Switching and Signalling

Telephony Basics

Telephony is the transmission and switching of voice signals between subscribers. A telephone network provides call setup, speech-path connection, supervision, charging and call release.

PSTN connection between two subscribers.

Part	Function
Telephone set	Converts speech to electrical signal and back (transducer + hybrid)
Local loop	Copper pair between subscriber and exchange
Local exchange	Detects dialling, switches calls, provides line supervision
Trunk exchange	Connects calls between exchanges
Signalling system	Carries control information for setup and release
Transmission system	Carries voice channels over copper, microwave, fibre or radio

Main parts of the PSTN

Line state	Meaning
On-hook	Handset down; loop open; no call current
Off-hook	Handset lifted; loop closed; exchange detects demand for service
Dialling	Digits sent by pulse or DTMF tone dialling
Ringing	Exchange applies ringing voltage to called line
Answer	Called party goes off-hook and the speech path is established
Release	One or both parties go on-hook; connection is cleared

Subscriber line states

Switching Functions

A switching system connects an incoming line or trunk to an outgoing one selected from the dialled address and routing rules. It allocates resources during setup, supervises the call, and releases those resources when the call ends.

Function	Explanation
Line scanning	Detect off-hook/on-hook condition of subscriber lines
Digit reception	Receive dial pulses or DTMF digits
Number analysis	Interpret dialled number and determine route
Path selection	Select a free path through the switching network
Call setup	Establish connection between calling and called parties
Supervision	Monitor answer, busy, release and faults
Charging	Record call duration, destination and tariff class
Signalling	Exchange control information with other exchanges
Maintenance	Alarm, testing, fault localisation, traffic measurement

Main functions of a switching system

Note

Switching-system requirements

• Capacity: Adequate switching paths and call-processing capacity are needed to meet busy-hour blocking and setup-delay targets.

• Call integrity: The exchange must connect the correct parties, maintain speech quality, detect answer and release accurately, and produce correct charging records.

• Availability: Redundant control, power, and transmission paths, together with fault detection and recovery, limit service interruption. Expansion must preserve these properties as traffic grows.

Electromechanical Switching

Electromechanical switches establish speech paths through mechanical contacts operated by electrical signals. Strowger selectors and crossbar matrices are the principal examples.

Strowger Step-by-Step Switch

• Selection: Dial pulses operate stepping magnets. A two-motion selector first moves vertically to a contact-bank level, then rotates to a contact on that level. Intermediate selectors commonly hunt for a free outgoing trunk; final selectors use the remaining digits to select the subscriber.

• Progressive control: Successive digits establish the path through a train of selectors, tying routing closely to the dialled number and exchange wiring.

• Limitations: Mechanical movement and contact wear make switching relatively slow and maintenance-intensive. Changing routes or services can require wiring changes.

Crossbar Switch

• Crosspoint operation: Electromagnets operate intersecting select and hold bars to close the contacts connecting a chosen inlet and outlet. The connection is held for the call.

• Common control: Registers receive digits, and shared control equipment selects an available path before operating the crosspoints. This separates digit reception from mechanical path selection and supports more flexible routing than direct step-by-step control.

Note

Crosspoint count of a full matrix

$$\text{Crosspoints}=N\times M,\qquad \text{(square exchange)}=N^{2}.$$

Strowger selector and crossbar switch.

Feature	Strowger	Crossbar
Control	Direct (dial pulses)	Common control possible
Element	Mechanical selector	Electromagnetic crosspoints
Advantage	Removed operator	Faster, more reliable
Limitation	Slow, noisy, high maintenance	Many crosspoints for large $N$

Electronic and SPC Switching

Stored Program Control (SPC) switching uses a computer-controlled processor to perform call processing. The switching path may be analog or digital, but control is carried out by stored software instructions.

Block diagram of a stored-program-controlled exchange.

Unit	Function
Line/trunk interface	Connects subscriber lines and trunks
Scanner	Detects line state changes
Distributor	Sends control signals to devices
Marker/path control	Selects the switching path
Central processor	Executes call-processing programs
Memory	Stores programs, subscriber data, routing tables
Switching network	Establishes the speech/data path
Maintenance terminal	Testing, alarms, operation and maintenance

Main units of an SPC exchange

• Centralised SPC: A central control complex performs call processing. It can use duplicated processors for availability; centralised control does not necessarily mean one unprotected CPU.

• Distributed SPC: Cooperating processors divide control among subsystems. This can improve scalability and fault containment, but requires coordination and recovery from interprocessor failures.

Note

Advantages of SPC

• Routing and services: Software and data-table changes can alter numbering, routes, tariffs, and services such as call forwarding or call waiting without rewiring the switching network.

• Supervision and records: Call-state tracking supports accurate charging, traffic measurement, alarms, and automatic tests.

• Maintenance: Remote operation and processor redundancy support fault recovery, provided software, stored data, and failover procedures are tested and protected.

Digital Switching Concepts

A PCM switch moves samples between input and output time slots and digital highways. Conventional voice produces one 8-bit sample every $125\,\mu\mathrm{s}$, giving a $64\,\mathrm{kbit/s}$ channel.

Note

PCM voice channel bit rate

$$R_b=f_s\times n = 8000~Sa/s\times 8~bit =64\,kbit/s.$$

E1 frame item	Value
Number of time slots	32
Bit rate per time slot	64 kbit/s
Total bit rate	2.048 Mbit/s
Bearer channels in PCM-30	30
TS0	Framing / synchronisation
TS16	Signalling in CAS systems

Space Switch

During each time slot, control memory enables crosspoints connecting selected input highways to output highways. A sample changes highway but retains its slot position; the switch does not buffer it for a different slot. Connections can change from one slot to the next.

Digital space-switching matrix.

Note

Space-switch crosspoints

$$\text{Crosspoints}=N\times M \quad (N~\text{input highways},~M~\text{output highways}).$$

Time Switch

A time-slot interchanger writes incoming PCM samples to speech memory and reads them in the order required by the output slots. Control memory stores the address mapping. Buffering changes the slot position and introduces a slot-dependent delay; a sample may be emitted in the following frame when necessary.

Time-slot interchanger with speech and control memories.

Mode	Operation
Sequential write, controlled read	Store in natural order, read in output order
Controlled write, sequential read	Store in output position, read in natural order

Multistage Digital Switching

Combining time and space stages provides alternative internal paths without a single enormous switching matrix. Hardware cost and blocking depend on the number of internal highways, available time slots, and route-selection algorithm. The letters TS, ST, TST, and STS give the stage order from input to output.

Time-Space and Space-Time Switching

• TS: The input time stage changes the slot; the following space stage connects the sample to its output highway during that slot.

• ST: The space stage first selects a highway without changing the slot; the output time stage then moves the sample to its required destination slot.

Time-Space-Time Switching

To connect $H_1$/slot 3 to $H_3$/slot 18, choose an internal slot, here slot 12, that is free on the relevant input and output paths through the space stage.

Time-space-time (TST) switching.

1. The input time switch stores the sample received in slot 3 and reads it in internal slot 12.

2. During slot 12, the space switch connects the input highway to the required output highway.

3. The output time switch stores the sample and emits it in slot 18. The mapping repeats each frame until the connection is released.

Space-Time-Space Switching

For the same $H_1$/slot 3 to $H_3$/slot 18 connection, select an intermediate path $H_2$ whose time switch and space-stage connections are available at the required slots.

Space-time-space (STS) switching.

1. The first space switch transfers the sample from $H_1$ to $H_2$ during slot 3.

2. The time switch on the intermediate path moves the sample from slot 3 to slot 18.

3. During slot 18, the final space switch connects the intermediate path to $H_3$.

Note

Internal blocking

A free destination does not guarantee a usable internal path. Blocking occurs when every candidate internal slot in TST, or every suitable intermediate path in STS, conflicts with existing connections. Adequate stage expansion and a suitable routing algorithm are needed for a stated nonblocking guarantee.

Network	Main use	Advantage	Limitation
TS/ST	Small systems	Simple	Limited flexibility/expansion
TST	Digital telephone exchanges	Flexible, hardware efficient	Precise timing/control
STS	Some multistage designs	Reduces crosspoints	More space-stage hardware

Comparison of multistage switching networks

Signalling Concepts

Signalling is the exchange of control information needed to establish, supervise, charge and release calls.

• Call information: Includes seizure, calling and called addresses, routing, alerting, answer, busy or congestion indications, and release causes. Answer and release events also drive charging records.

• Subscriber signalling: Occurs between the user terminal and exchange, for example loop-state changes, dial tone, DTMF digits, and ringing.

• Inter-exchange signalling: Carries control information between network nodes, for example SS5 tones or SS7 ISUP messages.

Feature	In-band	Out-of-band
Placement	Within the bearer signal band	Outside the bearer band or on a separate logical signalling channel
Interaction	User signals may interfere with control detection	Control is separated from the speech waveform
Example	SS5 tones in a voice circuit	SS7 links; E1 TS16 signalling outside the voice slots
Security	Exposed to signalling-like signals on the bearer	Avoids that exposure but still requires access control and message validation

Note

CAS vs CCS

• CAS (channel-associated signalling): Each bearer circuit has an assigned signalling resource. E1 CAS allocates fixed TS16 bits to particular voice channels across a multiframe, even though those bits are outside the speech slots.

• CCS (common-channel signalling): Addressed messages for many circuits share a signalling channel or network. SS7 messages identify the affected circuit or transaction rather than relying on a fixed per-channel bit position.

• Independent distinctions: CAS need not be in-band: E1 TS16 CAS is channel-associated but separate from the bearer slots. In-band/out-of-band describes placement; CAS/CCS describes how signalling resources are associated with calls.

Signalling System No. 5 (SS5)

SS5 is a legacy international telephone signalling system. It uses in-band line-signalling tones and multifrequency register signals to transfer call-control and address information over the associated speech circuit.

Feature	SS5
Type	In-band signalling
Use	International telephone trunks
Medium	Same channel as speech
Digit signalling	Multifrequency tones
Limitation	Interference/fraud prone, slower setup

• Bearer dependence: Tone-based setup occupies the associated circuit and requires reliable tone detection. The speech path can carry signals resembling control tones, creating interference and fraud risks.

• Limited information: Its signalling vocabulary is less extensible than addressed digital messages, making database services and rich call-control features difficult compared with SS7.

Signalling System No. 7 (SS7)

• Common-channel transport: SS7 carries addressed control messages on signalling links logically separate from speech channels. One link can control many bearer circuits, and the signalling route need not follow the speech route.

• Services: ISUP controls circuit calls. Other SS7 applications query databases for intelligent-network services, mobile roaming, and SMS.

Node	Full form	Function
SSP	Service Switching Point	Originates/terminates signalling at exchange
STP	Signal Transfer Point	Routes SS7 messages
SCP	Service Control Point	Service logic and database access, for example intelligent-network number translation

SS7 network architecture and protocol stack.

Layer/part	Function
MTP Level 1	Physical signalling data link
MTP Level 2	Reliable link transfer, error control
MTP Level 3	Message routing and network management
SCCP	Extended addressing, connectionless/connection-oriented service
ISUP	Call setup and release for telephone calls
TUP	Older telephone user part
TCAP	Transaction capabilities for database queries
MAP	Mobile application operations, such as location updates and SMS, carried using TCAP

SS7 protocol stack

Note

SS7 benefits and dependencies

• Efficiency and services: Digital messages carry richer control information than per-circuit tones, support database transactions without a speech connection, and allow signalling for many calls to share network resources.

• Resilience: Alternate signalling routes and network management support recovery, but common-channel failures can affect many calls. Diverse links and redundant signalling nodes are necessary.

• Security: Separating control from speech prevents users’ bearer tones from acting directly as SS7 messages. SS7 is not inherently secure against untrusted signalling peers; screening, access controls, and traffic monitoring remain essential.

ISUP call establishment and release.

1. IAM (Initial Address Message): The originating exchange selects a circuit and sends the called address and call information toward the destination.

2. ACM (Address Complete Message): The destination side reports that sufficient address information has been received. It may indicate that the called subscriber is free; ACM does not universally mean audible ringing has begun.

3. ANM (Answer Message): Reports answer. Conversation proceeds and answer-based charging normally begins.

4. REL (Release): Requests clearing and includes a cause value.

5. RLC (Release Complete): Confirms release so the circuit can return to the idle pool.

Solved Examples

Note

Example: Full crossbar crosspoints

A full matrix with 100 inlets and 100 outlets requires one crosspoint for each inlet–outlet pair:

$$\text{Crosspoints}=N\times N=100\times100=10{,}000.$$

[!NOTE]

Example: PCM voice-channel bit rate

At 8000 samples/s and 8 bits/sample, the PCM channel rate is

$$R_b=8000\times8=64{,}000~bit/s=64\,kbit/s.$$

Model Answers

Note

Example: Basic functions of a switching system (5 marks)

An exchange connects an incoming subscriber line or trunk to an outgoing line or trunk, maintains the call, and releases the resources when it ends.

• Detect service requests: Scan subscriber lines for off-hook/on-hook changes and receive seizure messages on trunks. Supply dial tone when ready to receive digits.

• Receive and analyse the address: Collect pulse or DTMF digits, or inter-exchange address messages; determine the destination, permitted service, and route.

• Select and establish a path: Reserve a free outlet and internal path, operate crosspoints or time-slot mappings, and exchange setup and alerting signals with the destination side.

• Supervise and charge: Track ringing, answer, busy, faults, and disconnect. Record answer time, release time, destination, and tariff information as required.

• Release: Clear the path and return ports, trunks, and time slots to the available pool.

• Maintain service: Test equipment, collect traffic measurements, report alarms, and recover from failures while meeting blocking, setup-delay, and availability targets.

Example: Strowger, crossbar and SPC switching (5–10 marks)

• Strowger: Dial pulses drive mechanical selectors progressively through a switching train.

  • A two-motion selector moves vertically to a bank level and rotates to a contact; intermediate stages may hunt for a free trunk.

  • It removed the need for an operator to connect each call, but mechanical motion, contact wear, and routing tied to selector wiring made it slow and difficult to modify.

• Crossbar: Select and hold magnets close contacts at the required intersections of a switching matrix.

  • Common registers and markers collect digits and choose a path before operating the switch, decoupling address reception from direct selector movement.

  • A full $N\times M$ matrix uses $NM$ crosspoints. Operation is faster than stepping through many selector positions, but the contacts remain mechanical and large matrices require substantial hardware.

• Stored Program Control: A processor executes software for call processing, routing, charging, and maintenance.

  • Scanners report line state; the processor uses subscriber and route data to allocate a path, control signalling, and update call records through answer and release.

  • Centralised SPC uses a central control complex, often duplicated; distributed SPC divides functions among cooperating processors.

  • SPC describes the control method, not the bearer technology. The controlled switching path can be electromechanical, analog electronic, or digital.

Feature	Strowger	Crossbar	SPC
Control	Direct pulse	Common control	Stored software
Path element	Stepping contacts	Crosspoints	Independent of control method
Speed	Low	Higher	High
Services	Basic	Improved	Rich (forwarding, ID)

Software control permits routing and service changes without rewiring, supports remote maintenance, and records detailed traffic and billing data. Its availability also depends on software correctness, protected data, reliable power, and tested failover.

Example: TS, ST, TST and STS digital switching (10 marks)

Conventional digital telephone switches move 8-bit PCM samples between highways and time slots. A $64\,\mathrm{kbit/s}$ voice channel supplies one sample every $125\,\mu\mathrm{s}$.

• Time stage (T): Stores samples in speech memory and retrieves them in the required output order using control-memory addresses. It changes slot position; buffering may defer output to the next frame.

• Space stage (S): Enables a crosspoint between an input and output highway during a slot. It changes highway without changing slot position. A full matrix has $N\times M$ crosspoints.

• TS and ST: TS changes the slot before selecting the output highway. ST selects a highway first and then changes the slot at the output time stage.

• TST: The input time switch moves a sample to a free internal slot, the space switch transfers it to the chosen output path in that slot, and the output time switch emits it in the destination slot.

• STS: The first space switch selects an intermediate highway, its time switch changes the slot, and the final space switch selects the destination highway during the new slot.

• TST example: To connect H1/TS3 to H3/TS18, select internal slot 12 if it is free on both relevant paths. Input T stores the slot-3 sample and reads it at slot 12; S transfers it toward H3; output T stores it and emits it at slot 18. This mapping repeats each frame until release.

Network	Strength	Limitation
TS/ST	Simple small system	Limited paths/expansion
TST	Many internal slots; efficient	Memory speed, frame control
STS	Distributes space paths	More crosspoints/highways

TST obtains path flexibility by choosing among internal time slots; STS chooses among intermediate highways and time switches. Neither name alone guarantees a nonblocking network: stage dimensions, existing occupancy, and routing determine whether a free internal path exists.

Example: Channel-associated signalling and SS7 (5–10 marks)

Signalling carries seizure, address, routing, alerting, answer, charging-related events, supervision, and release information.

• CAS: Each bearer circuit has an associated signalling resource. SS5 uses in-band tones on its speech circuit; E1 CAS instead assigns each channel fixed signalling bits in TS16. Thus CAS does not necessarily mean in-band signalling.

• CCS/SS7: A common signalling network carries addressed messages for many circuits and transactions. SSPs originate or terminate call signalling, STPs route messages, and SCPs execute service logic or query databases.

• Protocol parts: MTP1 provides the physical link, MTP2 reliable link transfer, and MTP3 routing and network management. ISUP controls circuit calls. SCCP adds addressing and services above MTP; TCAP supports transactions, with applications such as MAP supplying mobile-service operations.

• ISUP sequence:

  • IAM identifies the selected circuit and transfers address and call information.

  • ACM reports address completion; ANM reports answer and normally starts answer-based charging.

  • REL requests release with a cause; RLC confirms that clearing is complete.

• Benefits and limits: SS7 enables richer call control, database services, roaming, and SMS without carrying control tones through speech channels. Shared signalling requires redundant routes and congestion management; peer screening and access control are still necessary because separation from speech is not a complete security boundary.

Point	CAS	SS7/CCS
Association	Fixed signalling resource per bearer	Messages identify circuits or transactions
Transport	Bearer tones or assigned signalling bits	Common signalling links
Services	Limited per-channel information	Extensible call and database messages
Risks	In-band variants exposed to bearer interference	Untrusted signalling peers; shared-link failures