Wireless Mobile Networks

7. 搶旗看世界 - CTF - Hsun

CTF ( Capture the flag)

鄺世銘 劉孝恩 殷珮珊 周明鴻 許家維
 
 

0. 精神課程:

你想學甚麼?先決定自己完成要做甚麼

資訊收集Information gathering

網路安全Network security

網站與網頁應用程式安全Web security

系統安全System security

加密與解密Cryptography

惡意程式檢測Malware detection

逆向工程Reversing engineering

Fall08-網路概論課程: 

關於MIMO的一些問題

MIMO 可以在不需要增加頻寬或總發送功率耗損的情況下大幅地增加系統的資料吞吐量及傳送距離,容量隨著天線數量呈線性增加。

那這樣看起來好像就是增加多少的天線就會得到多大的容量?

以下是我在奇摩知識看到的:

在市面上所見的MIMO都是3隻天線的,是採3進2出的方式也就是3隻天線同時接收訊號,2隻天線同時傳輸訊號。

傳統基地台在802.11g傳輸模式下,天線一次送出的資料是54M,而2隻天線的機種也只是2隻天線同時送同一組訊號,所以也就只有54M,SUPER G的機種則是把1到11的頻道整合成一個頻道傳輸,可是相對的,會有蓋台缺點,但MIMO是同時送1組訊號並把資料1切為2,用2隻天線同時送出,像高架橋一樣,分上下2層,所以可以傳出2倍的資料,也就是一個頻道道重覆用2次,變成108M。

下面有人這樣評論我覺得滿有道理的:

『但MIMO是同時送1組訊號並把資料1切為2,用2隻天線同時送出,像高架橋一樣,分上下2層,所以可以傳出2倍的資料,也就是一個頻道道重覆用2次,變成108M』這樣的說法有點問題,亦即如果在一個時間內送了一組訊號,而只因為這一組訊號被切為兩段透過兩根不同的天線送出去,那麼基本上data rate並沒有增加,因為你單位時間內還是只送了同樣的資料量,而且原作者論點並沒有討論到接收端的天線數量,那也就是說只要傳送端的天線數量增加,就可以有效達到加快每Hz所能傳送的bits?那小弟我舉個反例好了,一個3*1的MISO系統,基本上他的channel capacity還是跟1*1一樣,多出來的只是trasmit diversity。

真正要解釋清楚MIMO的channel capacity,必須從數學上去看,你可以先找出capacity跟SNR的關係,然後將MIMO的channel帶入式子,然後把log裡面的channel部分作SVD分解,可以推出整個 channel capacity 會跟min(傳送端天線 ,接收端天線)成線性正比。

 

來源:奇摩知識

無線網卡

寒假打算買台NB,所以這陣子坐在電腦前面就會看一下NB的規格跟價錢,想找台CP值高的NB得慢慢挑。
因為我考慮購買的NB牌子能幫買方客製化,但過去都沒有碰過NB的規格,所以研究了些時間。其中無線網卡讓我研究了好久,最後才知道答案。


那台NB能夠選擇配上的無線網卡規格,給了:
Intel Wireless 5100 802.11a/g/n 1/2 Mini-Card
Intel Wireless 5300 802.11a/g/n 3/3 Mini-Card     這兩種來選擇。
其中我一直看不懂1/2跟3/3的意思,查了一下發現那代表的意思是5100有兩根天線,最大速度在300Mbps;
5300有三根天線,最大速度在450Mbps。


但是我還是不知道1/2的表示意思,所以轉而查天線的解釋介紹,查到了MIMO:(以下整理自維基百科、Intel網站和電子工程專輯網站)

多重輸入/多重輸出 ( MIMO;Multi-input Multi-output)

是一種用來描述多天線無線通訊系統的抽象數學模型。該技術最早是由馬可尼于1908年提出的,他利用多天線來抑制通道衰落(fading)。
根據收發兩端天線數量,相對於普通的單輸入單輸出系統(Single-Input Single-Output,SISO),MIMO 此類多天線技術尚包含早期所謂的「智慧型天線」,亦即單輸入多輸出系統(Single-Input Multi-Output,SIMO)和多輸入單輸出系統(Multiple-Input Single-Output,MISO)。

當無線電傳送訊號時,會反射其建立的多個無線電訊號副本。每個副本都是一個空間串流。使用單一輸入單一輸出(SISO) 的現行或舊式系統,在同一時間內僅能傳送或接收單一空間串流。MIMO 可讓多個天線在同一時間內各自獨立發送訊號和接收多個空間串流並恢複原信息。如此可讓天線同時進行傳送和接收。

MIMO通常用來搭配 IEEE  802.11n 使用,不過也可以搭配其他的 802.11 技術使用。MIMO 有時也稱為空間多樣,因為其使用多個空間通道進行資料的傳輸和接收。站台 (行動裝置) 或存取點 (AP) 必須支援 MIMO 才能夠執行。

由於 MIMO 可以在不需要增加頻寬或總發送功率耗損(transmit power expenditure)的情況下大幅地增加系統的資料吞吐量(throughput)及傳送距離,使得此技術於近幾年受到許多矚目。MIMO 的核心概念為利用多根發射天線與多根接收天線所提供之空間自由度來有效提升無線通訊系統之頻譜效率,以提升傳輸速率並改善通訊品質。MIMO 的優點是提高 WiFi 範圍和效能。連接到舊式 802.11g AP 的 802.11n 站台將能夠在較高的速度和較長的距離下進行連線。

MIMO容量隨著天線數量呈線性增加。不對稱的天線星座分佈(例如1x2或2x1)被稱為接收或發送分集。在這些情況下容量(CTx/Rx)隨天線的數量呈對數成長。

 

不過可惜的是雖然我想再繼續查「不對稱的天線星座分佈」的詳細情形,可惜也許是翻譯名詞翻得很爛還是知識太冷僻,找無此資料。

[筆記] 無線與行動網路

IEEE802.11 :

由IEEE所定義的無線通訊標準,無線區域網路的基本架構。而IEEE802即為區域網路的標準。

802.11架構中的基本單位稱做 BSS(basic service set),通常是由一個基地台與多個AP(Access Point - 存取點)所組成,而每個BSS再連結到網路,使得範圍內的使用者都能使用無線網路功能。

802.11有各種版本,最原始的就是802.11a,隨著網路技術的發展,逐漸發展出802.11b、c、...。
隨著版本發展,它們所提供的速度、效能也有提升。

WiMAX(Worldwide Interoperability for Microwave Access,全球互通微波存取):

IEEE802.16標準,其發展目的是為了讓無線網路能夠傳遞更大量的資料、範圍更加廣大,而且傳輸速度可以達到ADSL的標準。

目前WiMAX尚未普及,AP正在鋪設中,現在有些無線通訊業者,正準備擴展該技術,在未來幾年 WiMAX會逐漸發展。

行動電話技術,通常以G(Generation)來歸類。

1G:採用類比訊號傳輸,僅支援聲音訊號傳輸

2G:跟傳統類比訊號不同的是,2G系統已經採用數位訊號來傳遞,可是通常用來做聲音的傳輸,軟體、E-mail等無法支援直接傳輸。而2G手機可支援簡訊的功能。

2.5G:2G發展至3G的過渡技術,功能性介於2G與3G之間。通常傳輸速度小於等於3G。

3G:能夠同時傳送聲音及數據信息(例如E-Mail)。3G的代表特徵是提供高速數據業務,速率一般在幾百kbps以上。結合網際網路、行動通訊、多媒體等技術。

4G:除了3G功能以外,提供了更多不同的服務,不侷限於通信,金融、醫療等應用,並且能夠與網路做更多結合,將寬頻無線化,提供更快的速度、更方便的服務。

課後筆記

Mobile IP

在一般無線網路運作下,我們通常會在每一個router裡面建立個別的routing table,

用來檢視每一個router所負責的區域下有多少host,或是和Internet上其他router之間連繫的記錄表

但是在無線網路裡,host是會移動的, 所以要解決當host移動時要如何傳送訊息給已經移動到其他router範圍的host

方法有以下三種:

  1. Host-Specific Addess:隨著host移動在每一個router的routing table裡多加一個host的entry,用來記錄host的移動資訊
  2. Change IP Address:隨其移動而改變host的IP
  3. Moblie IP:在host裡存在兩種IP,一是固定的Home Address,另一個則是隨著移動而改變的care-of Address

方法1的缺點是因為網路上有太多的router而無法一一告知

方法二的缺點是即使回到原來的Home Agent範圍仍然要決定新的Address

在MobileIP的方法下,決定Care-of Address的方法有兩種

  1. Foreign agent care-of Address:由Foreign Agent 發出一個IP address,可同是讓多個host共用
  2. Collocated  care-of Address:利用DHCP來給予暫時性的IP

Mobile主要有三個機制

  1. Agent discovery:Agent每隔一段時間就會發送Agent advertisement  給所有涵蓋範圍內的host,host也藉由這個訊息知道自己到了哪個Agent管轄範圍
  2. Registration:要向原來的Home Agent回報現在在哪個Foreign裡面
  3. Routing

 

論文閱讀-無線網路提供差別服務的方法

無線網路所帶來的方便性以及龐大的商機成為下一波網路發展的重要技術 ,

而且網際網路及寬頻網路日趨成熟,整合無線和有線的網路技術成為研究的主要議題

所以無線網路的效能及封包遺失率問題成為現今重視服務品質的網際網路所要面臨的重要挑戰

無線網路的環境下,由於位元錯誤率較高導致封包遺失率相對偏高

對TCP而言,不僅是封包遺失的問題,更牽涉到TCP會將封包的遺失誤以為是網路壅塞,

而觸發壅塞控制(Congestion Control)系統,並且控制整體網路傳輸速度,進而使整體TCP的效能降低,

所以學術界的方法大致上分為兩類:在傳輸層進行改善、在資料鏈結層進行改善

傳輸層改善之方法
    在位元錯誤率高的環境下,TCP效能受到嚴重的影響。
    TCP層改善方法的研究多以更動TCP的機制,適應無線環境。這類的方法屬於end-to-end的機制
    Snoop Protocol
    ELN法

資料鏈結層改善方法
    在資料鏈結層上加上錯誤重傳的概念,無論在上行或下行都能有效增進效能
    當資料送出時,要收到資料接收端ACK(資料鏈結層),才能傳送下一個資料

而這篇論文提升效能的觀點有

依據網路狀態變化情形調整傳送封包:
(1)效能提升:高的位元錯誤率使整體效能下降,我們首先必須提升效能。
(2)對網路狀態做良好的估測:由於在無線區域網路中複雜的錯誤率變化,使得封包在傳送時效能容易受到網路中錯誤率變化的影響。因此,我們在每個鏈結上的通道品質做一個適當的估計以做為資源分配時的依據。
(3)有效分配網路資源:得知網路的狀態後,在資料傳送,對每個使用者做適當的頻寬資源分配。

所以提出了

可適性資料鏈結層架構
    (1)遺失封包傳送模組 (2)通道狀態估測器 (3)排程器
通道品質估測器設計
    以判定網路鏈結狀態的概念,決定送出資料的順序
    當位元錯誤率高時,延後傳送或是壓縮頻寬
    收集估測資訊,並傳送給排程器做有效的資源分配

來解決服務需求之間差異的問題,看完這篇論文以後,對於資料鏈結層運作的方式更為了解

而且對於排程的方法結合分析的結果,使得各種方法當有佐證。也讓我更清楚為何無線網路

對於Qos的重視。

 

 

Mobile Devices Networks

Mobile devices networks divided into:
1.GSM
2.PCS
3.D-AMPS

1. GSM
The GSM network is divided into three major systems: the switching system, the base station system, and the operation and support system. The cell phone connects to the base system station which then connects to the operation and support station; it then connects to the switching station where the call is transferred to where it needs to go. GSM is the most common standard and is used for a majority of cell phones.

Network Structure of a GSM:
* the Base Station Subsystem (the base stations and their controllers).
* the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
* the GPRS Core Network (the optional part which allows packet based Internet connections).
* all of the elements in the system combine to produce many GSM services such as voice calls and SMS.

Subscriber Identity Module (SIM Card)
The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking.

Security:
The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy.

2. PCS
PCS is a radio band that can be used by mobile phones in North America. Sprint happened to be the first service to set up a PCS.

Code Division Multiple Access (CDMA), GSM, and D-AMPS systems can be used on PCS frequencies.

3. D-AMPS
D-AMPS (Digital Advanced Mobile Phone Service) is an upgraded version of AMPS but it is being phased out due to advancement in technology. The newer GSM networks are replacing the older system.

Often referred to as TDMA (time division multiple access)

D-AMPS uses existing AMPS channels and allows for smooth transition between digital and analog systems in the same area. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots (hence time division) and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals. Calls were encrypted, although the algorithm used (CMEA) was later found to be weak.

IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol. SMS and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion.

Wireless MAN

Metropolitan Area Networks

Metropolitan area networks, or MANs, are large computer networks usually spanning a city. They typically use wireless infrastructure or Optical fiber connections to link their sites.

Implementation of Wireless MAN:
MAN links between LANs have been built without cables using either microwave, radio, or infra-red laser links. Most companies rent or lease circuits from common carriers due to the fact that laying long stretches of cable can be expensive.

DQDB, Distributed Queue Dual Bus, is the Metropolitan Area Network standard for data communication. It is specified in the IEEE 802.6 standard. Using DQDB, networks can be up to 20 miles (30km) long and operate at speeds of 34 to 155 Mbit/s.

Ethernet based MAN - Metro Ethernet

A Metro Ethernet is a computer network based on the Ethernet standard and which covers a metropolitan area. It is commonly used as a metropolitan access network to connect subscribers and businesses to a Wide Area Network, such as the Internet. Large businesses can also use Metro Ethernet to connect branch offices to their Intranet.

A typical service provider Metro Ethernet network is a collection of Layer 2 or 3 switches or routers connected through optical fiber. The topology could be a ring, hub-and-spoke (star), or full or partial mesh. The network will also have a hierarchy: core, distribution and access. The core in most cases is an existing IP/MPLS backbone, but may migrate to newer forms of Ethernet Transport in the form of 10G or 100G speeds.

Ethernet on the MAN can be used as:
- Pure Ethernet
Pure Ethernet-based deployments are cheap but less reliable and scalable, and thus are usually limited to small scale or experimental deployments.

- Ethernet over SDH
SDH-based deployments are useful when there is an existing SDH infrastructure already in place, its main shortcoming being the loss of flexibility in bandwidth management due to the rigid hierarchy imposed by the SDH network.

- Ethernet over MPLS
MPLS based deployments are costly but highly reliable and scalable, and are typically used by large service providers.

- Ethernet over DWDM
Five reasons to use it:
*Eliminate Network Layers While Reducing Complexities and Equipment
Costs
*Improve Resource Use to Achieve Optimal Bandwidth Efficiency
*Simplify End-to-End Provisioning to Speed Time to Market
*Automate Network Management for Scalability and Reduced Operating
Expenses
*Detect Problems Automatically and Resolve Them Faster Across the Entire
Network

Wireless LAN

Wireless LAN

A wireless LAN or WLAN or wireless local area network is the linking of two or more computers or devices using spread-spectrum or OFDM modulation technology based to enable communication between devices in a limited area. This gives users the mobility to move around within a broad coverage area and still be connected to the network.

Advantages of using WLAN:
- Convenience
The wireless nature of such networks allows users to access network resources from nearly any convenient location within their primary networking environment

-Mobility
With the emergence of public wireless networks, users can access the internet even outside their normal work environment.

-Productivity
Users connected to a wireless network can maintain a nearly constant affiliation with their desired network as they move from place to place.

-Deployment
Initial setup of an infrastructure-based wireless network requires little more than a single access point.

-Expandability
Wireless networks can serve a suddenly-increased number of clients with the existing equipment.

- Cost
Wireless networking hardware is at worst a modest increase from wired counterparts.

Disadvantages of using WLAN:
- Security
Wireless LAN transceivers are designed to serve computers throughout a structure with uninterrupted service using radio frequencies. Because of space and cost, the antennas typically present on wireless networking cards in the end computers are generally relatively poor. In order to properly receive signals using such limited antennas throughout even a modest area, the wireless LAN transceiver utilizes a fairly considerable amount of power. What this means is that not only can the wireless packets be intercepted by a nearby adversary's poorly-equipped computer.

- Range
The typical range of a common 802.11g network with standard equipment is on the order of tens of metres. To obtain additional range, repeaters or additional access points will have to be purchased. Costs for these items can add up quickly.

- Reliability
Like any radio frequency transmission, wireless networking signals are subject to a wide variety of interference, as well as complex propagation effects. One of the most insidious problems that can affect the stability and reliability of a wireless LAN is the microwave oven. In the case of typical networks, modulation is achieved by complicated forms of phase-shift keying (PSK) or quadrature amplitude modulation (QAM), making interference and propagation effects all the more disturbing. As a result, important network resources such as servers are rarely connected wirelessly.

- Speed
The speed on most wireless networks (typically 1-108 Mbit/s) is reasonably slow compared to the slowest common wired networks (100 Mbit/s up to several Gbit/s). There are also performance issues caused by TCP and its built-in congestion avoidance.

Architectures of WLAN
- Stations

All components that can connect into a wireless medium in a network are referred to as stations.

All stations are equipped with wireless network interface cards (WNICs).

Wireless stations fall into one of two categories: access points, and clients.

Access points (APs), normally routers, are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with.

Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface.

- Basic service set

The basic service set (BSS) is a set of all stations that can communicate with each other.

There are two types of BSS: Independent BSS (also referred to as IBSS), and infrastructure BSS.

Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS.

An independent BSS (IBSS) is an ad-hoc network that contains no access points, which means they can not connect to any other basic service set.

An infrastructure BSS can communicate with other stations not in the same basic service set by communicating through access points.

- Extended service set

An extended service set (ESS) is a set of connected BSSes. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string.

- Distribution system

A distribution system (DS) connects access points in an extended service set.

Types of WLAN:
- Peer-to-peer

An ad-hoc network is a network where stations communicate only peer to peer (P2P). There is no base and no one gives permission to talk. This is accomplished using the Independent Basic Service Set (IBSS).

A peer-to-peer (P2P) network allows wireless devices to directly communicate with each other. Wireless devices within range of each other can discover and communicate directly without involving central access points. This method is typically used by two computers so that they can connect to each other to form a network (as used in PSP - Playstation Portable).

- Bridge

A bridge can be used to connect networks, typically of different types. A wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to a wireless network. The bridge acts as the connection point to the Wireless LAN.

- Wireless distribution system

A Wireless Distribution System is a system that enables the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the need for a wired backbone to link them.

The notable advantage of WDS over other solutions is that it preserves the MAC addresses of client packets across links between access points.

WDS may also be referred to as repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). It should be noted, however, that throughput in this method is halved for all clients connected wirelessly to a router that is connected with WDS.

WLAN Roaming:
- Internal Roaming

The Mobile Station (MS) moves from one access point (AP) to another AP within a home network because the signal strength is too weak.

- External Roaming

The MS(client) moves into a WLAN of another Wireless Service Provider (WSP) and takes their services (Hotspot).

Wireless PAN & Bluetooth

Wireless PAN

Wireless Personal Area Network (WPAN) is a type of wireless network that interconnects devices within a relatively small area, generally within reach of a person. For example, Bluetooth provides a WPAN for interconnecting a headset to a laptop. ZigBee also supports WPAN applications.

Bluetooth
Bluetooth is a wireless protocol for exchanging data over short distances from fixed and mobile devices, creating personal area networks (PANs). It can connect several devices, overcoming problems of synchronization.

Bluetooth uses a radio technology called frequency-hopping spread spectrum, which chops up the data being sent and transmits chunks of it on up to 79 frequencies. In its basic mode, the modulation is Gaussian frequency-shift keying (GFSK). It can achieve a gross data rate of 1 Mb/s. Bluetooth provides a way to connect and exchange information between devices such as mobile phones, telephones, laptops, personal computers, printers, Global Positioning System (GPS) receivers, digital cameras, and video game consoles through a secure, globally unlicensed Industrial, Scientific, and Medical (ISM) 2.4 GHz short-range radio frequency bandwidth. The Bluetooth specifications are developed and licensed by the Bluetooth Special Interest Group (SIG). The Bluetooth SIG consists of companies in the areas of telecommunication, computing, networking, and consumer electronics.

Evolution of Bluetooth:
- Bluetooth 1.0 and 1.0B
- Bluetooth 1.1
- Bluetooth 1.2
- Bluetooth 2.0
- Bluetooth 2.1
- Bluetooth high speed (Not ready yet)
- Bluetooth 3.0 (Not ready yet)
- Bluetooth low energy (Not ready yet)

Mandatory protocols for all Bluetooth stacks:
- LMP (Link Management Protocol)

Used for control of the radio link between two devices. Implemented on the controller.

- L2CAP (Logical Link Control & Adaptation Protocol)

Used to multiplex multiple logical connections between two devices using different higher level protocols. Provides segmentation and reassembly of on-air packets.

- SDP (Service Discovery Protocol)

Used to allow devices to discover what services each other support, and what parameters to use to connect to them.

Two universally supported protocols:

- HCI (Host/Controller Interface)

Standardised communication between the host stack and the controller.

- RFCOMM (Cable replacement protocol)

Radio frequency communications (RFCOMM) is the cable replacement protocol used to create a virtual serial data stream. RFCOMM provides for binary data transport and emulates EIA-232 control signals over the Bluetooth baseband layer.

Communication and connection in bluetooth environment:
A master Bluetooth device can communicate with up to seven devices in a Wireless User Group. This network group of up to eight devices is called a piconet.

Security:
Bluetooth implements confidentiality, authentication and key derivation with custom algorithms based on the SAFER+ block cipher. In Bluetooth, key generation is generally based on a Bluetooth PIN, which must be entered into both devices.

Health Concerns:
Bluetooth uses the microwave radio frequency spectrum. The hazards can be compared to mobile phones hazard or lower.

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