Wireless Data Services

1. Wireless Data Services Lecture 1 ECS 289I

2. Wireless Networks • Wireless LANs • IEEE 802.11b CSMA/CA • Data rate upto 10Mbps • HIPERLAN/1 • Data rate upto 20Mbps • Wide Area Wireless Data Networks • CDPD (Cellular Digital Packet Data) • Upto 19.2Kbps • GPRS(General Packet Radio Service) • Upto 170Kbps • Cellular Networks • Circuit switched upto 19.2 Kbps

3. Characteristics of Wireless Environments that Affect TCP Performance • Limited Bandwidth • Wide area CDPD upto19.2Kbps • GPRS extension to GSM upto170Kbps • Wireless LANs based on IEEE 802.11b upto 10Mbps • Long Round Trip Delay • Wireless delays are longer than those in wired networks • TCP throughput is a function of the bandwidth delay product

4. Characteristics of Wireless Environments that Affect TCP Performance • Unfairness • TCP congestion window increases proportionally to the rate of incoming ACKs • Connections with longer round trip time will see slower window size increase and hence will experience smaller throughput • Random Losses • Wireless channels are prone to transmission losses due to fading channels and shadowing • BER in wireless channels can be of the order of 10-3 which implies a packet loss rate of upto 12% (assuming a 1500 byte packet) • Losses cause TCP to backoff

5. Characteristics of Wireless Environments that Affect TCP Performance • User Mobility • Supporting mobility requires handoff between Base Stations (BSs) or Access Points • Connection information must be transferred to new BS • Differentiate between losses due to • Congestion, Transmission, Mobility • Short Flows • Majority of Internet traffic is Web browsing and e-mail • Small amount of data • TCP transfers complete while the sender is in slow start phase. • Available bandwidth is not fully utilized

6. Characteristics of Wireless Environments that Affect TCP Performance • Power Consumption • Communicating over wireless channel requires more battery power than CPU processing • Energy efficiency depends on • Amount of avoidable extra data that is transmitted • Total duration of the connection

7. Solutions • Data Link Layer solutions • Modifications to TCP • New transport protocols optimized for wireless networks

8. Link Layer Solutions • Link layer protocols have immediate knowledge of dropped frames • Respond faster • Requires no change to TCP • Reliable delivery of packets at the link layer • Automatic Repeat Request (ARQ) • Forward Error Correction (FEC)

9. Link Layer Solutions • ARQ works well when the error rate is low • High error rate leads to many retransmissions: “connection blackout” • FEC has high bandwidth overhead • Raw bit rate of CDPD is 19.2 Kbps • Effective user bit rate is 9-13 Kbps • FEC schemes also require signal processing chips which require higher power.

10. Link Layer Solutions • TCP awareness • Link layer protocol being TCP unaware can lead to worse performance • Loss of packets cause link layer retransmission • TCP receiver will generate duplicate acknowledgement • TCP sender will retransmit segment • Duplicate effort leads to loss of bandwidth and power

11. TCP-aware Link Layer Protocols • Snoop Protocol • Requires no modification in the wired network • Deploy snoop agent at the base station • Snoop agent looks at every TCP segment sent to or received from mobile host • Snoop agent maintains a cache of unacknowledged data segment • Retransmits packet when a duplicate ack is received and suppresses the duplicate ack • Also uses timeouts to locally re-transmit segments • Uses timeouts that are less coarse than TCP timeouts

12. TCP-aware Link Layer Protocols • Sender can still timeout • Different version of TCP show different performance with snoop • Snoop protocol performs better when the RTT in the wireless part is small • Snoop does not perform well in wireless WANs which have low data rate and high RTT. Snoop retransmissions and TCP sender retransmissions can interfere

13. TCP-unaware Link Layer Protocols • TULIP (TCP Unaware Link Imporvement Protocol) • Developed for half-duplex wireless channels with limited b/w • Requires network layer to indicate the type of service requested • Like snoop, locally buffers packets and retarnsmits using an ARQ

14. TCP-unaware Link Layer Protocols • Delayed Duplicate Acknowledgements • Similar to snoop • Used link layer acks • Delays TCP Acks

15. Split Connections • Indrect-TCP (I-TCP) • Key idea: split the TCP connection into two at the peering point (base station) • Base station can acknowledge packets as soon as it receives the packet • Violates TCP semantics • Poor handoff performance • Since TCP state needs to be synchronized with the new base station • Performs poorly if the wireless part is not the “end-link.”

16. WAP • A new protocol stack for the wirless part • Transport protocol is very lightweight (similar to UDP) • TCP functionalities are pushed to the presentation and application layers • All connections go through a WAP proxy • WAP to the mobile • TCP/IP to the fixed host/ orgin server

17. TCP Modifications • TCP Selective Acknowledgement (SACK) • TCP Forward Acknowledgement (FACK) • TCP Santa Cruz

18. TCP SACK • Initially proposed to alleviate TCP inefficiency when multiple packets are dropped in the same window • Use TCP option fields to indicate to the sender multiple non-contiguous blocks of successfully received data • Kicks in when 3 duplicate acks are received • Transmits first un-SACKed data segment • Not much improvement if the bandwidthxdelay poduct is small or the window is small

19. TCP FACK • Makes more accurate estimation of transmitted but unacknowledged data • Introduces a new variable that keeps track of the “forward-most” data that arrived at the receiver • Sender keeps track of amount of retransmitted data

20. Changes to Other TCP Mechanics • Increase TCP’s initial congestion window • Explicit Congestion Notification (ECN) • Explicit Loss Notification (ELN) • Fast Retransmits • Ack Generation TCP Strategies

21. Increase Initial Window Size • Increase the initial window size to more than 1 • Initial study showed a 25% improvement for short flows • Other studies have shown that for low bandwidth links, initial window size of 4 will drop packets and hence not much improvement. • Current TCP specification still sets the initial window size to1

22. Explicit Congestion Notification • Functionality at the routers to indicate congestion to the sender • The congestion notification is generate before the router has to drop packets • Method to differentiate congestion and random losses at the sender • Not packets are dropped – hence power conserving

23. Explicit Loss Notofication • Proposed for wireless networks • Indicates the sender of losses in the wireless networks • There are no implementations or specifications

24. Fast Retransmits • Acks are used to trigger fast retransmit • When there are handoffs both packets and acks will be lost • Proposal to allow underlying network layer protocol to indicate handoff information to TCP

25. Ack Generation Strategies • TCP window growth is triggered by the ACKs • TCP Acks are cummulative • TCP follows a delayed acknowledgement scheme • Both these schemes lower throughput particularly for short flows • Various proposals • Acks every segment • Delayed Ack After Slow Start (DAASS • Unlimited byte counting