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CS-702 (C) · Wireless & Mobile Computing/Quick Revision Short Notes

Wireless & Mobile Computing (CS-702 (C)) - Unit 4 Short Notes

How unit 4 is examined

This unit covers why TCP fails on wireless links, the three fixes (I-TCP, S-TCP, M-TCP), and cellular/GSM networks; I-TCP, GSM architecture, cellular architecture and handover carry the marks.

Unsuitability of traditional TCP

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. The transport layer gives end-to-end delivery between processes; TCP does this reliably and assumes every lost packet means congestion. <mark>On a wireless link TCP wrongly treats corruption and disconnection as congestion, cuts its window and throughput collapses.</mark>

Key points.

  1. Transport layer functions are end-to-end process delivery using ports, segmentation and reassembly, flow control, error control and congestion control; TCP is reliable and connection-oriented, UDP is unreliable and connectionless.
  2. Wireless links have a high bit error rate, so segments are lost through corruption, yet TCP assumes congestion, halves its window and slows down needlessly.
  3. Mobility and handover cause temporary disconnections; TCP times out, backs off exponentially and stays idle long after the link returns.
  4. Wireless bandwidth is low and delay varies, so round-trip estimates and timers are wrong and cause spurious retransmissions.
  5. Hence mobile-aware solutions are needed: I-TCP (split), Snoop TCP (local retransmission) and M-TCP (window freeze).

Answer frame. Open with transport layer functions, then TCP versus UDP; develop points 2-4 as the failure causes; close by naming I-TCP, Snoop and M-TCP as remedies.

Asked: [7 marks] (Dec 2024) Discuss the function of transport layer. How protocols of traditional network are not suitable for wireless networks?

I-TCP

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Definition. <mark>Indirect TCP splits one end-to-end TCP connection into two at the Mobile Support Station (base station): a standard TCP connection over the fixed network and a separate wireless-tuned connection to the mobile host.</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u4-01" viewBox="0 0 510 80" width="510" height="80" role="img" aria-label="I-TCP. FH fixed host, MSS mobile support station (splits the connection), MH mobile host. Wired TCP1, wireless TCP2."><style>#dsfig-u4-01 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u4-01 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u4-01 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u4-01 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u4-01 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u4-01 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u4-01 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u4-01 .t{fill:#16181D;font-weight:500}#dsfig-u4-01 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u4-01 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u4-01 .dot{fill:#16181D}#dsfig-u4-01 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u4-01 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u4-01 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u4-01 .ah{fill:#454C5A}#dsfig-u4-01 .ah.hi{fill:#2340B8}#dsfig-u4-01 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u4-01 .wl .t{font-size:12px;font-weight:700}#dsfig-u4-01 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u4-01 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u4-01 .e{stroke:#B1B7C3}html.dark #dsfig-u4-01 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u4-01 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u4-01 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u4-01 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u4-01 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u4-01 .t{fill:#E6E8ED}html.dark #dsfig-u4-01 .t.inv{fill:#0F1115}html.dark #dsfig-u4-01 .kd{stroke:#E6E8ED}html.dark #dsfig-u4-01 .dot{fill:#E6E8ED}html.dark #dsfig-u4-01 .ann{fill:#8FA3FF}html.dark #dsfig-u4-01 .lbl{fill:#858D9C}html.dark #dsfig-u4-01 .ptr{fill:#8FA3FF}html.dark #dsfig-u4-01 .ah{fill:#B1B7C3}html.dark #dsfig-u4-01 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u4-01 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u4-01 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u4-01 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,40 L236,40"/><path class="e" d="M274,40 L451,40"/><g class="wl"><rect x="127.1" y="31" width="40.8" height="18" rx="9"/><text class="t" x="147.5" y="40" dy=".35em" text-anchor="middle">TCP1</text></g><g class="wl"><rect x="342.1" y="31" width="40.8" height="18" rx="9"/><text class="t" x="362.5" y="40" dy=".35em" text-anchor="middle">TCP2</text></g><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">FH</text><circle class="n" cx="255" cy="40" r="18"/><text class="t" x="255" y="40" dy=".35em" text-anchor="middle">MSS</text><circle class="n" cx="470" cy="40" r="18"/><text class="t" x="470" y="40" dy=".35em" text-anchor="middle">MH</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">I-TCP. FH fixed host, MSS mobile support station (splits the connection), MH mobile host. Wired TCP1, wireless TCP2.</figcaption></figure>

Key points.

  1. The fixed host talks to the MSS as if it were the mobile host, so the fixed network and its TCP need no change.
  2. The MSS acknowledges data to the sender itself and forwards it to the mobile host over the second connection, which may use a tuned or different protocol.
  3. Wireless losses are recovered locally between MSS and MH, so the fixed sender never shrinks its window.
  4. On handover the socket state is transferred from the old MSS to the new MSS.
  5. Advantages: no change to fixed hosts, wireless errors stay local, and the wireless part can be optimised.
  6. Disadvantages: end-to-end semantics are lost because an ACK may reach the sender before the data reaches the mobile host; a crashed MSS loses data, and handover is slow.
Point I-TCP S-TCP (Snoop) M-TCP
Idea Split connection Snoop and cache at base station Split with window freeze
Connection Two One, end-to-end Two
End-to-end semantics Lost Kept Kept for ACKs
Handoff State moved Cache rebuilt Supervisory host handoff
Drawback MSS failure, slow handover Fails if encrypted Needs special hosts

Answer frame. Open with the definition; draw the diagram; develop points 1-4, then advantages and disadvantages; close with handover handling. For the Dec 2020 question give one definition each for S-TCP and M-TCP, then the table.

Asked: [7 marks] (Dec 2020) Explain in detail I-TCP, S-TCP and M-TCP. Asked: [7 marks] (Jun 2025) Explain Indirect TCP (I-TCP) with the help of suitable diagram.

S-TCP (Snooping TCP)

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. Snooping TCP places a snoop agent at the base station that watches TCP packets and ACKs of the connection without splitting it.

Key points.

  1. The agent caches unacknowledged segments passing towards the mobile host and retransmits them locally when a wireless loss is detected by a duplicate ACK or a local timeout.
  2. It suppresses duplicate ACKs from the mobile host, so the fixed sender never sees the loss and does not shrink its window.
  3. End-to-end semantics are kept, because the connection stays one TCP connection.
  4. It fails when packets are encrypted, since the agent cannot read sequence numbers.

M-TCP

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Definition. Mobile TCP splits the connection at a supervisory host (SH) and, when the mobile host disconnects, freezes the sender by advertising a zero window.

Key points.

  1. The sender-to-SH part uses unmodified TCP; the SH-to-mobile part uses an optimised TCP.
  2. The SH does not acknowledge the last byte until the mobile host does, which keeps end-to-end semantics for ACKs.
  3. On disconnection the SH sets the window to zero, so the sender goes into persist mode instead of timing out and backing off.
  4. When the mobile host reconnects, the SH reopens the window and full-speed sending resumes at once.

Wireless cellular networks: cellular system

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Low weight</span>

Definition. <mark>A cellular system divides the service area into hexagonal cells, each served by a base station with its own frequency set, and reuses those frequencies in distant cells.</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u4-02" viewBox="0 0 603 209" width="603" height="209" role="img" aria-label="Cellular network. MS mobile station, BTS base transceiver station, BSC base station controller, MSC mobile switching centre, HLR/VLR home and visitor location registers."><style>#dsfig-u4-02 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u4-02 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u4-02 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u4-02 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u4-02 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u4-02 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u4-02 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u4-02 .t{fill:#16181D;font-weight:500}#dsfig-u4-02 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u4-02 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u4-02 .dot{fill:#16181D}#dsfig-u4-02 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u4-02 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u4-02 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u4-02 .ah{fill:#454C5A}#dsfig-u4-02 .ah.hi{fill:#2340B8}#dsfig-u4-02 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u4-02 .wl .t{font-size:12px;font-weight:700}#dsfig-u4-02 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u4-02 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u4-02 .e{stroke:#B1B7C3}html.dark #dsfig-u4-02 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u4-02 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u4-02 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u4-02 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u4-02 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u4-02 .t{fill:#E6E8ED}html.dark #dsfig-u4-02 .t.inv{fill:#0F1115}html.dark #dsfig-u4-02 .kd{stroke:#E6E8ED}html.dark #dsfig-u4-02 .dot{fill:#E6E8ED}html.dark #dsfig-u4-02 .ann{fill:#8FA3FF}html.dark #dsfig-u4-02 .lbl{fill:#858D9C}html.dark #dsfig-u4-02 .ptr{fill:#8FA3FF}html.dark #dsfig-u4-02 .ah{fill:#B1B7C3}html.dark #dsfig-u4-02 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u4-02 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u4-02 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u4-02 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah6" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh6" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,169 L150,169"/><path class="e" d="M188,169 L279,169"/><path class="e" d="M317,169 L408,169"/><path class="e" d="M446,169 L530,169"/><path class="e" d="M427,150 L427,59"/><path class="e" d="M416.5,153.2 L351.5,55.8"/><circle class="n" cx="40" cy="169" r="18"/><text class="t" x="40" y="169" dy=".35em" text-anchor="middle">MS</text><circle class="n" cx="169" cy="169" r="18"/><text class="t" x="169" y="169" dy=".35em" text-anchor="middle">BTS</text><circle class="n" cx="298" cy="169" r="18"/><text class="t" x="298" y="169" dy=".35em" text-anchor="middle">BSC</text><circle class="n" cx="427" cy="169" r="18"/><text class="t" x="427" y="169" dy=".35em" text-anchor="middle">MSC</text><rect class="n" x="531" y="154" width="50" height="30" rx="15"/><text class="t" x="556" y="169" dy=".35em" text-anchor="middle">PSTN</text><circle class="n" cx="427" cy="40" r="18"/><text class="t" x="427" y="40" dy=".35em" text-anchor="middle">HLR</text><circle class="n" cx="341" cy="40" r="18"/><text class="t" x="341" y="40" dy=".35em" text-anchor="middle">VLR</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Cellular network. MS mobile station, BTS base transceiver station, BSC base station controller, MSC mobile switching centre, HLR/VLR home and visitor location registers.</figcaption></figure>

Key points.

  1. The MS connects over radio to the BTS of its cell; several BTSs are controlled by one BSC.
  2. The MSC switches calls, connects to the PSTN, and uses HLR and VLR to track subscribers.
  3. Frequency reuse lets the same frequencies serve different cells that are far apart, which raises capacity.
  4. When the MS moves to another cell the call is handed over without breaking (see handover below).

Answer frame. Draw the diagram first, explain each block in a line, then frequency reuse, then define handover with its types.

Asked: [7 marks] (Dec 2024) With the help of a neat diagram explain the architecture of cellular network. What is the concept of handover?

Cellular networks v/s WLAN

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Definition. A cellular network is a wide-area, licensed-spectrum, operator-run network for mobile voice and data; a WLAN is a short-range, unlicensed-spectrum local network.

Key points.

  1. Cellular range is kilometres per cell; WLAN range is tens of metres.
  2. Cellular uses licensed bands and needs a subscription; WLAN uses free ISM bands.
  3. Cellular supports seamless handover and roaming; WLAN mobility is limited.
  4. Cellular is circuit-oriented for voice with centralised control; WLAN is packet-based with higher data rates at low cost.

GSM - services

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Definition. GSM (Global System for Mobile communications) is the digital cellular standard offering three service classes.

Key points.

  1. Bearer services carry data, such as circuit and packet data up to 9.6 kbit/s, and later GPRS.
  2. Tele services are end-user services: telephony, emergency calling (112), fax and SMS.
  3. Supplementary services add features such as call forwarding, call waiting, call barring, caller ID and conference calling.

System architecture

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Definition. <mark>GSM architecture has three subsystems: the Mobile Station (MS), the Base Station Subsystem (BSS) and the Network and Switching Subsystem (NSS), managed by the Operation Subsystem (OSS).</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u4-03" viewBox="0 0 596 252" width="596" height="252" role="img" aria-label="GSM architecture. BSS = BTS + BSC; NSS = MSC, VLR, HLR, AUC, EIR; OSS = OMC."><style>#dsfig-u4-03 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u4-03 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u4-03 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u4-03 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u4-03 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u4-03 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u4-03 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u4-03 .t{fill:#16181D;font-weight:500}#dsfig-u4-03 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u4-03 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u4-03 .dot{fill:#16181D}#dsfig-u4-03 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u4-03 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u4-03 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u4-03 .ah{fill:#454C5A}#dsfig-u4-03 .ah.hi{fill:#2340B8}#dsfig-u4-03 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u4-03 .wl .t{font-size:12px;font-weight:700}#dsfig-u4-03 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u4-03 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u4-03 .e{stroke:#B1B7C3}html.dark #dsfig-u4-03 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u4-03 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u4-03 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u4-03 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u4-03 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u4-03 .t{fill:#E6E8ED}html.dark #dsfig-u4-03 .t.inv{fill:#0F1115}html.dark #dsfig-u4-03 .kd{stroke:#E6E8ED}html.dark #dsfig-u4-03 .dot{fill:#E6E8ED}html.dark #dsfig-u4-03 .ann{fill:#8FA3FF}html.dark #dsfig-u4-03 .lbl{fill:#858D9C}html.dark #dsfig-u4-03 .ptr{fill:#8FA3FF}html.dark #dsfig-u4-03 .ah{fill:#B1B7C3}html.dark #dsfig-u4-03 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u4-03 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u4-03 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u4-03 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah7" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh7" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,126 L150,126"/><path class="e" d="M188,126 L279,126"/><path class="e" d="M317,126 L408,126"/><path class="e" d="M427,107 L427,59"/><path class="e" d="M442.8,115.5 L540.2,50.5"/><path class="e" d="M442.8,136.5 L540.2,201.5"/><path class="e" d="M556,59 L556,107"/><path class="e" d="M427,145 L427,193"/><circle class="n" cx="40" cy="126" r="18"/><text class="t" x="40" y="126" dy=".35em" text-anchor="middle">MS</text><circle class="n" cx="169" cy="126" r="18"/><text class="t" x="169" y="126" dy=".35em" text-anchor="middle">BTS</text><circle class="n" cx="298" cy="126" r="18"/><text class="t" x="298" y="126" dy=".35em" text-anchor="middle">BSC</text><circle class="n" cx="427" cy="126" r="18"/><text class="t" x="427" y="126" dy=".35em" text-anchor="middle">MSC</text><circle class="n" cx="427" cy="40" r="18"/><text class="t" x="427" y="40" dy=".35em" text-anchor="middle">VLR</text><circle class="n" cx="556" cy="40" r="18"/><text class="t" x="556" y="40" dy=".35em" text-anchor="middle">HLR</text><circle class="n" cx="556" cy="126" r="18"/><text class="t" x="556" y="126" dy=".35em" text-anchor="middle">AUC</text><circle class="n" cx="556" cy="212" r="18"/><text class="t" x="556" y="212" dy=".35em" text-anchor="middle">EIR</text><circle class="n" cx="427" cy="212" r="18"/><text class="t" x="427" y="212" dy=".35em" text-anchor="middle">OMC</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">GSM architecture. BSS = BTS + BSC; NSS = MSC, VLR, HLR, AUC, EIR; OSS = OMC.</figcaption></figure>

Key points.

  1. The MS is the handset with a SIM card; the BTS holds the radio transceivers and the BSC manages radio channels and handover.
  2. The MSC switches calls and links the network to the PSTN.
  3. HLR stores permanent subscriber data and current location; VLR stores temporary data of visiting subscribers.
  4. AUC holds keys for authentication and encryption; EIR holds IMEI lists to block stolen phones.
  5. OSS (OMC) handles operation, maintenance and billing. Interfaces: Um (MS-BTS), Abis (BTS-BSC), A (BSC-MSC).

Answer frame. List subsystems, draw the diagram, explain blocks in the order MS, BTS, BSC, MSC, registers, then interfaces and a call-flow line.

Asked: [7 marks] (Jun 2025) Explain in detail about the system architecture of GSM.

Localization and calling

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Definition. Localization is tracking which location area a mobile is in, using HLR and VLR, so incoming calls can be routed to it.

Key points.

  1. The MS registers via a location update whenever it enters a new location area; the VLR records it and informs the HLR.
  2. For a mobile-terminated call, the gateway MSC asks the HLR for the current MSC/VLR, and the call is routed there and paged.
  3. For a mobile-originated call, the MS requests a channel, is authenticated, and the MSC sets up the call to the dialled number.

Handover and roaming

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Definition. <mark>Handover (handoff) is the transfer of an ongoing call from one channel or cell to another without interrupting the call, needed because the user moves or signal quality drops.</mark>

Key points.

  1. Hard handover breaks the old link before making the new one; GSM uses it. Soft handover makes the new link before breaking the old one; CDMA uses it.
  2. Intra-cell handover changes the channel within the same BTS to escape interference; the BSC controls it.
  3. Inter-cell intra-BSC handover moves the call between two BTSs under one BSC; the BSC controls it.
  4. Inter-BSC (intra-MSC) handover moves the call between BSCs under one MSC; the MSC controls it.
  5. Inter-MSC handover moves the call between MSCs; both MSCs are involved, for example a user driving across a city boundary.
  6. Steps: the MS reports signal measurements, the BSC decides, a channel is reserved in the target cell, the MS is told to switch, and the old channel is released.
  7. Roaming lets a subscriber use service in a visited network; the VLR registers the visitor and the HLR of the home network is updated.

Answer frame. Define handover and its need; list the four GSM types with controlling entity and a scenario each; give the steps; end with roaming.

Asked: [7 marks] (Dec 2020) Define Handoff. What are its types? What are the four types of handover available in GSM?

Last-minute revision

  • Traditional TCP treats every loss as congestion; wireless loss is corruption or disconnection.
  • I-TCP splits the connection at the MSS: fixed TCP plus wireless TCP.
  • I-TCP loses end-to-end semantics; S-TCP keeps them.
  • S-TCP snoops, caches and retransmits locally, and suppresses duplicate ACKs.
  • M-TCP freezes the sender with a zero window on disconnection.
  • GSM subsystems: MS, BSS (BTS+BSC), NSS (MSC, HLR, VLR, AUC, EIR), OSS.
  • HLR is permanent; VLR is temporary; AUC keys; EIR IMEI.
  • GSM interfaces: Um, Abis, A.
  • Four GSM handovers: intra-cell, inter-cell intra-BSC, inter-BSC, inter-MSC.
  • GSM services: bearer, tele and supplementary.

Memory hooks

  • I-TCP: "I split it" at the MSS.
  • Snoop: spy, store, resend.
  • M-TCP: Mute the sender with a zero window.
  • GSM order: MS, BTS, BSC, MSC, then registers.
  • Handover ladder: cell, BSC, MSC, then across MSCs.

Coverage checklist

  • unsuitability of Traditional TCP: Dec 2024 transport layer and unsuitability question.
  • I-TCP: Dec 2020 I-TCP, S-TCP, M-TCP; Jun 2025 I-TCP with diagram.
  • S-TCP: covered in the Dec 2020 question.
  • M-TCP: covered in the Dec 2020 question.
  • Wireless Cellular networks: Cellular system: Dec 2024 cellular architecture and handover.
  • Cellular networks v/s WLAN: no past question.
  • GSM - Services: no past question.
  • system architecture: Jun 2025 GSM architecture.
  • Localization and calling: no past question.
  • handover and Roaming: Dec 2020 handoff types and four GSM handovers.
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