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 ms Bandwidth Mbps 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 Goodput High 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 link Presence 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 categories 1.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

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