83中无线局域网(IEEE 802[1].11)仿真指南(英文).

2021年1月25日发(作者：seller是什么意思)

Wireless LAN (IEEE 802.11 Model Guide
Purpose
This document describes Wireless LAN (WLAN simulation model shipped as part
of standard OPNET model release 10.0A for IT DecisionGuru and OPNET Modeler with
radio module. Wireless LAN Background
Wireless LAN protocol is based on the IEEE 802.11 standard. The standard defines
a medium access control (MAC sublayer and three physical (PHY goal of the
IEEE 802.11 protocol is to describe a wireless LAN that delivers services commonly
found in wired networks, e.g., throughput, reliable data delivery, and continuous network
architecture of the IEEE 802.11 WLAN is designed to support a network
where most decision making is distributed across the mobile stations. Some of the
components that are the basic building blocks of the 802.11 based network are described
below:
1. Station: In IEEE 80
2.11 network a station is the component that connects to the wireless medium. The
station may be mobile, portable, or station supports all station services
which include authentication,deauthentication, privacy, and delivery of the data (MAC
service data unit. 2. Basic Service Set (BSS: The IEEE 802.11 WLAN architecture is
built around a BSS. A BSS is a set of stations that communicate with one
all the stations in the BSS can communicate directly with each other and there is no
connection to a wired network, the BSS is called an independent BSS (IBSS. IBSS is also
know as adhoc network which is typically a short-lived network with small number of
stations in direct communication a BSS includes an access point (AP, the
BSS is no longer independent and is called an infrastructure BSS or simply BSS. In an
infrastructure BSS, all mobile stations communicate with the AP. The AP provides both
the connection to the wired LAN, if any, and the local relay function within the BSS.


3. Extended Service Set (ESS: An ESS is a set of infrastructure BSSs, where the
APs communicate among themselves to forward traffic from one BSS to another. The
APs perform this communication via distribution system (DS. The DS is the backbone of
the WLAN and may be constructed of either wired or wireless networks.
Medium Access Control
The IEEE 802.11 medium access control (MAC supplies the functionality required
to provide a reliable delivery mechanism for user data over wireless first
function of the WLAN MAC is to provide a reliable data delivery serviceto the users.
This is achieved through a frame exchange protocol at the second
function of the WLAN MAC is to provide fair mechanism to control access of shared
wireless media. It performs this function through two different access mechanisms: the
contention-based mechanism, called the distributed coordination function (DCF, and a
centrally controlled access mechanism,called the point coordination function (
third function of the WLAN MAC is to provide protection to the data it is
done through a privacy service, called Wired Equivalent Privacy (WEP which encrypts
the data sent over the wireless medium.
MAC Frame Exchange
The minimal MAC frame exchange consists of two frame, a data frame sent from
the source to the destination and an acknowledgement (Ack frame from the destination to
the source. If the source doesn’t
receive an acknowledgment then it attempts to retransmit
a data frame after observing appropriate are retry limits associated with
the retransmission of the protocol also suggests an optional use of request to
send (Rts and clear to send (Cts frame exchange between source and destination stations
for media reservation. Rts is transmitted from source to destination station and Cts is a
response initiated by the destination station to the source station. This initial handshake is
followed by the minimal MAC frame exchange.


Basic Access Mechanism
The basic access mechanism is carrier sense multiple access with collision
avoidance (CSMA/CA with binary exponential backoff. In this type of access mechanism,
a station will listen to the medium before beginning a the medium is
already carrying a transmission, the station that is listening will not begin its own
transmission. This is the CSMA portion of the access two or more stations
began their transmission at the same time then there will be a collision which
may cause one or more frames to be order to avoid collision, a station
listens to the medium before beginning its own transmission. If it detects an existing
transmission then it will enter into a deferral period determined by binary exponential
backoff algorithm. The binary exponential backoff mechanism chooses a random number
which represents the amount of time that must elapse while the medium is idle i.e. the
listening station may attempt to begin its transmission IEEE 802.11 MAC uses
collision avoidance rather than collision detection in order to be able to simultaneously
transmit and receive. For this reason, the IEEE 802.11 MAC implements a network
allocation vector (NAV. The NAV is a value that indicates to a station the amount of time
that remains before the medium will become available. The NAV is kept current through
duration values that are transmitted in all frames. By combining the virtual carrier sensing
mechanism (using NAV with the physical carrier sensing mechanism, the MAC
implements the collision avoidance portion of the CSMA/CA access
buted coordination function (DCF is a basic access mechanism
described in the protocol. It uses physical and virtual carrier sense mechanisms. If both
mechanisms indicate that medium in not in use for an interval of DIFS (distributed
interframe space then the station will begin to transmit the frame. However,if the medium
is not busy then the backoff algorithm is applied. The transmission is considered to be
unsuccessful if there is no acknowledgement received from the destination. This may
result in the retransmission of the frame. Centrally Controlled Access Mechanism


The centrally controlled mechanism uses a poll and response protocol to eliminate
the possibility of connection for the medium. This access mechanism is called the point
coordination function (PCF. A point coordinator (PC controls the PCF. The PC is always
located in an AP. Generally, the PCF operates by stations requesting that the PC register
them on a polling list, and the PC then regularly polls the stations for traffic while also
delivering traffic to the PCF is an optional part of the IEEE 802.11 standard.
It is built over the DCF and both operate simultaneously.
Model Scope and Limitations
This section discusses the implementation choices made in developing the
simulation model for the WLAN MAC protocol. Certain parts of the protocol have been
simplified, omitted or deferred for later development in view of the fact that it is intended
primarily for DCF based MAC performance estimation.
Wireless LAN Model Applications
Some of the studies that you can perform using the Wireless LAN protocolmodel are
listed below:
1. Study the possibility of having WLAN as an alternative to local area network
access technology
(e.g. compared to ethernet.
2. Analyze network performance by varying the capacity (or number of nodes in the
network.
3. Performance evaluation of protocol specific features like fragmentation and
reassembly or
RTS/CTS frame exchange.


4. Setting up independent and infrastructure BSS network and evaluate their
performances.
5. Research new media access control mechanism to reduce bandwidth overhead
resulting from
RTS/CTS section discusses the key elements of the WLAN
simulation model suite including model attributes and statistics. The idea is to show that
how the model simulates the protocol behavior, so some of the implementation specific
details are not mentioned before there are two types of wireless network
configuration:Infrastructure BSS and Independent BSS. The current WLAN model
supports both the configurations and their setup is as follows:
1. Infrastructure BSS



Workstations can communicate outside of the BSS using Access Point (AP which is
connected to the distribution system. Also, this AP acts as a relay within a BSS.
2. Independent BSS

This is an adhoc network of several stations. The workstations can have a peer-to-
peer connection with other stations in the BSS but the communication will be limited to
the BSS.
“wireless_lan” Object Palette

Following are the models which are contained in the palette:
1. Wireless Workstation (fixed and mobile
2. Wireless Server (fixed and server
3. Wireless terminal station (WLAN MAC without IP


4. Wireless Router (with one wireless interface and one or more wireline interface
like ethernet, FDDI, token ring, ATM etc.

Both modeler and ITDG have several wireless nodes defined in the object
models can be created and added to this palette as needed.
Node Architecture
Node models of some commonly used Wireless LAN objects are as follows:
1. Wireless Workstation



The Wireless LAN modeling of the MAC and the physical layer is comprised of the
wireless_lan_mac
process, transmitter, receiver, and the channel streams (as shown by the dotted box
in the figure above. The ARP (address resolution protocol is an interface between the
MAC and the higher layers.
2. Wireless Router (or Access Point



The illustrated access point contains one wireless interface and one Ethernet
interface. It can be used to connect a BSS with a wireline distribution that
the wireless interface receives the data frame and sends it to the higher layer for address
resolution if the destination station does not belong to the same BSS.
3. BSS: Wireless LAN-based network is configured using OPNET subnet object.
Each OPNET subnet can be considered as a single BSS if the user intends to auto
configure the IP addresses. Also, this object is used to represent logical and physical
grouping of nodes within a larger topology. Once all the subnet are appropriately
configured then they can be connected through a distribution system (e.g any wireline
technology.
WLAN Parameters
This section discusses the WLAN-related configurable parameters available as part
of the simulation model suite. Note that there are two types of attributes:model attributes,