Wireless and mobility: impact on­ higher-layer protocols

Wireless networks differ from wired networks because the characteristics of the wireless medium affect multiple layers of the protocol stack.

Link layer

At the link layer, wireless networks are affected by the characteristics of the wireless channel, including:

• fading
• multipath propagation
• hidden terminals

These factors can increase interference, packet loss, and transmission errors.

Network layer

At the network layer, mobility introduces new challenges because mobile users:

• change their points of attachment to the network
• may require mobility-management mechanisms and routing changes when they move between access points or networks

Transport layer

At the transport layer, the main differences appear in terms of performance.

In particular, TCP can behave very differently in wired and wireless networks.

TCP retransmits segments that are considered lost or corrupted. For mobile users, packet loss can occur for several reasons:

• network congestion: e.g., a router buffer may overflow
• handover: e.g., delays may occur while rerouting packets to a mobile user's new point of attachment
• bit errors: these are much more common in wireless networks than in wired networks

The problem is that the TCP sender cannot determine the actual cause of a loss. It only knows that a segment was not successfully delivered.

Therefore, TCP treats all losses in the same way:

• it retransmits the lost segment
• it reduces its congestion window
• it decreases its sending rate

By unconditionally reducing the congestion window after a loss, TCP assumes that the loss was caused by congestion rather than corruption or handover.

When a loss is caused by bit errors or handover, there's really no reason for the TCP sender to decrease its congestion window.

In these situations, the network may not be congested at all. Router buffers may be empty, and packets may otherwise flow normally without congestion.

Researchers recognized in the early to mid-1990s that TCP's congestion-control mechanism could be problematic in wireless environments.

Three main approaches were developed to address this problem:

Approach	Description
Local recovery protocols	recover from bit errors when and where they occur (e.g., at the wireless link) examples: the 802.11 ARQ protocol techniques combining ARQ and Forward Error Correction (FEC) the TCP sender is unaware that a wireless link exists
TCP sender awareness of wireless links	the TCP sender and receiver are made aware of the wireless link this allows them to: distinguish between congestion losses occurring in the wired network and corruption/loss occurring at the wireless link apply congestion control only when the loss is actually due to network congestion
Split-connection approaches	the connection between the mobile and the end point is broken into two transport-layer connections: from the mobile host to the wireless access point from the wireless access point to the other communication end point the end-to-end connection is formed by the concatenation of a wireless part and a wired part the transport layer over the wireless segment can be: a standard TCP connection a specialised error-recovery protocol built on top of UDP split TCP connections have been widely used in cellular data networks

Application layer

At the application-layer, because the wireless spectrum is shared among many users:

• bandwidth is a limited resource
• applications that operate over wireless links must treat bandwidth as a scarce commodity
• protocols and applications should be designed with the scarcity of wireless capacity in mind