephone exchanges. We will cover the structure of the
switching fabrics only briefly;
• Two main types of switching fabric are used: the crossbar
switch and the interconnection network.
Crossbar switches
• Often used in small scale
exchanges
• Usually has an electronic
switch for each line
– To connect n lines, n^2
switches are needed
– A typical space division switch
– Uses physical space for
connections
– Solid state technology
Interconnection Network
• Used in larger switches
– Different types available
• Interconnection networks are made from a
number of smaller switching elements.
• The idea in interconnection networks is to save in
the number of crossbars using special topologies.
Time Division Switching
• Time Division Multiplexing is used in combination
with the above mentioned structures. There are
different structures: Time-Space, Space-Time,
Time-Space-Time etc.
• The idea in Time division Multiplexing is to
rearrange the time slots in a PCM frame (E1).
• Time slot interchanger (TSI):
Time-Space-Time switching
• In Time-Space-Time switching, the internal
crossbar works at a higher rate than the input line
rate
– Any time slot can be switched to any time slot
– The crossbar configuration changes for each time slot
SS#7
T-110.300
Jouni Karvo, Timo Kiravuo
Signaling
• The network made of exchanges and links must
be co-ordinated in order to make calls possible
• The purpose of signaling is just to do this: it
– establishes calls,
– monitors the calls, and
– tears down the calls.
• In-band signaling was used earlier
– MF tones waste of resources
– Security problems
• A new signaling system was created:
– Common Channel Signaling Subsystem #7
Common Channel Signaling
Subsystem #7
• SS#7 is a packet switched network on top of the telephone
network
• It is used to transport signaling information
– It controls the trunk connections
– SS#7 does not transport user data
• SS#7 packets use either a dedicated 56 or 64kbps channel
(such as E1 TS16) or dedicated links.
• If signaling does not work, the whole telephone network is
"dead''. Thus, stringent reliability requirements.
– There is a lot of redundancy; every part of the signaling system
has at least one spare
SS#7 elements
SS#7 overlay network:
Operator A
Operator B
• The actual telephone exchanges switching the actual
telephone traffic are called Signal Switching Points (SSP)
(circles).
• The packet switches (routers) of the SS#7 network are
called Signal Transfer Points (STP) (squares with a
diagonal).
• The databases for advanced call-processing capabilities are
called Signal Control Points (SCP) (cylinders).
SS#7 Protocol Stack
• SS#7 uses a packet switched protocol set to carry
its mission.
• The protocol stack generates a packet switched
network over a circuit switched one
• MTP-x protocols (Message Transfer Part)
protocols provide transport of SS#7 messages
– MTP-2 is the link level protocol
– MTP-3 provides network level functionality
– Routing to specific elements
SS#7 Protocol Stack
TUP
DUP BISUP ISUP
OMAP
TCAP
SCCP
MTP-3
MTP-2
MTP-1
PDH/SDH/ATM
Twisted pair / fiber
• The upper parts (layers) provide functions for call management
– Telephone User Part (TUP) for traditional telephone signaling
– Data User Part (DUP) for datacom users
– ISDN User Part (ISUP) and Broadband ISDN User Part (BISUP)
for ISDN and ATM users.
Signaling Connection
Control Part (SCCP)
• Provides connections for the parts above it
• Incremental routing
– Global Title Translation, GTT (compare to DNS)
• Sub-addressing for applications within an element
– SSN = Subsystem Number (compare to ports in UDP
and TCP).
• Four classes of service available:
– [0] basic connectionless class,
– [1] sequenced connectionless class (messages arrive in
order),
– [2] basic connection oriented class, and
– [3] flow control connection oriented class.
SS#7 Protocol Stack
Above SCCP
• Operations, Maintenance and Administration Part
(OMAP) provides services for the operator
– Include updating routing tables, and diagnostics
• Transactions Capabilities Application Part (TCAP)
provides database transaction service for the
upper level protocols (such as INAP).
SS#7 Environment Adaptation
• SS#7 is used for the core network
• For the access network, there are different signaling stacks:
• For ISDN: LAPB/LAPD corresponding MTP-2 on top of
which Q.931 corresponding to ISUP.
• For B-ISDN:
– MTP layers 1-2 can be replaced by a specific Signaling AAL
protocol (SAAL)
– Three layer protocol: bottom: AAL5, then SSCOP (Service
Specific Connection Oriented Peer-to-Peer Protocol), then
SSCF (Service Specific Coordination Function).
SSCOP+SSCF = Service Specific Convergence Sublayer.
– BISUP can be replaced with ATM Forum UNI or ITU-T Q.2931.