Transferring Internet Data on Wireless Networks

Presented by : Mohamed Gamal

Presented to : Prof. Dr. Mohab Mangoud

The Internet

It is the largest network where users share resources:

• Web pages

• File transfers

• Voice communication

• Video Streaming

The OSI model

TCP

Transmission Control Protocol is a reliable protocol• Flow control• Windowing• Congestion Control

Transmission Control Protocol is designed for Wired networks

TCP Congestion control

• AIMD (additive increase multiplicative decrease)

• Affected by• RTT (round Trip Time)• Retransmit timeout• Packet losses

• Assumes packet losses are due to congestion

Wired vs. Wireless links

Wired• Constant delay• Almost constant bandwidth• Packet losses due to congestion

Wireless• Varying delay• Asymmetric variable bandwidth• Loss of connectivity and high bit error rate• Packet losses due to corruption

Types of Wireless links used

We have three main types of links• Cellular• WLAN• Satellite

Cellular Links

• Coverage radius of cell varies 200m-30km• In GPRS downlink

• 40kbps bandwidth and 400ms delay

• In GPRS upnlink• 10kbps bandwidth and 200ms delay

• Protected by FEC and retransmits which cause delay

• Acquiring channel access causes excessive delay

WLANs

• Small coverage area

• Low Latency 3-100ms

• High BW 2-108Mbps

• Uplink and downlink channels are not independent

• Shared bandwidth

Satellite links

• Very Large coverage area• High Latency 50-300ms• Bandwidth 0.01-50Mbps• GEO sats have latency 270ms, dowlink

bandwidth of 40Mbps, uplink bandwidth of 1Mbps

• LEO sats have 100ms latency, 1Mbps bandwidth but handover interval of 4 seconds

Topologies

• Performance is affected by number and locations of wireless links in the path

• General topologies• Bus• Ring• Mesh• Star

• Common wireless topologies• Wireless link as last hop• Wireless link in the middle• 2 wireless links at the end (mobile and laptop)• Wireless links at both ends (VoIP)

• Mobile users transfer more data downlink than uplink• WAP (Wireless Application Protocol)

Performance metrics

Throughput

Delay

Fairness

Dynamics

GoodputHigh Goodput = lower power and reduced expenses

1 Mbps duplex link

1Mbps WLAN

Wireless Link characteristics

1. Error Losses and Corruption

2. Delay Variation

3. Packet Reordering

4. On-Demand Resource Allocation

5. Bandwidth variation

6. Asymmetry in Bandwidth and Latency

Error Loss and Corruption

Effect on TCP• Reduce sending rate• Burst losses trigger lengthy retransmission

timeouts

Presence in wireless links• Handovers and mobility cause lots of

packet losses• Losses have been decreased due to use of

FEC and link layer retransmissions

Delay Variation

Effect on TCP• Abrupt delay trigger spurious timeouts that cause

unnecessary retransmissions and false congestion control• Persistent delay variation can inflate the retransmission

timeout

Presence in wireless links• Occur due to link-layer error recovery and handovers• Sudden change in radio conditions (entering a tunnel)• Delay can occur in one direction if a path is asymmetric

Packet Reordering

Effect on TCP• Triggers packet retransmissions

Presence in wireless links• WLAN do not introduce reordering• Cellular links include an option for out of order

delivery• Reordering on satellite links with a high

bandwidth-delay product is attractive because it reduces the per-packet delay for other traffic on the link

On-Demand Resource allocation

Effect on TCP• Causes delay variation that depends on traffic

patterns

Presence in wireless links• GPRS requires 200ms to allocate a channel for

uplink and 80ms delay for downlink• For WLAN and satellite links a new data burst

triggers MAC contention• Subsequent data can often be transmitted without

delay

Bandwidth variation

Effect on TCP• Periods of low link bandwidth can result in

congestion• Periods of high link bandwidth could result in

underutilization of that linkPresence in wireless links

• Bandwidth oscillation can occur in CDMA2000 and UMTS (with High Speed Downlink Packet Access) links for certain configurations*

*M. Yavuz and F. Khafizov. TCP over wireless links with variable bandwidth.

Asymmetry in Bandwidth and latency

Effect on TCP• Causes congestion for TCP ACKs

Presence in wireless links• Cellular links have moderate bandwidth

asymmetry with factors of 2 to 5• Satellite Links are often asymmetric in

bandwidth and in latency

Improving TCP or wireless ?

Improving TCP performance over wireless

There are three categories1. End-to-end protocols, where sender is

aware of the wireless link

2. Link-layer protocols, that provide local reliability

3. Split-connection protocols, that break the end to end connection into two parts at the base station

New Technologies made available by wireless links

• Wireless VoIP

• Mobile TV

• Mobile Holograms

Summary

During the last years, both internet and mobile systems grew extremely fast.

Nowadays these two worlds are converging.

Wired Links are still ahead of wireless links but wireless links progressed quickly.

The END

References

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Qian Zhang, Wenwu Zhu, and Ya-Qin Zhang. Network-adaptive Scalable Video Streaming Over 3G Wireless Network. In IEEE International Conference on Image Processing(ICIP’01), Oct., 2001

Lin Cai, Xuemin Sherman, Jon W. Mark and Jianping Pan, Performance Analysis of AIMD-Controlled Multimedia Flows in Wireless IP Networks

Ramon Caceres and Liviu Iftode. Improving the performance of reliable Transport Protocols in Mobile Computing Environments. In IEEE Journal on Selected Areas in Communications, Vol. 13, No. 5, June 1995

Trista Pei-chun Chen and Tsuhan Chen. Fine-Grained Rate Shaping for Video Streaming Over Wireless Networks. In EURASIP JASP 2004