1. IT351: Mobile & Wireless Computing
Wireless Transport Layer
Objective:
To study the TCP functionality and problem domain as it relates specifically to integrated wireless networks.
To provide an insight into available solutions to enhance TCP operation in wireless networking.
To provide an overview of the Wireless Application Protocol (WAP) as a solution following the split-connection model.

2. Outline Transmission Control Protocol - TCP (recap)
Congestion window
Slow start
Fast retransmit
Problem of TCP in wireless networks
Solutions:
Indirect TCP (I-TCP)
Wireless Application Protocol (WAP)

3. Overview of the main chapters
Support for Mobility
Chapter 9:
Mobile Transport Layer
Chapter 8:
Mobile Network Layer
Chapter 4:
Telecommunication Systems
Chapter 5:
Satellite
Systems
Chapter 6:
Broadcast
Systems
Chapter 7:
Wireless
LAN
Chapter 3:
Medium Access Control
Chapter 2:
Wireless Transmission

4. Recap – Transmission Control Protocol
Transport protocols typically designed for
Fixed end-systems
Fixed, wired networks
TCP:
Provides “reliable, connection oriented transport protocol”
It is an “end-to-end protocol that supports flow and congestion control”
Used for Web, , FTP and many other Internet applications (majority of Internet traffic is TCP)
Has been designed and optimised for wired networks

5. TCP in Wireless Networks
Flow and Congestion Control
TCP Transmission Policy – TCP can manage the flow of data between applications and react to congestion across the Internet
TCP Congestion Control – network congestion
Ask senders to slow down (or stop altogether)
However, first it must be possible to detect that congestion has occurred.
Assumptions made about why packets aren’t arriving -- congestion

6. Motivation TCP congestion control
packet loss in fixed networks typically due to (temporary) overload situations
router have to discard packets as soon as the buffers are full
TCP recognizes congestion only indirect via missing acknowledgements…..retransmissions unwise, they would only contribute to the congestion and make it even worse
Sender alters the Congestion Window according to the way the network is currently performing
The Congestion Window will keep increasing until segments timeout and then start reducing
slow-start algorithm as reaction

7. Motivation TCP slow-start algorithm
sender calculates a congestion window for a receiver
start with a congestion window size equal to one segment
exponential increase (double) of the congestion window up to the congestion threshold, then linear increase (increase by 1)
missing acknowledgement causes the reduction of the congestion threshold to one half of the current congestion window
congestion window starts again with one segment

8. Influences of mobility on TCP-mechanisms
TCP assumes congestion if packets are dropped
typically wrong in wireless networks, here we often have packet loss due to transmission errors
furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible
The performance of an unchanged TCP degrades severely
however, TCP cannot be changed fundamentally due to the large base of installation in the fixed network, TCP for mobility has to remain compatible
the basic TCP mechanisms keep the whole Internet together

9. TCP in Wireless Networks
Effects of dropped packets
Node A sends data to Node B (wireless)
Node B drops some packets randomly
Node B will ask for retransmissions
Node A assumes congestion
TCP may make performance worse

10. TCP in Wireless Networks
Solutions
New Transport Protocols
Cross-layer design: Interaction between transport and network layers to know the reason for packet loss – violate layering structure & hinder Internet stability.
Split Connections
Have two TCP connections, one for the wireless link and one for the wired link
Wireless TCP (I-TCP) – split connection protocol with new mechanisms for rate-based congestion control
Acknowledgements can arrive at the sender before the segment reaches the receiver

11. Indirect TCP (split connection solution)
Indirect TCP or I-TCP segments the connection
no changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol
optimized TCP protocol for mobile hosts
splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer
hosts in the fixed part of the net do not notice the characteristics of the wireless part
mobile host
„wired“ Internet
access point
(foreign agent)
standard TCP
„wireless“ TCP

12. I-TCP socket and state migration
access point1
socket migration
and state transfer
Internet
access point2
mobile host

13. Indirect TCP Advantages Disadvantages
no changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work
transmission errors on the wireless link do not propagate into the fixed network
simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host
therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known
Disadvantages
loss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crash
higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

14. Wireless Application Protocol - WAP
Upper layer protocols for Wireless Internet
Based on the Split connections solution

15. WAP Requirements Ensure interoperability Observe market needs
Ease of use, price sensitivity, tasks, usage patterns
Observe network needs
Less bandwidth, more latency, less predictable
Observe device needs
Less CPU power, less memory, limited display/input

16. WAP
WSP: Wireless Session Protocol
WML: Wireless Markup Language