---
title: "IP and soft switching for PC-to-PSTN calls"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:ip-soft-switching-skype-pstn"
source: https://www.bctengineer.com/q/semester_8/telecommunications/ip-soft-switching-skype-pstn
site: BCT Engineering
---

# IP and soft switching for PC-to-PSTN calls

Telecommunications · Chapter 8: Data Communication

- Asked in: 2076 Asoj, 5 marks
- Video: [Tutorial: What is a Softswitch?](https://www.youtube.com/watch?v=qsiOb8Xazfw) — Teracom Training Institute
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/ip-soft-switching-skype-pstn
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Explain how IP switching and soft switching allow a call to be made from a personal computer with Internet access and running Skype to a telephone connected to PSTN?

## Short answer

Skype sends compressed voice packets over IP to a gateway, which converts them to SIP/RTP. A softswitch controls call setup, routing, and billing, and directs a media gateway to convert packets to 64 kbps PCM. SS7 signalling connects the call to the PSTN exchange.

## Answer

A call from a PC running Skype to an ordinary telephone crosses two different networks: the **packet-switched Internet (IP switching)** and the **circuit-switched PSTN**. A **softswitch** and **gateways** join them. (Skype itself uses its own proprietary protocol; standard SIP/RTP is used from Skype's gateway onwards, inside the operator network.)

```text
PC (Skype client)
   | voice + signalling in Skype's own
   | (proprietary) protocol over UDP/IP
   v
Internet routers (IP switching)
   |
   v
Skype "SkypeOut" gateway --> SIP + RTP
   |
   v
+---------------+  H.248  +------------------+
| Media Gateway |<------->|    Softswitch    |
| pkt <-> TDM   |         | call control,    |
+---------------+         | routing, billing |
   | E1 / PCM             +------------------+
   v                               | SIGTRAN
PSTN exchange <--SS7 ISUP-- Signalling Gateway
   |
   v
Telephone
```

#### Role of IP switching
- The Skype client digitises and compresses the voice (e.g. SILK, or G.729 for calls to phones) and puts about 20 ms of speech in each packet, sent over UDP/IP using Skype's own protocol.
- Internet routers switch these packets hop by hop using the destination IP address, with no fixed circuit. The packets reach Skype's "SkypeOut" gateway, which passes the call as standard SIP signalling and RTP voice to a partner telecom operator.

#### Role of the softswitch
A softswitch is call-control software running on servers. It does the job of the control part of a classic exchange, separate from the media path.
- Receives the call request (dialled number) from Skype's gateway as a SIP INVITE.
- Authenticates the user and checks credit (billing).
- Chooses the route and the gateway nearest the called number.
- Controls the media gateway using MGCP/H.248 (Megaco).
- Maps SIP signalling into **SS7 ISUP** messages (IAM, ACM, ANM, REL), which the **signalling gateway** delivers to the PSTN.

#### Call steps
1. The user dials +977-1-xxxxxxx in Skype; the request goes over IP to Skype's servers, which forward it as SIP to the operator's softswitch.
2. The softswitch verifies the account and selects a gateway and trunk.
3. The signalling gateway sends an ISUP **IAM** to the PSTN exchange; the exchange rings the phone and returns **ACM**.
4. When the phone is answered, **ANM** comes back; the softswitch tells the media gateway to connect the RTP stream to a TDM time slot.
5. The media gateway converts packets to 64 kbps PCM (G.711) and back, adding jitter buffering and echo cancellation.
6. At hang-up, BYE/REL messages release the slot, and the softswitch records the call for billing.

So IP switching carries the voice packets across the Internet, while the softswitch provides the intelligence (signalling, routing, billing) and controls the gateways that join the Internet to the PSTN.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the softswitch do in a PC-to-PSTN call?
   A. Provides call control: authentication, billing, routing, and controls the gateways
   B. Digitises the voice
   C. Carries the voice packets
   D. Acts as the telephone
   Answer: A. It is call-control software separate from the media path. (Key point: Softswitch role)

2. What does the media gateway do?
   A. Converts packets to 64 kbps PCM (G.711) TDM and back, with jitter buffering and echo cancellation
   B. Runs the Skype client
   C. Converts SIP to ISUP
   D. Bills the user
   Answer: A. The signalling gateway handles SIP to SS7 ISUP; the media gateway handles voice. (Key point: Media gateway)

3. How do Internet routers handle the voice packets?
   A. By TDM time slots
   B. Via a fixed circuit
   C. Through SS7
   D. Hop by hop using the destination IP address, with no fixed circuit
   Answer: D. Voice is carried in RTP/UDP/IP packets. (Key point: IP switching in call)

4. In the PSTN leg, what does ANM signal?
   A. Call release
   B. Address complete, phone ringing
   C. The called phone has answered
   D. Call setup request
   Answer: C. IAM starts the call, ACM means ringing, ANM means answered, REL releases. (Key point: ISUP messages)

## Related in Telecommunications

- [PC-to-phone calls via internet and PSTN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apc-to-phone-call-internet) — Telecommunications, chapter 8
- [Short note: Telephone numbering in PSTN](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-pstn-numbering) — Telecommunications, chapter 7, asked 2×
- [Soft switching architecture and management](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asoft-switching-architecture) — Telecommunications, chapter 8, asked 4×
- [Short note: Soft switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-soft-switching) — Telecommunications, chapter 8, asked 3×
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×
- [Switching hierarchy and evolution until the telephone](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-telephone-evolution) — Telecommunications, chapter 1, asked 3×

## Related in other subjects

- [PC and SP in CALL, RET, PUSH, POP](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Apc-sp-call-ret-push-pop) — Microprocessor, chapter 2
- [Report on internet access for students](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Areport-internet-access-students-committee) — Communication English, chapter 9
- [Internet-connected system threats](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Ainternet-security-threats) — Operating System, chapter 8