Computer Networking — Chapter 7: Wireless and Mobile Networks

Introduction

Wireless networks are broadly classified by:

• Single hop vs multi-hop: one wireless hop vs ad hoc routing
• Infrastructure-based vs infrastructure-less: base station present vs no base station

Wireless Links and Network Characteristics

Wireless link characteristics:

• Signal attenuation: signal strength decreases with distance (path loss)
• Multipath propagation: reflections cause multiple copies of signal to arrive
• Interference from other sources
• SNR (Signal-to-Noise Ratio) and BER (Bit Error Rate) tradeoff: higher SNR = lower BER; higher data rate = higher BER at same SNR

Hidden terminal problem: Node A and C are both in range of B but out of range of each other; both transmit to B simultaneously, causing collision.

CDMA (Code Division Multiple Access)

• Each sender gets a unique code
• Sender encodes data by multiplying with spreading code
• Receiver decodes by multiplying with the same code
• Many senders can transmit simultaneously (codes are orthogonal)

WiFi: 802.11 Wireless LANs

802.11 Architecture

• BSS (Basic Service Set): group of stations communicating via an AP (Access Point)
• AP connects the BSS to the distribution system (wired Ethernet)
• Channels: 802.11 operates in ISM bands; 11 channels in 2.4 GHz (3 non-overlapping: 1, 6, 11)

WiFi Standards Comparison:

802.11 MAC Protocol: CSMA/CA

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):

1. If channel idle for DIFS time: transmit entire frame (no collision detection — can’t listen while transmitting)
2. If channel busy: choose random backoff, decrement while idle, freeze when busy, transmit when counter hits 0
3. Receiver sends ACK after SIFS time
4. If sender doesn’t receive ACK, retransmit (binary exponential backoff)

Why collision avoidance instead of detection? Because in wireless, the transmitter can’t reliably detect collisions (signal strength is too weak compared to its own transmission).

RTS/CTS (Optional):

• RTS (Request to Send) — small frame from sender
• CTS (Clear to Send) — small frame from AP/receiver
• Nearby stations hear CTS and defer transmission (solves hidden terminal problem)

802.11 Frame Structure

Four address fields are needed because frames can traverse APs on both sides.

Advanced Features in 802.11

• MIMO (Multiple Input Multiple Output): multiple antennas for spatial multiplexing
• Channel bonding: combining multiple channels for higher throughput
• Beamforming: focusing signal toward specific receivers
• MU-MIMO: serving multiple clients simultaneously (802.11ac)

Bluetooth and Zigbee

• Bluetooth (802.15.1): short-range, low-power, personal area network; master-slave piconets
• Zigbee (802.15.4): very low-power, low-rate; used in IoT sensor networks; mesh topology

Cellular Internet Access

3G Cellular Architecture:

• Base station (NodeB) connected to RNC (Radio Network Controller)
• RNC connects to SGSN and GGSN for Internet access

4G LTE (Long-Term Evolution):

• All-IP core network (EPC — Evolved Packet Core)
• eNodeB: evolved base station (combines NodeB and RNC roles)
• OFDMA (Orthogonal Frequency Division Multiple Access) on downlink
• SC-FDMA on uplink (better power efficiency)
• Data rates: 100+ Mbps downlink, 50+ Mbps uplink

Mobility Management: Principles

Terminology:

• Home network: permanent home of mobile node
• Foreign network: network the mobile node is visiting
• Home agent: entity in the home network that tracks the mobile node
• Foreign agent: entity in the foreign network that assists the mobile node
• Care-of address (COA): temporary address in the foreign network

Addressing

Each mobile node has a permanent home address (used by higher-layer applications). When visiting a foreign network, it obtains a care-of address.

Routing to a Mobile Node

Indirect Routing:

• Sender sends to home address; home agent intercepts and forwards via tunnel to COA
• Foreign agent delivers to mobile node
• Transparent to sender but triangle routing problem (inefficient)

Direct Routing:

• Sender queries a database to get mobile’s COA
• Sender tunnels directly to COA
• More efficient but not transparent (sender needs support)

Mobile IP

• Standard for supporting mobility in the Internet
• Agent discovery: home/foreign agents advertise their presence (ICMP router advertisement extensions)
• Registration: mobile node registers its COA with its home agent
• Tunneling: home agent encapsulates datagrams to COA (IP-in-IP tunneling)

Managing Mobility in Cellular Networks

GSM handoffs:

1. Mobile-assisted handoff: mobile measures signal strength of nearby base stations
2. Reports to MSC
3. MSC initiates handover to new base station
4. Connection path is rerouted

Anchor MSC: maintains the connection with the mobile as it moves across MSCs; prevents re-routing for every cell change.

Wireless and Mobility: Impact on Higher-Layer Protocols

• Wireless effects on TCP: high BER on wireless links causes TCP to incorrectly interpret losses as congestion; solutions: link-layer retransmission, split-connection, Snoop TCP
• Applications: streaming, VoIP affected by variable bandwidth and delay on wireless links