AEiE0903 Switching systems and Traffic engineering¶
Analog, digital, and soft switching¶
| Switching type | Main basis | Recognition cue |
|---|---|---|
| Analog switching | Direct analog path connection | Older exchange context |
| Digital switching | Time-slot or digital path switching | PCM, TDM exchange |
| Soft switching | Call/session control separated from media transport | IP telephony core |
- Softswitch is a control-oriented architecture in packet networks.
- It replaces many functions of monolithic circuit-switch exchanges with software control over distributed gateways.
Routing and signaling¶
- Routing decides where traffic should go.
- Signaling exchanges control information to establish, supervise, and release connections.
- Common channel signaling carries signaling on a channel separate from the user voice path.
- SS7 is the classic common channel signaling example in telephony.
Teletraffic parameters¶
| Quantity | Meaning | Usual unit or cue |
|---|---|---|
| Busy hour | One-hour period with maximum traffic load | Design reference hour |
| Traffic intensity | Occupancy demand | Erlang |
| Grade of service (GoS) | Probability of call blocking or delay measure | Smaller is better |
| Service level | Performance target to users | Often delay or completion target |
Traffic intensity:
\[
A = \lambda h
\]
- \(A\): traffic intensity in Erlangs.
- \(\lambda\): average call arrival rate per unit time.
- \(h\): mean holding time.
One Erlang means one channel occupied continuously during the observation interval.
Erlang engineering cues¶
- More offered traffic or fewer trunks increases blocking probability.
- Busy hour traffic drives dimensioning.
- Erlang B is the standard loss-system model without waiting.
- Erlang C is used when waiting is possible.
Traffic routing in wireless networks¶
- Wireless routing also considers mobility, handover, and radio resource limits.
- In cellular systems, channel allocation and location updates affect effective traffic handling.
- Location information identifies the serving network or exchange, mobile-terminated traffic is routed toward it, and handover reroutes an active bearer as the user moves.
- Traffic routing chooses the path toward the serving location; radio-channel assignment allocates air-interface resources. They are related but distinct functions.
ISDN essentials¶
- ISDN integrates voice, data, and signaling on a digital network.
- Basic rate interface (BRI) is commonly summarized as 2B + D.
- Primary rate interface depends on regional standard.
- A B channel in ISDN is 64 kbit/s.
- The D channel carries signaling and control.
Packet versus circuit switching for personal communication networks¶
In this syllabus context, PCN is read as personal communication network. Keep the abbreviation unexpanded if a source or question uses a different local convention.
| Aspect | Circuit switching | Packet switching |
|---|---|---|
| Path | Dedicated end-to-end path | Shared links, packets forwarded individually or by VC |
| Setup | Required before transfer | Optional, depends on packet model |
| Delay variation | Low after setup | Variable |
| Efficiency for bursty traffic | Lower | Higher |
- Voice was classically circuit-switched.
- Data traffic is generally better matched to packet switching.
Protocol for network access cues¶
- There is no single universal network-access protocol; the exact procedure depends on the radio or access technology.
- A typical initial-access sequence is synchronization, access request or random access, identity/authentication, and channel or bearer assignment.
- On a shared medium, access protocols can be contention based (ALOHA, slotted ALOHA, CSMA), controlled/reservation based, or channelized through FDMA, TDMA, CDMA, or OFDMA. Technology-specific random-access channels are often called RACH.
- Access protocol decides how a terminal obtains use of the network or shared channel. Signaling controls sessions/calls, routing selects a path, and the bearer carries user traffic; these are related but not interchangeable roles.
- If an exam expects one named legacy PCN protocol, verify the intended course source; the syllabus phrase alone does not identify one unique standard.
Recognition cues for switching systems and traffic engineering¶
- "Probability of blocked call" points to grade of service.
- "Separate signaling network" points to common channel signaling.
- "2B + D" points to ISDN BRI.
- "64 kbit/s bearer" points to B channel.
Common traps in switching systems and traffic engineering¶
- Busy hour is not the entire day average.
- Traffic intensity in Erlangs is dimensionless in origin but conventionally reported as Erlangs.
- Softswitch is not just any digital switch; its hallmark is software-based call/session control with media gateways.
- GoS and service level are related but not identical wording.
One-step examples for switching systems and traffic engineering¶
- If 30 calls per hour each last 3 minutes, then \(A = 30 \times 3/60 = 1.5\) Erlangs.
- A 2B + D line provides two 64 kbit/s bearer channels plus one signaling channel.
- A signaling channel separate from the speech channel is common channel signaling.
Switching-and-traffic-engineering revision box¶
Traffic intensity is measured in Erlangs. Busy hour is the design hour. GoS measures blocking or delay quality. ISDN B channel is 64 kbit/s. Softswitch separates control from media transport.