Wireless Internet: An In-Depth Briefing Document

Introduction

The transition from wired to wireless internet connectivity introduces several challenges and complexities that need to be addressed to ensure seamless mobile communication. Traditional internet protocols were designed for static, wired environments and often struggle in the dynamic wireless context. This document reviews key themes related to wireless internet, focusing on the issues surrounding mobile IP, address mobility, tunneling, and handoffs.

Main Themes

1. Address Mobility

• Problem Overview: The traditional IP addressing scheme is designed for fixed nodes within a wired network, using a hierarchical structure that reduces the size of routing tables in core routers. This scheme, however, is not directly applicable to wireless environments where mobile hosts frequently change their point of attachment to the network.
• Issue with Mobility: When a mobile host moves from one subnet to another, packets intended for it may still be routed to the old subnet due to the fixed network identifier in the IP address. This can cause significant delays or loss of connectivity.
• Mobile IP as a Solution: To address these challenges, Mobile IP was introduced. It allows mobile devices to maintain a permanent home address while also using a temporary care-of address (COA) at their current location. The Home Agent (HA) and Foreign Agent (FA) play crucial roles in tracking and routing packets to the mobile host.

2. Mobile IP Mechanisms

• Essence of Mobile IP: The core concept of Mobile IP involves maintaining the old IP address with additional mechanisms to support mobility. The Mobile Node (MN) acquires a COA, which could either be a Foreign Agent-based COA or a co-located COA. This ensures that data is correctly routed to the mobile device, regardless of its current network location.
• Tunneling and Encapsulation: When a packet is sent to the mobile device's permanent IP address, the HA encapsulates it within another packet addressed to the COA, a process known as tunneling. This mechanism, while functional, introduces inefficiencies like triangular routing, where data takes a non-optimal path through the network.

3. Challenges with Traditional Protocols

• TCP Inefficiencies: TCP, the dominant transport layer protocol, was originally designed for reliable communication over wired networks. In wireless environments, TCP's performance can degrade due to signal fluctuations, mobility, and the inherent nature of wireless channels. The document discusses proposals for enhancing TCP's performance in wireless contexts.If a data packet or an ACK packet is lost, then TCP assumes that the loss is due to congestion and reduces the size of the congestion window byhalf. With every successive packet loss the congestion window is reduced, and hence TCP provides a degraded performance in wireless links. Even in situations where the packet loss is caused by link error or collision, the TCP invokes the congestion control algorithm leading to very low throughput. The identification of the real cause that led to the packet loss is important in improving the performance of the TCP over wireless links. Some of the solutions for the transport layer issues include indirect-TCP (ITCP), snoop TCP, and mobile TCP.
• **Inefficiency of Application Layer Protocols:**Traditional application layer protocols used in the Internet such as HTTP,3 TELNET, simple mail transfer protocol (SMTP), and several markup languages such as HTML were designed and optimized for wired networks. Many of these protocols are not very efficient when used with wireless links. The major issues that prevent HTTP from being used in wireless Internet are its stateless operation, high overhead due to character encoding, redundant information carried in the HTTP requests, and opening of a new TCP connection with every transaction. Wireless bandwidth is limited and much more expensive compared to wired networks. Also, the capabilities of the handheld devices are limited, making it difficult to handle computationally and bandwidth-wise expensive application protocols. Wireless application protocol (WAP) and optimizations over traditional HTTP are some of the solutions for the application layer issues.

4.Handoffs

Complexity of Handoffs

• Definition: A handoff refers to the process where a mobile device transitions its connection from one base station to another as it moves. This is critical for maintaining seamless connectivity in a wireless network.

Types of Handoffs

• Based on Functionality:
  1. Mobile-Initiated Handoff:
     • The handoff is managed by the Mobile Node (MN).
     • The MN measures the signal strength, selects the target base station (BS), and triggers the handoff.
  2. Mobile-Evaluated Handoff:
     • Similar to mobile-initiated, but the decision on the handoff lies within the network, typically at the BS.
  3. Network-Initiated Handoff:
     • The network (BS) decides when and where the MN should be handed over.
     • Only the network measures the uplink signal strength, with minimal involvement from the MN.
  4. Mobile-Assisted Handoff:
     • The MN assists the network in a network-initiated scenario by measuring the downlink signal strength.
     • This helps avoid the "black hole" scenario, where asymmetrical channel properties cause a significant drop in throughput in one direction.
• Based on Number of Active Connections:
  • Hard Handoff:
    • Only one active connection exists during the handoff, either to the old or new Foreign Agent (FA).
  • Soft Handoff:
    • The MN maintains connections with both the old and new FAs during the transition, ensuring a smoother switch and reduced risk of dropped packets.