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

Wireless & Mobile Computing (CS-702 (C)) - Unit 3 Short Notes

How unit 3 is examined

This unit covers WLAN problems and 802.11, Mobile IP and ad hoc routing; the marks sit in Mobile IP terms, MAC problems, ad hoc networks, 802.11 architecture and AODV.

Transmission Medium For WLANs

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Definition. A WLAN carries data between stations over infrared light or radio waves instead of cables.

Key points.

  1. Infrared needs line of sight, cannot pass walls and works only inside one room, but it is free of radio interference and licence.
  2. Radio in the licence-free 2.4 GHz and 5 GHz ISM bands passes through walls and gives longer range.
  3. Radio suffers interference from other devices, multipath fading and easy eavesdropping.
  4. Wireless links have higher error rates and lower bandwidth than wired links.

MAC problems

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Definition. Wireless MAC problems are the difficulties in sharing one radio channel that wired CSMA/CD never faced, because a wireless node cannot hear every other node or its own collisions.

Key points.

  1. Hidden and exposed terminals make carrier sensing unreliable, so collisions occur or transmissions are wrongly blocked.
  2. Bandwidth is scarce and shared, so the protocol must have low overhead and waste little on control packets.
  3. Battery power is limited, so the protocol must let idle nodes sleep and avoid needless retransmissions.
  4. Mobility keeps changing who is in range, so neighbour information goes stale quickly.
  5. Without a base station, synchronisation and channel allocation must be distributed, with no central scheduler.
  6. Radio errors and a node's own strong signal drowning others (near-far) make collisions undetectable while sending.

Comparison: wired vs wireless media access.

Basis Wired (Ethernet) Wireless (802.11)
Method CSMA/CD CSMA/CA with optional RTS/CTS
Collision Detected while sending Cannot be detected, so avoided
Sensing Every node hears the cable Range-limited: hidden and exposed nodes
Reliability Rare errors, no ACK High errors, link-layer ACK needed
Topology Fixed Changing with mobility
Related protocols None MACA, MACAW

Answer frame. Open by defining an ad hoc network as infrastructure-less and mobile; list issues 1-6 with one sentence each; for the wired comparison use the table; close that MAC design must be distributed, collision-avoiding and power-saving.

Asked: [7 marks] (Dec 2020, Dec 2024) List and explain the issues in designing a MAC protocol for adhoc networks. How media access is different in wireless networks as compared to wired networks?

Hidden and Exposed terminals

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Definition. A hidden terminal is a node out of a sender's range that can still collide with it at a common receiver; an exposed terminal is a node that needlessly defers because it hears a neighbouring sender.

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Key points.

  1. Hidden case: A and C both sense an idle channel, both send to B, and their frames collide at B.
  2. Exposed case: B is sending to A, C hears B and defers, although a transmission from C to D would not disturb A, so bandwidth is wasted.
  3. RTS/CTS reduces hidden collisions: the receiver's CTS is heard by the hidden node, which then stays silent.
  4. Near and far terminals: see the next topic.

Asked: [7 marks] (Dec 2020) Discuss hidden and exposed terminal problem and near and far terminal problem.

Near and Far terminals

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Definition. The near-far problem is when a nearby terminal's strong signal drowns a distant terminal's weak signal at the receiver.

Key points.

  1. The receiver cannot decode the far node because signal strength falls with distance.
  2. It is severe in CDMA and spread-spectrum systems where all share one frequency.
  3. The fix is power control: the base station makes near nodes reduce transmit power so all signals arrive at similar strength.
  4. The related term is capture effect in 802.11.

Infrastructure and Ad hoc Networks

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Definition. An infrastructure network communicates through a fixed access point; an ad hoc network is an infrastructure-less, self-organising set of mobile nodes that talk directly and relay for each other.

Key points.

  1. Ad hoc nodes act as both host and router, forwarding packets over multiple hops.
  2. Topology is dynamic because nodes move, join and leave.
  3. Setup is quick and cheap with no central administration, but bandwidth and battery are limited.
  4. Applications: battlefield, disaster relief, conferences, sensor and vehicular networks.
  5. Infrastructure MACs (such as PCF) rely on the AP to poll, allocate and synchronise, so they fail when no AP exists.
  6. Without central control, hidden and exposed terminals and changing neighbours cannot be handled by a coordinator, and contention and timing have to be distributed.
Basis Infrastructure network Ad hoc network
Central unit Access point / base station None
Routing By AP, single wireless hop Multi-hop by nodes
Topology Fixed backbone Dynamic
Setup Planned Spontaneous
Control Centralised Distributed

Answer frame. Open with the definition; draw the two topologies side by side; give the table, then applications; for the MAC justification, argue points 5 and 6 and close that ad hoc MACs must be distributed.

Asked: [7 marks] (Dec 2024) What are Adhoc networks? How are they different from traditional networks? Asked: [7 marks] (Jun 2025) Why "MAC protocol designed for infrastructure based wireless network may not work satisfactory in infrastructure less environment" - Justify?

IEEE 802.11- System arch

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Definition. The 802.11 architecture is built from stations (STA), access points (AP), basic service sets (BSS), an extended service set (ESS) and a distribution system (DS).

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Key points.

  1. A BSS is a group of stations under one AP; without an AP it is an independent (ad hoc) BSS.
  2. The DS (usually Ethernet) interconnects APs, and several BSSs joined by the DS form an ESS.
  3. Services: association, reassociation, authentication, distribution and integration to wired LANs.
  4. Infrastructure mode uses an AP; ad hoc mode lets stations talk directly.

Asked: [7 marks] (Jun 2025) Describe the architecture of IEEE 802.11 wireless LAN.

Protocol arch

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Definition. 802.11 defines the bottom two OSI layers: a physical layer and a MAC sublayer under the 802.2 LLC.

Key points.

  1. The PHY has PLCP and PMD sublayers and does sensing and transmission.
  2. The MAC does access control, fragmentation and encryption.
  3. The station and DS management planes handle roaming and power saving.

Physical layer

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Definition. The PHY transmits bits by infrared or by radio using FHSS, DSSS or OFDM.

Key points.

  1. FHSS hops among 79 channels of 1 MHz in the 2.4 GHz band, giving 1-2 Mbps.
  2. DSSS spreads each bit with an 11-chip Barker code; 802.11b reaches 11 Mbps.
  3. OFDM is used by 802.11a (5 GHz, up to 54 Mbps) and 802.11g (2.4 GHz).

Concept of spread spectrum

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Definition. Spread spectrum spreads a signal over a much wider bandwidth than needed, using a code known to the receiver.

Key points.

  1. It resists narrow-band interference and jamming.
  2. It gives low probability of interception and multipath tolerance.
  3. DSSS multiplies data by a fast chip code; FHSS jumps between carrier frequencies in a pseudo-random pattern.
  4. Different codes let several users share a band (CDMA).

MAC and its management

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Definition. The 802.11 MAC controls channel access with DCF (contention) and optional PCF (polled), and manages association and roaming.

Key points.

  1. DCF uses CSMA/CA: sense the medium for DIFS, back off a random time, send, and wait for an ACK.
  2. Interframe spaces order priority: SIFS is shortest, then PIFS, then DIFS.
  3. RTS/CTS with the NAV timer reserves the channel against hidden nodes.
  4. PCF lets the AP poll stations during contention-free periods.
  5. Management covers scanning, association, reassociation and synchronisation using beacons.

Power management

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Definition. Power management lets stations sleep to save battery while the AP buffers their frames.

Key points.

  1. A station announces sleep by setting the power-management bit in its frame.
  2. The AP buffers frames for sleeping stations.
  3. Stations wake at each beacon and read the TIM to see whether frames wait for them.
  4. In ad hoc mode, ATIM windows serve the same purpose.

Security

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Definition. WLAN security provides authentication and confidentiality over an open radio channel.

Key points.

  1. WEP uses the RC4 stream cipher with a shared key and a 24-bit IV; it is weak and easily broken.
  2. WPA fixed WEP with TKIP and per-packet keys.
  3. WPA2 (802.11i) uses AES-CCMP and 802.1X authentication.
  4. Other measures are SSID hiding and MAC filtering, which are weak.

Mobile IP: unsuitability of Traditional IP

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Definition. Mobile IP lets a host keep its IP address and stay reachable while moving between networks.

Key points.

  1. Traditional IP routes by the network prefix in the address, so a moved host with the same address gets no packets, and changing the address breaks TCP connections.
  2. Mobile IP solves this with a home agent, foreign agent, care-of address and tunneling.
  3. Problems: triangular routing (packets go via the home agent, replies go directly), handover latency, and security threats such as spoofed registration.
  4. Tunneling adds header overhead and reduces performance.

Asked: [7 marks] (Jun 2025) What is Mobile IP used for? What are the major problems of Mobile IP?

Goals, Terminology

<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">High weight</span>

Definition. Mobile IP (RFC 3344) lets a mobile node move between networks while keeping one permanent IP address, transparently to TCP and applications.

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Key points.

  1. Goals: transparency to applications, compatibility with existing IP and hosts, and no need to change routers or correspondent nodes.
  2. Home network: the network whose prefix matches the mobile node's permanent address.
  3. Home address: the permanent IP address assigned to the node in its home network.
  4. Foreign network: any network the mobile node visits away from home.
  5. Foreign agent: a router on the foreign network that serves the visiting node, offers the care-of address and delivers tunnelled packets.
  6. Care-of address (COA): the temporary address marking the node's current location, either the foreign agent's address or a co-located address got by DHCP.
  7. Home agent: a router on the home network that registers the node's COA, intercepts its packets and tunnels them to the COA.
  8. Example: a laptop with home address 10.1.1.5 visits a foreign network, registers COA 20.2.2.1 with its home agent, and gets packets from correspondents by tunnel.

<mark>Mobile IP keeps the home address fixed and uses a care-of address and tunneling from the home agent to deliver packets to the moving node.</mark>

Answer frame. Open with the purpose of Mobile IP; define the six terms one line each in the order asked; draw the diagram, then show the registration and tunneling relationship with the example; close that the home address gives identity and the COA gives location.

Asked: [14 marks] (Dec 2020) Explain the following terms associated with mobile IP: Home Network, Home address, Foreign Network, Foreign Agent, Care of Address, Home Agent.

Agent advertisement and discovery

<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. Agents announce their presence by periodic ICMP router advertisement messages with a mobility extension.

Key points.

  1. The message carries the COA, lifetime and flags marking a home or foreign agent.
  2. A node detects it has moved when it hears a foreign agent or stops hearing its home agent.
  3. A node may send an agent solicitation instead of waiting.

Registration

<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. Registration tells the home agent the mobile node's current COA.

Key points.

  1. The node sends a registration request, directly or via the foreign agent, over UDP port 434.
  2. The home agent creates a binding of home address to COA and replies with a registration reply.
  3. Requests carry a lifetime and are authenticated with a shared secret to stop spoofing.
  4. On returning home, the node deregisters.

Tunneling techniques

<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. Tunneling encapsulates the original packet inside a new IP packet addressed to the COA.

Key points.

  1. IP-in-IP adds a full outer header with source home agent and destination COA.
  2. Minimal encapsulation shrinks the added header.
  3. GRE is a more general encapsulation.
  4. At the tunnel end, the outer header is removed and the inner packet is delivered.

Ad hoc network routing: Ad hoc Network routing v/s Traditional IP routing

<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. Ad hoc routing finds multi-hop paths between mobile nodes with no fixed routers.

Key points.

  1. Traditional routing assumes fixed routers, stable links and static topology; ad hoc topology changes constantly.
  2. Ad hoc nodes must be routers and periodic table exchange wastes bandwidth and battery.
  3. Ad hoc protocols therefore add on-demand discovery, sequence numbers and loop freedom.
  4. Traditional protocols like RIP and OSPF do not scale to such mobility.

types of routing protocols

<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. Ad hoc routing protocols are classified by when routes are found into proactive, reactive and hybrid.

Key points.

  1. Proactive (table-driven) protocols such as DSDV and OLSR keep routes to all nodes at all times, with low latency but high overhead.
  2. Reactive (on-demand) protocols such as AODV and DSR find a route only when needed, with low overhead but discovery delay.
  3. Hybrid protocols such as ZRP are proactive within a zone and reactive outside it.
Class Approach Overhead Latency Examples
Proactive Periodic table updates High Low DSDV, OLSR
Reactive Discovery on demand Low High first packet AODV, DSR
Hybrid Zone-based mix Medium Medium ZRP

Asked: [7 marks] (Dec 2024) Give a classification of Adhoc routing protocols.

Examples: OADV

<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. AODV (written OADV in the paper) is Ad hoc On-demand Distance Vector routing, a reactive protocol that discovers a route only when a source needs it.

Key points.

  1. Route discovery: the source broadcasts a route request (RREQ) carrying source and destination sequence numbers, and each node keeps a reverse route to the source.
  2. The destination, or a node with a fresh route, returns a unicast route reply (RREP) along the reverse path, setting up the forward route.
  3. Sequence numbers keep routes fresh and prevent loops.
  4. Maintenance: when a link breaks, the detecting node sends a route error (RERR) to sources, which rediscover the route.
  5. Example: S wants D via A and B; S floods RREQ, D replies RREP over B, A, S; if B-D breaks, B sends RERR and S floods again.

Asked: [7 marks] (Jun 2025) Explain the mechanism of OADV protocol with an example.

DSDV

<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. DSDV is a proactive distance-vector protocol where every node keeps a table with a next hop and hop count to every destination.

Key points.

  1. Each entry carries a destination-generated sequence number, even when the route is valid and odd when broken.
  2. A higher sequence number wins; on equal numbers the shorter metric wins.
  3. Tables are exchanged by periodic full dumps and triggered incremental updates.

DSR

<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. DSR is a reactive protocol using source routing, where the packet header lists the whole path.

Key points.

  1. Route discovery floods an RREQ that collects node addresses, and the RREP returns the full path.
  2. Nodes cache routes and intermediate nodes need no routing tables.
  3. Route errors remove broken routes from caches.

ZRP etc.

<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. ZRP, the Zone Routing Protocol, is hybrid: proactive inside a zone of radius r hops and reactive between zones.

Key points.

  1. IARP keeps routes to nodes inside the zone.
  2. IERP discovers outside routes by bordercasting queries to border nodes.
  3. This reduces both table overhead and flooding.

Last-minute revision

  • Hidden terminal: out of sender's range but collides at the receiver; exposed: needlessly defers.
  • RTS/CTS reduces hidden-node collisions; near-far is fixed by power control.
  • Wireless uses CSMA/CA because collisions cannot be detected.
  • 802.11 has BSS, ESS, DS, AP and STA; ad hoc mode is an IBSS.
  • SIFS < PIFS < DIFS.
  • WEP uses RC4; WPA2 uses AES-CCMP.
  • Mobile IP terms: home address is identity, care-of address is location.
  • Registration uses UDP port 434; tunneling is IP-in-IP.
  • Mobile IP problems: triangular routing, handover latency, security.
  • Proactive: DSDV, OLSR; reactive: AODV, DSR; hybrid: ZRP.
  • AODV uses RREQ, RREP and RERR with sequence numbers.

Memory hooks

  • HEN: Home agent, Encapsulate, Notify (registration) for Mobile IP.
  • Hidden hears nothing, exposed hears too much.
  • Proactive = Prepared tables; Reactive = Request when needed.
  • RRR for AODV: Request, Reply, Error.

Coverage checklist

  • Transmission Medium For WLANs: no past questions.
  • MAC problems: MAC issues in ad hoc networks; wired vs wireless media access (Dec 2020, Dec 2024).
  • Hidden and Exposed terminals: hidden, exposed and near-far discussion (Dec 2020).
  • Near and Far terminals: covered in the hidden and exposed question.
  • Infrastructure and Ad hoc Networks: what are ad hoc networks (Dec 2024); MAC for infrastructure-less (Jun 2025).
  • IEEE 802.11- System arch: 802.11 architecture (Jun 2025).
  • Protocol arch: no past questions.
  • Physical layer: no past questions.
  • Concept of spread spectrum: no past questions.
  • MAC and its management: no past questions.
  • Power management: no past questions.
  • Security: no past questions.
  • Mobile IP: unsuitability of Traditional IP: uses and problems of Mobile IP (Jun 2025).
  • Goals, Terminology: Mobile IP terms (Dec 2020).
  • Agent advertisement and discovery: no past questions.
  • Registration: no past questions.
  • Tunneling techniques: no past questions.
  • Ad hoc network routing: Ad hoc Network routing v/s Traditional IP routing: no past questions.
  • types of routing protocols: classification of ad hoc routing (Dec 2024).
  • Examples: OADV: AODV mechanism (Jun 2025).
  • DSDV: no past questions.
  • DSR: no past questions.
  • ZRP etc.: no past questions.
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