[Network]Wireless and Mobile

Wireless

1 Introduction

1.1 Elements

1. Wireless Hosts

Wireless does not mean mobility.

2. Base Station

Typically connect to wired network.

1.2 Modes

1. Infrastructure mode

base station connects mobiles into wired network.

Handoff: mobile changes base station

2. ad hoc mode

no base station

nodes orgnize themselves into a network: route among themselves.

2 Characteristics

2.1 Differences from wired link

1. signal strength

2. interference

3. multipath propagation

2.2 SNR versus BER

SNR: signal-to-noise rate

BER: bit error rate

The higher SNR, the lower BER.

2.3 Multiple Access

1. Hidden terminal problem

2. Signal attenuation

2.4 CDMA: Code Division Multiple Access

The algorithm is as what I have recapped before

调制（modulation）
(1)先将资料的0转成 -1
A = [1,-1,1], B = [1,1,-1]
好处是在解调时，更能够区别0和1，而使解调错误率下降。
(2-1) A使用Walsh Transform的第一个channel [1,1,1,1,1,1,1,1]（即其第一个basis,矩阵的第一个row）来做调制
A_m = [1,1,1,1,1,1,1,1,|-1,-1,-1,-1,-1,-1,-1,-1,|1,1,1,1,1,1,1,1,]。
(2-2) B使用Walsh Transform的第二个channel [1,1,1,1,-1,-1,-1,-1]（矩阵的第二个row）来做调制
B_m = [1,1,1,1,-1,-1,-1,-1,|1,1,1,1,-1,-1,-1,-1,|-1,-1,-1,-1,1,1,1,1,]。
(3)将调制的结果相加M = A_m + B_m
M = A_m + B_m = [2,2,2,2,0,0,0,0,0,0,0,0,-2,-2,-2,-2,0,0,0,0,2,2,2,2]
(4)最后传送出去的信号为M,共有24个bit。

解调（demodulation）
(1)将接收到的资料分别和channel做内积
(1-1) M和 第一个channel [1,1,1,1,1,1,1,1]做内积得到
前八码内积：[2,2,2,2,0,0,0,0] \cdot [1,1,1,1,1,1,1,1] = 8
中间八码内积：[0,0,0,0,-2,-2,-2,-2] \cdot [1,1,1,1,1,1,1,1] = -8
后八码内积：[0,0,0,0,2,2,2,2] \cdot [1,1,1,1,1,1,1,1] = 8
(1-2) M和 第二个channel [1,1,1,1,-1,-1,-1,-1]做内积得到
前八码内积：[2,2,2,2,0,0,0,0] \cdot [1,1,1,1,-1,-1,-1,-1] = 8
中间八码内积：[0,0,0,0,-2,-2,-2,-2] \cdot [1,1,1,1,-1,-1,-1,-1] = 8
后八码内积：[0,0,0,0,2,2,2,2] \cdot [1,1,1,1,-1,-1,-1,-1] = -8
(2)内积结果出来若为8，则解调为1;若为 -8，则解调为 -1
(2-1)第一个channel解调出信号为[8, -8, 8] \to [1, -1, 1]
(2-2)第二个channel解调出信号为[8, 8, -8] \to [1, 1, -1]
(3)最后，将 -1还原回0
(3-1)因此，第一个channel成功还原信号为[1, 0, 1]
(3-2)因此，第二个channel成功还原信号为[1, 1, 0]

3 Wi-Fi: IEEE 802.11 Wireless LAN

There is a family of 802 wireless LAN. They all:

1) use CSMA/CA for multiple access

2) have base-station and ad-hoc network versions

3.1 Architecture

1. Basic Service Set: contains 

1) wireless hosts

2) access point: base station

3) ad hoc mode: hosts only

2. Channel

1) divided into 11 channels

2) interference possible: neighboring AP chooses same channel.

3. Association

host must associates to a AP

4. Passive/Active Scanning

1) Passive

beacon -> request -> response

2) Active

request -> reponse -> assocation request -> assocation response

3.2 Multiple Access - Avoid Collisions

1. reasons: why not collision detection

1) signal weak

2) can not sense all collisions: hidden terminal problem

2. Avoiding collisions

1) Idea: allow sender to reverse channel. Sender first send request-to-send(RTS) packets to base station using CSMA. BS broadcast clear-to-send(CTS) in response to RTS.

CTS heard by all nodes.

3.3 Frame

1. Addressing

1) Address 1: receiver

2) Address 2: sender

3) Address 3: router

2. In frame control

1) Frame type: RTS, CTS, ACK, data

3.4 Other Characteristics

1. Mobility within same subnet

Self-learning: switch will see frame from H1 and remember which switch can be used to reach H1

2. advanced capabilities

1) Rate adaptation

BER vs. SNR

2) Power management

If there is no msg to be sent to this node. This node will sleep. When the node receive ‘beacon frame’, the node will wake up.

3. Personal area network

ad box mode: master/slave/parked device

3.5 802.16: WiMax

antenna: unlike 802.11, it has a large range

downlink/uplink scheduling

4 Cellular Internet Access

4.1 Architecture

like the other architecture, it is hierarchical and consists a list of base station, users, and links or protocols.

In Cellular Internet: the hierarchical arch is formed by Mobile Switching Center. And Cell denotes the components, BS, users, air-interface(protocols)

1. Hops: two techniques

1) FDMA/TDMA: divide spectrum

2) CDMA: bit dot algorithm(see before)

4.2 Standards

2G, 2.5G, 3G

GSM -> GPRS 

Mobility

1 Principles: addressing and routing

1.1 Address

First I need to clarify some address definations:

1) mobile: 

care-of-address, in visited network known by home network

permanent address, in home network known by correspondent to communicate

2) home:

home agent: function entity for mobile

3) correspondent:

who mobile want to contact with.

1.2 Routing

1. Registration

Mobile moves to a visited network, and connects to home agent telling it some mobility information.

Here are three important components:

1) mobile

2) foreign agent

3) home agent

2. Indirect Routing

Mobile requests to connect with correspondent through home agent. Then correspondent transfers pkt to mobile through home agent. Mobile replies directly to correspondent.

Triangle routing Problem

On-going connection can be maintains 

3. Direct Routing

Correspondent need to get care-of-address when mobile moves to another visited network.

Some Improvement

Accommdation mobility with direct routing

when mobile moves, new FA gets pkt from old FA(Chaining)

 2 Mobile IP

2.1 Indirect routing

In indirect or direct routing network, frame should be encapsulated into addresses.

Such as indirect routing network:

1) correspondent: mobile permanent address

2) home agent: add care-of-address(the address of foreign agent address) to find visited network 

3) foreign agent: dissolve to a permanent address.

2.2 Agent discovery

agent advertisement: foreign/home agents advertise service by broadcasting ICMP msg.

Registration Example

foreign agent broadcast ICMP msg. And a new mobile will put out registration request to foreign agent. And then foreign agent forwards the request to the home agent and gets the reply for the mobile.

NOTE:

Mobile maintains the home agent address consistantly.

3 Handoff within Mobile Switching Center

3.1 Common: 

like mobile move to another foreign network and update the information between BSS and MSC

3.2 Anchor MSC: 

like the machanism mentioned above.