[Computer Networking] 17. Wireless Networks and Mobile Communications

Wireless Network Components
==============================

[Wired Internet]
      |
[Base Station/AP] ----Wireless---- [Wireless Host]
      |                Link        [Wireless Host]
      |                            [Wireless Host]
[Wired Network]

Components:
  1. Wireless Hosts: Smartphones, laptops, tablets
  2. Base Station: AP, cell tower
  3. Wireless Link: Communication channel between host and base station
  4. Network Infrastructure: Connects base stations to wired network

Wireless Network Classification
==================================

                 | Infrastructure-based  | Infrastructure-less
-----------------+-----------------------+--------------------
Single Hop       | WiFi, 4G LTE          | Bluetooth
                 | (Via AP/Base Station)  | (Direct device-to-device)
-----------------+-----------------------+--------------------
Multi Hop        | Wireless Mesh          | MANET, VANET
                 | (Using relays)         | (Ad hoc networks)

Key Characteristics of Wireless Links
========================================

1. Path Loss
   - Signal strength decreases rapidly with distance
   - Free space: inversely proportional to square of distance
   - Indoors: additional attenuation from walls, furniture, etc.

2. Multipath Fading
   - Signals arrive via multiple reflected/diffracted paths
   - Multiple signals combine to strengthen or cancel

3. Interference
   - Interference from other devices using same frequency
   - Microwaves, Bluetooth, etc. (2.4GHz band)

4. SNR (Signal-to-Noise Ratio)
   - Higher SNR: fewer errors
   - High SNR: high modulation rate, high data rate
   - Low SNR: low modulation rate, low data rate

Hidden Terminal Problem
========================

      Obstacle
[A] .....|..... [B] ---------- [C]
 |       |       |              |
 |  A's range    |  B's range   |
 |<----->|       |<----------->|
 |               |
 A cannot sense C
 C cannot sense A

Problem:
  A is transmitting to B
  C senses channel: "Idle!" (cannot sense A's signal)
  C also starts transmitting to B --> Collision at B!

This is why CSMA/CD cannot be used in wireless

802.11 Network Structure
===========================

BSS (Basic Service Set):
  [AP (Access Point)]
   /    |    \
 [H1]  [H2]  [H3]
  Wireless hosts

ESS (Extended Service Set):
  [AP1] --- Wired --- [AP2] --- Wired --- [AP3]
   / \                / \                / \
 [H1][H2]          [H3][H4]          [H5][H6]

Key Components:
  - BSS: Group of wireless hosts associated with one AP
  - AP: Bridge connecting wired network and wireless hosts
  - SSID: Network identification name (e.g., "MyWiFi")
  - Channel: 1~11 (2.4GHz) or 36~165 (5GHz)

WiFi Association Process
==========================

1. AP Discovery
   - Passive Scan: AP periodically sends beacon frames
   - Active Scan: Host sends probe request, AP responds

2. Authentication and Association
   Host                            AP
     |--- Authentication Request -->|
     |<-- Authentication Response --|
     |--- Association Request ----->|
     |<-- Association Response -----|

3. Obtain IP address via DHCP

4. Begin data transfer

CSMA/CA Operation
====================

1. Carrier Sense
   - If channel is idle during DIFS (Distributed Inter-Frame Space), transmit

2. If channel is busy:
   - Start random backoff timer
   - Countdown timer only when channel is idle
   - Transmit when timer reaches 0

3. ACK Confirmation
   - Receiver sends ACK after SIFS (Short Inter-Frame Space)
   - Retransmit if ACK not received (increase backoff value)

Timeline:
  Sender:   [DIFS Wait][Data Transmission]....[Timeout]
  Receiver:                              [SIFS][ACK]

CSMA/CD vs CSMA/CA:
  CD: Detect collision then stop immediately (Ethernet)
  CA: Avoid collision + ACK confirmation (WiFi)

RTS/CTS Operation
====================

Sender A               AP                Nodes in range
    |                   |                       |
    |-- RTS ----------->|                       |
    |   (Request to Send)|                      |
    |                   |---- CTS ------------->|
    |<-- CTS -----------|   (Channel reservation)|
    |                   |                       |
    |== Data =========>|                       |
    |                   |                       | (Refrain from transmitting)
    |<-- ACK -----------|                       |
    |                   |                       |

RTS: Request to Send
CTS: Clear to Send

All nodes receiving CTS refrain from transmitting for the specified duration
  --> Hidden terminal problem solved

802.11 Frame Structure
========================

+-------+------+------+------+------+-----+------+------+-----+
|Frame  |Dur-  |Addr  |Addr  |Addr  |Seq  |Addr  |Data  | CRC |
|Control|ation | 1    | 2    | 3    |Ctrl | 4    |      |     |
| 2B    | 2B   | 6B   | 6B   | 6B   | 2B  | 6B   |0-2312| 4B  |
+-------+------+------+------+------+-----+------+------+-----+

4 MAC Address Fields:
  Address 1: MAC of receiving wireless interface
  Address 2: MAC of sending wireless interface
  Address 3: MAC of router interface connected to AP
  Address 4: Used only in ad hoc mode

Why 3 addresses are needed:
  H1 --> AP --> Router (R1)
  Address 1: AP's MAC (wireless receiver)
  Address 2: H1's MAC (wireless sender)
  Address 3: R1's MAC (toward final destination)

WiFi Standard Evolution
=========================

Standard    | Release | Frequency  | Max Speed    | Features
------------+---------+------------+--------------+------------------
802.11b     | 1999    | 2.4 GHz    | 11 Mbps      | Early adoption
802.11a     | 1999    | 5 GHz      | 54 Mbps      | OFDM
802.11g     | 2003    | 2.4 GHz    | 54 Mbps      | Backward compat. with b
802.11n     | 2009    | 2.4/5 GHz  | 600 Mbps     | MIMO
(WiFi 4)    |         |            |              |
802.11ac    | 2013    | 5 GHz      | 6.9 Gbps     | MU-MIMO
(WiFi 5)    |         |            |              |
802.11ax    | 2020    | 2.4/5/6    | 9.6 Gbps     | OFDMA
(WiFi 6)    |         | GHz        |              |

Cellular Network Basic Structure
===================================

  [Cell 1]    [Cell 2]    [Cell 3]
   / \         / \         / \
  /   \       /   \       /   \
 / BTS \     / BTS \     / BTS \
(Base    )  (Base    )  (Base    )
(Station )  (Station )  (Station )
   |          |          |
   +----------+----------+
              |
         [BSC/RNC]
         (Base Station Controller)
              |
         [MSC/MME]
         (Mobile Switching Center)
              |
     [Core Network] --- [Internet]

Cell: Area covered by one base station
Frequency Reuse: Same frequency used in non-adjacent cells

Cellular Network Generational Evolution
==========================================

Gen    | Technology | Speed           | Features
-------+-----------+-----------------+------------------
1G     | AMPS      | 2.4 Kbps        | Analog voice
2G     | GSM       | 14.4 Kbps       | Digital voice
2.5G   | GPRS      | 56-114 Kbps     | Packet data
2.75G  | EDGE      | 384 Kbps        | Enhanced data
3G     | UMTS      | 2 Mbps          | Mobile Internet
3.5G   | HSPA      | 14 Mbps         | High-speed packet
4G     | LTE       | 100 Mbps~1Gbps  | All-IP network
4.5G   | LTE-A     | 3 Gbps          | Carrier aggregation
5G     | NR        | 20 Gbps         | Ultra-low latency, IoT

4G LTE Network Structure
===========================

[UE] --Wireless-- [eNodeB] --- [S-GW] --- [P-GW] --- [Internet]
                    |           |           |
                    |       [MME]       [PCRF]
                    |     (Mobility     (Policy/
                    |      Management)  Charging)
                    |
                [HSS]
              (Subscriber DB)

UE:      User Equipment
eNodeB:  Base Station (evolved Node B)
S-GW:    Serving Gateway (data path management)
P-GW:    Packet Gateway (Internet connectivity)
MME:     Mobility Management Entity
HSS:     Home Subscriber Server (subscriber info)

Key Features:
  - All traffic transmitted as IP packets (All-IP)
  - Voice also handled as VoLTE (Voice over LTE)
  - Flat architecture (reduced layers for minimum latency)

Mobility Levels
=================

No mobility                                   High mobility
    |-------|--------|--------|--------|---------|
    Fixed   Low      Medium   High     Ultra-high
    Desktop WiFi     Pedestrian Vehicle High-speed
            Roaming  Handoff  Handoff  Train

Handoff Process
=================

Mobile device moving from one base station to another:

  [Old BS] .... [Mobile Device] .... [New BS]
      |                              |
      |  Signal weakening   Signal strengthening
      |                              |
      |<--- Handoff Decision ------->|
      |                              |
      |---- Traffic Path Switch ---->|
      |                              |

Handoff Types:
  1. Hard Handoff: Disconnect old, connect new (GSM)
  2. Soft Handoff: Maintain both connections simultaneously (CDMA)

Handoff Criteria:
  - Signal strength (RSSI)
  - Signal quality (SINR)
  - Load balancing

Mobility Support: Indirect Routing
=====================================

Home Network                    Visited Network
[Home Agent]                    [Foreign Agent]
     |                              |
     |<---- Mobile Registration ----|
     |                              |
Correspondent --> Home Agent --> Foreign Agent --> Mobile Device
                  (Tunneling)    (Forwarding)

Disadvantage: Triangle Routing
  Even if correspondent is right next to visited network,
  traffic must go through home network

Direct Routing:
  Correspondent checks current location from Home Agent
  Then sends directly to Foreign Agent (avoids triangle routing)

WiFi Security Protocol Evolution
===================================

Protocol | Release | Encryption    | Security Level
---------+---------+---------------+-----------
WEP      | 1999    | RC4 (40bit)   | Very weak (do not use)
WPA      | 2003    | TKIP          | Transitional
WPA2     | 2004    | AES-CCMP      | Good
WPA3     | 2018    | SAE + AES     | Strong

WPA2 Operation (4-way Handshake):
  1. AP sends random value
  2. Client sends random value + MIC
  3. AP sends group key + MIC
  4. Client sends confirmation
  --> Both sides securely share session key