Telephone Networks & Signalling: Calls, Switches & Protocols

TAKEAWAYS:

• Telephone networks use circuit switching (PSTN) and packet switching (VoIP) to connect calls, with signalling protocols (SS7, SIP) managing call setup/teardown.
• Central offices (COs) and switches (e.g., Class 4/5) route calls via hierarchical networks, balancing load and cost.
• Signalling System No. 7 (SS7) enables real-time call control, while Session Initiation Protocol (SIP) powers modern VoIP (e.g., WhatsApp calls).
• Analog vs. digital signalling: Analog uses frequency division (e.g., dual-tone multifrequency, DTMF), while digital uses ISDN or VoIP codecs (e.g., G.711).
• Nepal’s telephony: NTC’s PSTN, Ncell’s GSM, and eSewa’s VoIP all rely on these principles for call routing and billing.
• Exam focus: Trace a call path (e.g., Pokhara→Kathmandu), compare PSTN/VoIP, and explain SS7/SIP message flows.

1. Telephone Network Architecture: How Calls Travel

Telephone networks connect users via switches and transmission media. The core components are:

• End offices (Class 5 switches): Local switches handling subscriber lines (e.g., your home phone → local CO).
• Tandem offices (Class 4 switches): Intermediate switches routing long-distance calls (e.g., Pokhara CO → Kathmandu CO).
• Toll offices (Class 3 switches): High-level switches for international calls (e.g., Nepal→India via undersea cables).

graph TD
    A["User A\n(Pokhara)"] -->|"Local Call"| B["Class 5 Switch\n(Pokhara CO)"]
    B -->|"Toll Call"| C["Class 4 Switch\n(Kathmandu CO)"]
    C --> D["User B\n(Kathmandu)"]
    B -->|"International"| E["Class 3 Switch\n(NTC Gateway)"]
    E -->|"Undersea Cable"| F["India CO"]

Worked Example: A Call from Pokhara to Kathmandu

1. You dial 9800123456 (Ncell number).
2. Your Class 5 switch (Pokhara CO) checks the prefix (98 = mobile) and routes to Ncell’s mobile switching center (MSC).
3. The MSC queries the home location register (HLR) to locate the subscriber in Kathmandu.
4. The call travels via fiber optic cables to Kathmandu’s MSC, then to the destination’s base transceiver station (BTS).
5. The recipient’s phone rings; the call is circuit-switched end-to-end until hung up.

Labelled diagram of a 1950s PSTN switchboard showing manual patch cords (Image: Malcolm Carruthers, CC BY-SA 2.0, via Wikimedia Commons)

2. Circuit Switching vs. Packet Switching in Telephony

Why PSTN Still Matters in Nepal:

• NTC’s landline network (used by banks, hospitals) relies on Time Division Multiplexing (TDM) over copper pairs.
• Ncell’s 2G/3G uses circuit-switched data for voice, while 4G/5G uses VoIP (IMS core).
• VoIP in Nepal: eSewa’s customer service uses SIP trunking to route calls over the internet, reducing costs.

3. Signalling Protocols: The Invisible Handshake

Signalling protocols control call setup, routing, and teardown. The two key systems are:

A. SS7 (Signalling System No. 7)

• Used in PSTN and mobile networks (e.g., Ncell’s call routing).
• Operates over separate signalling links (not the same as voice data).
• Message types:
  • Initial Address Message (IAM): "Start call from A to B."
  • Address Complete Message (ACM): "B’s number is valid."
  • Answer Message (ANM): "B picked up."
  • Release Message (REL): "Call ended."

sequenceDiagram
    participant A as User A (Pokhara)
    participant CO1 as Pokhara CO (Class 5)
    participant CO2 as Kathmandu CO (Class 4)
    participant B as User B (Kathmandu)
    A->>CO1: Dial 9800123456 (DTMF)
    CO1->>CO2: IAM (SS7)
    CO2->>B: Ring (via MSC)
    B->>CO2: Off-hook (ANM)
    CO2->>CO1: ACM (SS7)
    CO1->>A: Call connected
    A->>CO1: Hang up (REL)
    CO1->>CO2: REL (SS7)

Real-World Use in Nepal:

• When you call Ncell customer care (100), the SS7 network routes you to their Intelligent Network (IN) server, which plays pre-recorded menus.
• NTC’s 198 call center uses SS7 to log call durations for billing.

B. SIP (Session Initiation Protocol)

• Used in VoIP (e.g., WhatsApp calls, Zoom).
• Works over IP networks (like the internet).
• Message flow:

  sequenceDiagram
      participant A as User A (VoIP)
      participant SIPServer as SIP Proxy Server
      participant B as User B (VoIP)
      A->>SIPServer: INVITE (SIP)
      SIPServer->>B: INVITE
      B->>SIPServer: 200 OK
      SIPServer->>A: 200 OK
      A->>B: RTP (Voice Data)

Worked Example: WhatsApp Call from Kathmandu to India

1. You start a call → WhatsApp sends SIP INVITE to Google’s servers.
2. Google’s SIP proxy routes to the recipient’s device via WebRTC.
3. Voice is compressed (Opus codec) and sent as RTP packets.
4. If the call drops, SIP sends BYE to tear down the session.

4. Signalling Methods: Analog vs. Digital

Worked Example: Dialing a Bank’s IVR (e.g., NMB Bank)

1. You call 9800123456 → DTMF tones 1 (for English) are sent.
2. The Class 5 switch forwards DTMF to the bank’s IVR server.
3. The IVR plays: "Press 1 for balance inquiry" → You press 1 → DTMF 1 triggers a database query.

5. Telephone Network Topologies in Nepal

Nepal’s telephony uses hierarchical and mesh topologies:

1. Hierarchical (PSTN):

   • Local calls: Star topology (all users → central CO).
   • Long-distance: Tree topology (CO → tandem → toll office).

   graph TD
       A["Pokhara CO"] --> B["Kathmandu CO"]
       A --> C["Bhaktapur CO"]
       B --> D["India Gateway"]

2. Mesh (Mobile Networks):

   • Ncell’s 4G/5G uses a partial mesh where BTSs connect to multiple MSCs for redundancy.
   • Fiber backhaul: NTC’s undersea cable (connecting Nepal to India) uses a ring topology for fault tolerance.

6. Specialized Services and Signalling

A. Intelligent Network (IN)

• Adds extra services (e.g., Ncell’s "Missed Call Alert").
• Uses SS7 to trigger actions:
  • If you miss a call → SMS sent via SS7 to your phone.
  • If you call a premium number (e.g., 16600) → IN routes to a special server.

B. Number Portability (Nepal’s Example)

• Since 2018, Nepal allows mobile number portability (switch carriers without changing number).
• How it works:
  1. You request porting from Ncell to NTC.
  2. SS7’s HLR updates the new MSC’s database.
  3. Incoming calls are automatically routed to your new network.

7. Challenges in Nepal’s Telephone Networks

Real-World Fix: NTC’s Kathmandu Ring

• A 1,200 km fiber optic loop around Kathmandu to prevent single-point failures.
• Uses DWDM (Dense Wavelength Division Multiplexing) to carry 160+ channels on one fiber.

In the Real World

1. Ncell’s Call Routing

   • When you call a Ncell number from a landline, the Class 5 switch uses SS7 to query Ncell’s MSC, which then pages the nearest BTS to ring your phone.
   • Why it matters: Ensures calls reach you even if you’re moving (e.g., from Pokhara to Chitwan).

2. eSewa’s VoIP Customer Service

   • eSewa uses SIP trunking to route calls from their Kathmandu call center to your phone over the internet.
   • How it saves money: Avoids NTC’s per-minute landline charges (costs ~Rs. 10/min vs. ~Rs. 2/min over VoIP).

3. NTC’s 198 Emergency Service

   • When you dial 198, the SS7 network automatically routes you to the nearest police station’s IVR, even if you’re roaming.
   • Tech used: Location Register (LR) updates in real-time via SS7.

Exam Tip

1. Trace a call path: Always draw a sequence diagram for:

   • Pokhara→Kathmandu (PSTN).
   • Kathmandu→India (international via SS7).
   • WhatsApp call (SIP + RTP).

2. Compare PSTN vs. VoIP:

   • PSTN: Circuit switching, SS7, copper wires.
   • VoIP: Packet switching, SIP, internet.

3. SS7/SIP messages:

   • Memorize IAM, ACM, ANM, REL for SS7.
   • For SIP: INVITE, 200 OK, BYE.

4. Nepal-specific:

   • NTC’s fiber backhaul and DWDM.
   • Ncell’s MSC + HLR for roaming.
   • Number portability (SS7’s role).

5. Worked examples:

   • Always quantify: "A call from Pokhara to Kathmandu takes 200ms for SS7 signalling + 50ms for circuit setup."
   • Use real numbers: NTC’s 64 kbps for voice, Ncell’s 16 kbps for compressed VoIP.

Final Visual Summary:

mindmap
  root((Telephone Networks & Signalling))
    PSTN
      Circuit Switching
      SS7 (IAM/ACM/ANM)
      Copper/Wireless
    VoIP
      Packet Switching
      SIP (INVITE/RTP/BYE)
      Internet
    Nepal Examples
      NTC: Fiber + SS7
      Ncell: MSC + HLR
      eSewa: SIP Trunking
    Challenges
      Call Drops (Fiber)
      Fraud (SS7 Weakness)
      Solutions (5G/Diameter)