无线网络 3G标准 Bluetooth WiFi

3G 3G：全称为3rd Generation，中文含义就是指第三代数字通信。 1995年问世的第一代模拟制式手机（1G）只能进行语音通话(FDMA)； 1996到1997年出现的第二代GSM、TDMA等数字制式手机（2G）便增加了接收数据的功能，如接收电子邮件或网页； 国际电联ITU在2000年5月确定WCDMA、CDMA2000、TD-SCDMA以及WiMAX四大主流无线接口标准，写入3G技术指导性文件《2000年国际移动通讯计划》（简称IMT—2000）

3G WCDMA，全称为Wideband CDMA，也称为CDMA Direct Spread，意为宽频码分多路存取，这是基于GSM网发展出来的3G技术规范，是欧洲提出的宽带CDMA技术，它与日本提出的宽带CDMA技术基本相同，目前正在进一步融合。 CDMA2000是由窄带CDMA(CDMA IS95)技术发展而来的宽带CDMA技术，也称为CDMA Multi-Carrier，它是由美国高通北美公司为主导提出，摩托罗拉、Lucent和后来加入的韩国三星都有参与，韩国现在成为该标准的主导者。

3G Time Division - Synchronous CDMA(时分同步CDMA)，该标准是由中国大陆独自制定的3G标准，1999年6月29日，中国原邮电部电信科学技术研究院（大唐电信）向ITU提出。该标准将智能无线、同步CDMA和软件无线电等当今国际领先技术融于其中，在频谱利用率、对业务支持具有灵活性、频率灵活性及成本等方面的独特优势。 WiMAX 的全名是微波存取全球互通(Worldwide Interoperability for Microwave Access)，又称为802.16无线城域网，是又一种为企业和家庭用户提供“最后一英里”的宽带无线连接方案。2007年10月19日，WiMAX正式被批准成为继WCDMA、CDMA2000和TD-SCDMA之后的第四个全球3G标准。

Bluetooth Bluetooth 技术在 2.4 GHz 波段运行，该波段是一种无需申请许可证的工业、科技、医学 (ISM) 无线电波段； 蓝牙目前暂时共有四个版本 V1.1/1.2/2.0/2.1； 以通讯距离可分为 Class A(1)/Class B(2)，ClassA通讯距离大约在 80~100M 距离之间，ClassB 8~30M 之间； UWB超宽带版本，版本于2008年中发布。整合了UWB技术的新版蓝牙将使用户能够对大量数据同速进行和传输，UWB技术在10米的有效范围内速率可达到480Mbps，超过了许多应用中最高要求的200Mbps，将MP3播放器或高画质数码相机的同速进行即是此技术的应用实例。

Wireless Networking(802.11)

wifi常见标准有以下几种： 　IEEE 802.11a ：使用5GHz频段，传输速度54Mbps，与802.11b不兼容 IEEE 802.11b ：使用2.4GHz频段，传输速度11Mbps IEEE 802.11g ：使用2.4GHz频段，传输速度主要有54Mbps、108Mbps，可向下兼容802.11b IEEE 802.11n草案：使用2.4GHz频段，传输速度可达300Mbps，目前标准尚为草案，但产品已层出不穷 目前IEEE 802.11b最常用，但IEEE 802.11g更具下一代标准的实力，802.11n也在快速发展中。

Overview Physical layer Link layer challenges Internet mobility

Physical layer 规定工作频段 数据收发时的调制方法

Cellular Reuse Transmissions decay over distance Spectrum can be reused in different areas Different “LANs” Decay is 1/R2 in free space, 1/R4 in some situations

Overview Physical layer Link layer challenges Internet mobility

WiFi工作模式 从形成的无线网络是否存在中心访问节点来看，分为机会网络（Opportunity Network）和基础设施网络（Infrastructure Network）两种类型。

机会网络（Opportunity Network ） 无线终端自主形成通信网络 Ad Hoc Mode 和 Delay Tolerate Network

CSMA/CD Does Not Work Carrier sense problems Relevant contention at the receiver, not sender Hidden terminal Exposed terminal A C B

Media access control (a) The hidden station problem. Why not use CSMA/CD? CSMA/CA (a) The hidden station problem. (b) The exposed station problem.

MACA Multiple Access Collision Avoidance Sender send RTS (require to send) with a time to use the radio media Receiver send CTS (clear to send) to sender Any other terminal who get the CTS will not send data Multi-RTS Collision： Senders can’t receive the CTS

MACA

工作模式（基础设施模式） 无线终端通过AP接入有线网络 Infrastructure Mode

AP Finding-Active Scanning Host broadcast Probe All APs which get that Probe return a Probe Response Host select a AP as its associate AP and send Association Request The selected AP returns a Associate Response

Overview Physical layer Link layer challenges Internet mobility (Mobility IP)

Mobility IP Addressing Dynamic Host Configuration (DHCP) Host gets new IP address in new locations Problems Host does not have constant name/address  how do others contact host Naming Use DHCP and update name-address mapping whenever host changes address

Mobile IP (RFC 2290) Interception Delivery Terminology Typically home agent – hosts on home network Delivery Typically IP-in-IP tunneling Endpoint – either temporary mobile address or foreign agent Terminology Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA) home address

Mobile IP (MH at Home) Correspondent Host (CH) Internet Packet Correspondent Host (CH) Internet Visiting Location Home Mobile Host (MH)

Mobile IP (MH Moving) Correspondent Host (CH) Internet Packet Correspondent Host (CH) Internet Visiting Location Home Home Agent (HA) Mobile Host (MH) I am here

Mobile IP (MH Away – Foreign Agent) Packet Correspondent Host (CH) Mobile Host (MH) Internet Visiting Location Home Encapsulated Home Agent (HA) Foreign Agent (FA)

Overview Physical layer Link layer challenges Internet mobility

无线网络的传输距离与那些因素相关? 802.11b的蜂窝半径？ 3G标准中三种标准（TD-SCDMA,WCDMA,CDMA2000)的工作频段？

The end of wireless END

Challenge #1: Wireless Bit-Errors Router Computer 1 Computer 2 2 3 2 2 1 Wireless Burst losses lead to coarse-grained timeouts Result: Low throughput

Performance Degradation Best possible TCP with no errors (1.30 Mbps) TCP Reno (280 Kbps) Sequence number (bytes) Time (s) 2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN

Proposed Solutions End-to-end protocols Split-connection protocols Selective ACKs, Explicit loss notification Split-connection protocols Separate connections for wired path and wireless hop Reliable link-layer protocols Error-correcting codes Local retransmission

Approach Styles (End-to-End) Improve TCP implementations Not incrementally deployable Improve loss recovery (SACK, NewReno) Help it identify congestion (ELN, ECN) ACKs include flag indicating wireless loss Trick TCP into doing right thing  E.g. send extra dupacks Wired link Wireless link

End-to-End: Selective Acks 4 3 X 6 5 2 1 Correspondent Host Mobile Host Base Station

End-to-End: Selective Acks Correspondent Host Mobile Host Base Station ack 1 ack 1,3 ack 1,3-4 ack 1,3-5 ack 1,3-6

Approach Styles (Split Connection) Split connections Wireless connection need not be TCP Hard state at base station Complicates mobility Vulnerable to failures Violates end-to-end semantics Wired link Wireless link

X X Split Connection 3 2 1 D C B A Correspondent Host Mobile Host Base Station ack 0 ack 0 sack A sack A,B sack A,B,D

Split-Connection Congestion Window 10000 20000 30000 40000 50000 60000 20 40 60 80 100 120 Time (sec) Congestion Window (bytes) Wired connection Wireless connection Wired connection does not shrink congestion window But wireless connection times out often, causing sender to stall

Approach Styles (Link Layer) More aggressive local rexmit than TCP Bandwidth not wasted on wired links Adverse interactions with transport layer Timer interactions Interactions with fast retransmissions Large end-to-end round-trip time variation FEC does not work well with burst losses Wired link Wireless link ARQ/FEC

Hybrid Approach: Snoop Protocol Transport-aware link protocol Modify base station To cache un-acked TCP packets … And perform local retransmissions Key ideas No transport level code in base station When node moves to different base station, state eventually recreated there

Snoop Protocol: CH to MH 4 3 2 1 Snoop Agent 6 5 1 Correspondent Host Mobile Host Base Station Snoop agent: active interposition agent Snoops on TCP segments and ACKs Detects losses by duplicate ACKs and timers Suppresses duplicate ACKs from FH sender

Snoop Protocol: CH to MH 6 3 Snoop Agent 5 2 4 1 Correspondent Host Mobile Host Base Station Transfer of file from CH to MH Current window = 6 packets

Snoop Protocol: CH to MH 6 5 Snoop Agent 4 3 2 1 Correspondent Host Mobile Host Base Station Transfer begins

Snoop Protocol: CH to MH 4 3 2 1 Snoop Agent 6 5 1 Correspondent Host Mobile Host Base Station Snoop agent caches segments that pass by Difference #1 from pure link-layer – does not add a new header uses existing TCP header to identify losses

Snoop Protocol: CH to MH 4 3 2 1 Snoop Agent 6 5 3 2 1 Correspondent Host Mobile Host Base Station 1 Lost Packets Packet 1 is Lost

Snoop Protocol: CH to MH 5 4 3 2 1 Snoop Agent 6 4 3 2 ack 0 Correspondent Host Mobile Host Base Station 1 Lost Packets Packet 1 is Lost Duplicate ACKs generated

Snoop Protocol: CH to MH 6 5 4 3 2 1 Snoop Agent 4 6 5 1 3 2 ack 0 Correspondent Host Mobile Host Base Station ack 0 1 Lost Packets Packet 1 is Lost Duplicate ACKs generated Packet 1 retransmitted from cache at higher priority

Snoop Protocol: CH to MH 1 6 5 4 3 2 1 Snoop Agent 4 6 5 3 2 ack 4 Correspondent Host Mobile Host Base Station X ack 0 Duplicate ACKs suppressed Difference #2 from pure link-layer – tries to prevent sender from noticing loss Sender may still timeout though – fortunately timeouts are typically long (500ms+)

Snoop Protocol: CH to MH 5 1 6 5 Snoop Agent 4 6 3 2 ack 5 Correspondent Host Mobile Host Base Station ack 4 Clean cache on new ACK

Snoop Protocol: CH to MH 6 5 1 6 Snoop Agent 4 3 2 ack 4 ack 6 Correspondent Host Mobile Host Base Station ack 5 Clean cache on new ACK

Snoop Protocol: CH to MH 6 5 1 9 Snoop Agent 4 8 7 3 2 Correspondent Host Mobile Host Base Station ack 5 ack 6 Active soft state agent at base station Transport-aware reliable link protocol Preserves end-to-end semantics

Snoop Data Processing Sender retransmission Congestion loss Packet arrives No 1. Forward pkt New pkt? 2. Reset local rexmit counter Yes Sender retransmission No 1. Mark as cong. loss In-sequence? 2. Forward pkt Congestion loss Yes 1. Cache packet 2. Forward to mobile Common case

Snoop ACK Processing Common case Spurious ack Later dup acks Ack arrives (from mobile host) 1. Free buffers 2. Update Yes New ack? RTT estimate 3. Propagate ack to sender No Common case No Discard Dup ack? Spurious ack Yes No Yes Retransmit Discard > threshold lost packet Later dup acks Next pkt lost for lost packet

Overview Link layer challenges Internet mobility TCP Over Noisy Links Adapting Applications to Slow Links

Adapting Applications Applications make key assumptions Hardware variation E.g. how big is screen? Software variation E.g. is there a postscript decoder? Network variation E.g. how fast is the network? Reason why we are discussing in this class  Basic idea – distillation Transcode object to meet needs of mobile host

Transcoding Example Generate reduced quality variant of Web page at proxy Must predict how much size reduction will result from transcoding How long to transcode? Send appropriate reduced-size variant Target response time? Adapting content to available bandwidth Identifying slow transfers Requesting lower quality for slow transfers

Source Adaptation Can also just have source provide different versions Common solution today No waiting for transcoding Full version not sent across network Can’t handle fine grain adaptation