Introduction

IEEE 802.11 is designed for a limited geographical area (homes, offices, campuses, stations). Also known as Wi-Fi.

IEEEE 802.11 supports additional features(功能):

Power management.(电量管理)
Security mechanisms.(安全机制)

Original 802.11 standard defined two radio based physical layer standard(物理层标准):

Frequency hopping.(跳频)
Direct sequence.(11-bit chipping sequence, 切片序列)

Hidden node problem

Suppose both A and C want to communicate with B and so they each send it a frame.

But A and C are unaware of each other since their signals do not carry that far.
These two frames collide with each other at B.
- But unlike an Ethernet, neither A nor C is aware of this collision.
A and C are said to hidden nodes with respect to(关于) each other.

双方虽然听不到对方的信号，但同时传送给相同对象会造成碰撞。

Exposed node problem

Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission.

It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission.
Suppose C wants to transmit to node D.
This is not a problem since C’s transmission to D will not interfere(干扰) with A’s ability to receive from B.

双方虽然听得到对方的信号，但可以同时传送给不同的对象。

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Key Idea:

Sender and receiver exchange control frames with each other before the sender actually transmits any data.
This exchange informs(通知) all nearby nodes that a transmission is about to begin.
Sender transmits a Request to Send (RTS) frame to the receiver.
- The RTS frame includes a field that indicates how long the sender wants to hold the medium.
Receiver replies with a Clear to Send (CTS) frame.

Then:

Any node that sees the CTS frame:

is close to the receiver, therefore cannot transmit for the period of time specified in the CTS frame.

Any node that sees the RTS frame but not the CTS frame:

is not close enough to the receiver to interfere with it, so is free to transmit.

RTS frame collision:

If two or more nodes detect an idle link and try
to transmit an RTS frame at the same time:

Their RTS frame will collide with each other.
So the senders realize the collision has happened when they do not receive the CTS frame after a period of time.
Each sender waits a random amount of time before trying again.
The amount of time is defined by the same exponential backoff algorithm used on the Ethernet.

Frame Types:

Class 1 frames

Control Frames
1. RTS
2. CTS
3. ACK(收到Data的回复)
4. Poll(轮询)
Management Frames
1. Probe Request/Response
2. Beacon(信标)
3. Authentication(认证)

Distributed System(分散式系统)

Nodes are free to move around, And directly reachable nodes may change dynamically. To deal with mobility and partial connectivity(移动性与部分连通性), 802.11 defines additional structures on a set of nodes:

some nodes are allowed to roam(漫游).
some are connected to a wired network infrastructure(有线网络基础设施) called Access Points (AP) and connected to each other by a so-called distribution system (DS).

A distribution system that connects many APs, And the distribution network runs at layer 2 of the ISO architecture.

The technique for selecting an AP is called scanning:

The node sends a Probe frame.
All APs within reach reply with a Probe Response frame.
The node selects one of the APs and sends that AP an Association Request frame.
The AP replies with an Association Response frame.
And a node runs this protocol whenever:
- it joins the network, and
- when it wants to change another AP (better signal).

Active scanning: As node moves, it sends Probe frames, which eventually result in Probe Responses from APs.

Passive Scanning: APs also periodically send a Beacon frame(信标帧) that advertises the capabilities of the AP; these include the transmission rate(传输速率) supported by the AP.

IEEE 802.11 Frame format

2	6	6	6	6	2	6	0-2312	4	bytes
Frame Control	Duration(持续时间)/Conn ID	Addr1	Addr2	Addr3	Sequence Control	Addr4	Data	FCS

Frame Control:

2	2	4	1	1	1	1	2	1	1	bits
Protocol Version	Type	Subtype	To DS	From DS	Last Flag	Retry	Power Mang.	EP	Rsvd

How these four addresses are interpreted(理解) depends on the settings of the ToDS and FromDS bits in the frame’s Control field:

To DS	From DS	Addr 1	Addr 2	Addr 3	Addr 4
0	0	DA	SA	BSSID	N/A
0	1	DA	BSSID	SA	N/A
1	0	BSSID	SA	DA	N/A
1	1	DA	TA	RA	SA

BSSID: MAC address of an AP.
SA: Original Source address.
DA: Final Destination address.
TA: Transmitter address.
RA: Receiver address.

IEEE 802.11 MAC Architecture

Distributed Coordination Function (DCF, 分布式协调)
- 竞争式服务，非同步传输！
- The fundamental access method for the 802.11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
- Shall be implemented in all stations and APs. And used within both configurations:
  - Ad hoc(随意连接网络，无APs).
  - Infrastructured(结构连接网络，APs+DS).
Point Coordination Function (PCF, 点协调)
- 免竞争式服务，具时限传输！
- An alternative access method, shall be implemented on top of the DCF through the use of an access priority mechanism(机制).
- A point coordinator (polling master, 点协调器) is used to determine which station currently has the right to transmit.
- Different accesses of traffic can be defined through the use of different values of IFS (Inter-Frame Space). (PCF轮询的频率更快)
Coexistence(共存) of DCF and PCF
- Both the DCF and PCF shall coexist without interference.
- Superframe : A contention-free burst(无争) occurs at the beginning, followed by a contention period.

Distributed Coordination Function (DCF)

Carrier Sense

Carrier Sense shall be performed both through physical and virtual mechanisms.

Physical Carrier Sense Mechanism
- A physical carrier sense mechanism shall be provided by the PHY.
Virtual Carrier Sense Mechanism
- Provided by the MAC, named Net Allocation Vector (NAV, 净分配向量), which maintains a prediction of future traffic based on duration information announced in RTS/CTS frames.

MAC-Level Ack (Positive Acknowledgment)

To allow detection of a lost or errored frame an ACK frame shall be returned immediately following a successfully received frame.

The gap between the received frame and ACK frame shall be SIFS.

Data, Poll, Request, Response should be acknowledged with an ACK frame. The lack of an ACK frame means that an error has occurred.

Priority levels: Inter-Frame Space (IFS)

Short-IFS (SIFS)
- Used for: an ACK frame, a CTS frame, or by a station responding to any polling.
PCF-IFS (PIFS)
- Used only by the PCF to send any of the Contention Free Period frames.
DCF-IFS (DIFS)
- Used by the DCF to transmit asynchronous(非同步) MPDUs(MAC Protocol Data Unit, 协议数据单元).

DCF Access Procedure

In DCF, if busy, the STA shall defer until after a DIFS gap is detected, and then generate a random backoff period for an additional deferral time(延期时间) (resolve contention, 解决竞争).

Backoff time = INT(CW * Random()) * Slot time

CW(Contention Window): An integer between CWmin and CWmax, and doubled when retransmission.
Slot Time(网路最长传输时间) = Transmitter turn-on delay + medium propagation delay + medium busy detect response time.

Backoff Procedure

The Backoff Timer shall be frozen while the medium is sensed busy and shall decrement only when the medium is free (resume whenever free period > DIFS).
Transmission shall commence(开始) whenever the Backoff Timer reaches zero.

Control of the Channel

The IFS is used to provide an efficient MSDU delivery mechanism.
Once a station has contended for the channel, it will continue to send fragments until either: (The gap between last ack and following fragment is a SIFS)
- all fragments of a MSDU have been sent,
- an ack is not received, or
- the station can not send any additional fragments due to a dwell time boundary.
If the source station does not receive an ack frame, it will attempt to retransmit the fragment at a later time (according to the backoff algorithm).

RTS/CTS Usage with Fragmentation

The RTS/CTS frames defines the duration of the first frame and ack.
The duration field in the data and ack frames specifies the total duration of the next fragment and ack.
The last Fragment and ACK will have the duration set to zero.
Each Fragment and ACK acts as a virtual RTS and CTS.
STA shall use an RTS/CTS exchange for directed frames only when the length of the MPDU is greater than the RTS_Threshold.

Point Coordination Function (PCF)

The PCF protocol is based on a polling scheme controlled by one special STA called the Point Coordinator (PC).

At the beginning of the SF, the PC shall
sense the medium. If it is free the PC shall wait a PIFS time and transmit a Data frame with the CF-Poll(contention-free) Subtype bit set, to the next station on the polling list, or a CF-End frame, if a null CF period is desired.

When a STA is being polling, it should send CF-Up frame within a SIFS, and following a CF-Down frame to poll next STA; or waits for a PIFS but no frame to send, directly poll next STA.

想了解更多关于计算机网络架构与网络安全：计算机网络架构与网络安全专栏

IEEE 802.11 无线局域网络 (Wireless LAN)相关推荐

《分布式系统：概念与设计》一3.5.2　IEEE 802.11无线LAN
3.5.2 IEEE 802.11无线LAN 本节将总结无线LAN技术中必须解决的无线网络的特殊特征,同时解释IEEE 802.11是如何处理这些特征的.IEEE 802.11(WiFi)标准扩展了以 ...
《计算机网络》——IEEE 802.11 无线局域网、无线局域网的分类、广域网、PPP协议、HDLC协议、链路层设备、冲突域和广播域
文章目录 IEEE 802.11 无线局域网 802.11的MAC帧头格式无线局域网的分类广域网 PPP协议 PPP协议应满足的要求: PPP协议无需满足的要求 PPP协议的帧格式 HDLC协议 ...
【计算机网络】数据链路层 : IEEE 802.11 无线局域网 ( 802.11 MAC 帧头格式 | 无线局域网分类 )
文章目录一.802.11 MAC 帧头格式二.无线局域网分类一.802.11 MAC 帧头格式 IEEE 802.11 是无线局域网通用标准 , 有 IEEE 置顶 ; 802.11 MA ...
计算机网络：IEEE 802.11无线局域网
文章目录 1.无线局域网的组成 (1)有固定基础设施无线局域网 (2)无固定基础设施移动自组织网络 2.802.11局域网的MAC帧 1.无线局域网的组成无线局域网可分为两大类:有固定基础设施的无线 ...
802.11a/b/g/n无线协议详解 802.11 无线认证和加密有什么区别开放系统
802.11a/b/g/n,其实指的是无线网络协议,细分为802.11a.802.11b.802.11g.802.11n等.这几种不同的无线协议.都是由802.11演变而来的.802.11是IEEE最 ...
无线射频专题《协议类，IEEE 802.11/802.11b/802.11a/802.11g/802.11n/802.11ac标准简介》
IEEE 802.11 标准和格式 IEEE 802 是指处理局域网和城域网的一系列IEEE 标准.IEEE 802 家族标准由IEEE 802 局域网/ 广域网标准委员会(LMSC) 维护.各个工作 ...
无线射频专题《IEEE 802.11协议讲解2@WiFi抓包分析之MAC_Header》
前言: IEEE 802.11是IEEE最初制定的一个无线局域网标准,主要用于解决办公室局域网和校园网中,用户与用户终端的无线接入,业务主要限于数据存取,速率最高只能达到2Mbps.由于802.11在 ...
802.11基本概念介绍【802.11 无线网络权威指南学习总结1】
802.11基本概念介绍[802.11 无线网络权威指南学习总结1] 1.802.11网络技术介绍 IEEE 802 规格的重心放在 OSI 模型最底下的两层,因为它们同时涵盖了实体(physical ...
802.11 无线网络权威指南-第1章无线网络导论
相关背景知识 1.IEEE 802网络(Ethernet) 以太网是当今现有局域网采用的最通用的通信协议标准.该标准定义了在局域网(LAN)中采用的电缆类型和信号处理方法.以太网在互联设备之间以10~ ...

IEEE 802.11 无线局域网络 (Wireless LAN)