Understanding Wireless Networks: Elements, Taxonomy, and Mobility

Posted on Feb 15, 2025 in International Business Economics

Elements of a Wireless Network

Wireless Hosts: Laptops, smartphones that run applications. They may be stationary (non-mobile) or mobile. Wireless does not always mean mobility.

Base Station: Typically connected to a wired network. It relays packets between the wired network and wireless host(s) in its area (e.g., cell towers, 802.11 access points).

Wireless Link: Typically used to connect mobile devices to a base station. It can also be used as a backbone link. A multiple access protocol coordinates link access with various data rates and transmission distances.

Infrastructure Mode: A base station connects mobiles into a wired network. Handoff occurs when a mobile changes the base station providing the connection into the wired network.

Ad Hoc Mode: No base stations are present. Nodes can only transmit to other nodes within link coverage. Nodes organize themselves into a network and route among themselves.

Wireless Network Taxonomy

Single Hop:

  • Infrastructure: A host connects to a base station (WiFi, WiMAX, cellular) which connects to the larger Internet.
  • No Infrastructure: No base station, no connection to the larger Internet (Bluetooth, ad hoc nets).

Multiple Hops:

  • Infrastructure: A host may have to relay through several wireless nodes to connect to the larger Internet (mesh net).
  • No Infrastructure: No base station, no connection to the larger Internet. May have to relay to reach another given wireless node (MANET, VANET).

Wireless Link Characteristics

Decreased Signal Strength: Radio signal attenuates as it propagates through matter (path loss).

Interference from Other Sources: Standardized wireless network frequencies (e.g., 2.4 GHz) are shared by other devices (e.g., phones). Devices (motors) interfere as well.

Multipath Propagation: Radio signals reflect off objects and the ground, arriving at the destination at slightly different times.

These factors make communication across a wireless link much more difficult.

SNR: Signal-to-Noise Ratio

A larger SNR makes it easier to extract the signal from noise.

SNR versus BER Trade-offs:

  • Given a physical layer: increase power -> increase SNR -> decrease BER.
  • Given SNR: choose a physical layer that meets the BER requirement, giving the highest throughput.
  • SNR may change with mobility: dynamically adapt the physical layer (modulation technique, rate).

Code Division Multiple Access (CDMA)

A unique code is assigned to each user (i.e., code set partitioning).

All users share the same frequency, but each user has their own “chipping” sequence (i.e., code) to encode data.

This allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal).

Encoding: 1 encoded signal = (original data) X (chipping sequence)

Decoding: Inner-product of encoded signal and chipping sequence

Avoiding Collisions

The idea is to allow the sender to reserve a channel rather than randomly accessing data frames to avoid collisions of long data frames.

  • The sender first transmits small request-to-send (RTS) packets to the BS using CSMA.
  • RTSs may still collide with each other (but they’re short).
  • The BS broadcasts clear-to-send (CTS) in response to RTS.
  • CTS is heard by all nodes. The sender transmits the data frame, and other stations defer transmissions.
  • This avoids data frame collisions completely using small reservation packets.

Components of Cellular Network Architecture

MSC: Connects cells to the wired telephone network, manages call setup, and handles mobility.

Cell: Covers a geographical region.

Base Station (BS): Analogous to an 802.11 AP. Mobile users attach to the network through the BS.

Air-Interface: The physical and link layer protocol between the mobile and the BS.

Cellular Networks: The First Hop

Two techniques for sharing mobile-to-BS radio spectrum:

  • Combined FDMA/TDMA: Divide the spectrum into frequency channels and divide each channel into time slots.
  • CDMA: Code Division Multiple Access.

Mobility

Spectrum of mobility, from the network perspective:

  • No Mobility: Mobile wireless user using the same access point.
  • Mobile user connecting/disconnecting from the network using DHCP.
  • High Mobility: Mobile user passing through multiple access points while maintaining ongoing connections (like a cell phone).

Mobility Approaches

  • Let Routing Handle It: Routers advertise the permanent address of mobile-nodes-in-residence via usual routing table exchange. Routing tables indicate where each mobile is located. No changes to end-systems.
  • Let End-Systems Handle It:
    • Indirect Routing: Communication from the correspondent to the mobile goes through the home agent, then is forwarded to the remote.
    • Direct Routing: The correspondent gets the foreign address of the mobile and sends directly to the mobile.

Mobility Registration

  • Home Network: The original network where the mobile device is registered. It has a home agent that tracks the mobile’s location.
  • Visited Network: The network the mobile device moves to. It has a foreign agent that facilitates communication with the home network.
  • Wide Area Network (WAN): The connection between the home and visited networks.

Steps in Mobility Registration

  1. Step 1: Mobile contacts the foreign agent: When the mobile device enters a new (visited) network, it identifies the foreign agent in this network. The mobile sends a registration request to the foreign agent, notifying it of its presence in the visited network.
  2. Step 2: Foreign agent informs the home agent: The foreign agent relays this information to the home agent in the mobile’s home network. The message essentially says: “This mobile device is now in my network.”

End Result

  • The foreign agent:
    • Now knows about the mobile device’s presence in the visited network.
    • Acts as an intermediary for the mobile device while it is in this network.
  • The home agent:
    • Updates its records to know the current location of the mobile device.
    • Can route traffic to the mobile via the foreign agent in the visited network.

Indirect Routing Comments

  • The mobile uses two addresses:
    • Permanent address: Used by the correspondent (hence the mobile location is transparent to the correspondent).
    • Care-of-address: Used by the home agent to forward datagrams to the mobile.
  • Foreign agent functions may be done by the mobile itself.
  • Triangle routing: correspondent-home-network-mobile. Inefficient when the correspondent and mobile are in the same network.

Indirect Routing: Moving Between Networks

Suppose a mobile user moves to another network:

  • Registers with the new foreign agent.
  • The new foreign agent registers with the home agent.
  • The home agent updates the care-of-address for the mobile.
  • Packets continue to be forwarded to the mobile (but with the new care-of-address).
  • Mobility, changing foreign networks transparent: ongoing connections can be maintained!

Mobility via Direct Routing: Comments

  • Overcomes the triangle routing problem.
  • Non-transparent to the correspondent: the correspondent must get the care-of-address from the home agent.

Accommodating Mobility with Direct Routing

  • Anchor foreign agent: FA in the first visited network, data is always routed first to the anchor FA.
  • When the mobile moves: the new FA arranges to have data forwarded from the old FA (chaining).

Mobile IP

RFC 3344 has many features we’ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet).

Three components to the standard: indirect routing of datagrams, agent discovery, and registration with the home agent.

Indirect Routing Process: Packet Sent by Correspondent

  • A sender (correspondent) sends a packet to the mobile device’s permanent address (128.119.40.186).
  • The packet reaches the home agent in the home network.

Home Agent Forwards the Packet

  • The home agent encapsulates the packet into another packet (this is called tunneling).
  • The encapsulated packet’s outer header has the destination set to the care-of address (79.129.13.2), allowing it to reach the mobile’s location in the visited network.

Foreign Agent Delivers the Packet

  • The packet arrives at the foreign agent in the visited network.
  • The foreign agent removes the encapsulation and forwards the original packet (with the permanent address 128.119.40.186) to the mobile device.

Handling Mobility in Cellular Networks

  • Home Network: The network of the cellular provider you subscribe to (e.g., Sprint PCS, Verizon).
  • Home Location Register (HLR): A database in the home network containing the permanent cell phone number, profile information (services, preferences, billing), and information about the current location (could be in another network).
  • Visited Network: The network in which the mobile currently resides.
  • Visitor Location Register (VLR): A database with an entry for each user currently in the network (could be the home network).

GSM: Handoff with Common MSC

Handoff goal: route the call via the new base station (without interruption).

Reasons for handoff:

  • Stronger signal to/from the new BSS (continuing connectivity, less battery drain).
  • Load balance: free up a channel in the current BSS.

GSM doesn’t mandate why to perform handoff (policy), only how (mechanism).

Handoff initiated by the old BSS:

  • The old BSS informs the MSC of the impending handoff and provides a list of 1+ new BSSs.
  • The MSC sets up a path (allocates resources) to the new BSS.
  • The new BSS allocates a radio channel for use by the mobile.
  • The new BSS signals the MSC and the old BSS: ready.
  • The old BSS tells the mobile: perform handoff to the new BSS.
  • The mobile and the new BSS signal to activate the new channel.
  • The mobile signals via the new BSS to the MSC: handoff complete.
  • The MSC reroutes the call.
  • MSC-old-BSS resources are released.

GSM: Handoff Between MSCs

  • Anchor MSC: The first MSC visited during the call. The call remains routed through the anchor MSC.
  • New MSCs add on to the end of the MSC chain as the mobile moves to the new MSC.
  • Optional path minimization step to shorten the multi-MSC chain.

Wireless, Mobility: Impact on Higher Layer Protocols

Logically, the impact should be minimal. The best-effort service model remains unchanged. TCP and UDP can (and do) run over wireless and mobile networks.

However, performance-wise:

  • Packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff.
  • TCP interprets loss as congestion and will decrease the congestion window unnecessarily. Delay impairments for real-time traffic. Limited bandwidth of the wireless link.