07-10-2016, 03:36 PM
Design and Implementation of Wi-Fi Medium Access Control Layer for Transmitter with VHDL
1458227710-DesignandImplementationofWiFiMediumAccessControlLayerforTransmitterwithVHDL1.pdf (Size: 659.19 KB / Downloads: 13)
Abstract: For the wireless communication in radio frequency range, IEEE 802.11 is one of the many standard available. IEEE
802.11b defines the Medium Access Control Layer [MAC] for wireless local area networks. The wireless local area network, WLAN is
dominated by IEEE 802.11 standard. It becomes one of the main focuses of the WLAN research. Now most of the ongoing research
projects are simulation based as their actual hardware implementation is not cost effective. The main core of the IEEE 802.11b standard
is the CSMA\CA, Physical and MAC layers. But only MAC layer for transmitter is modeled in this paper using the VHDL. The VHDL
(Very High Speed Hardware Description LANguage) is defined in IEEE as a tool of creation of electronics system because it supports
the development verification synthesis and testing of hardware design, the communication of hardware design data and the
maintenance, modification and procurement of hardware. It is a common LANguage for electronics design and development
prototyping. The main purpose of the IEEE 802.11 standard is to provide wireless connectivity to devices that require a faster
installation, such as Laptops, PDA’s or generally mobile devices inside a WLAN. MAC procedures are defined here for accessing the
physical medium, which can be infrared or radio frequency. Here Wi-Fi MAC Transmitter module is divided in to 5 blocks i.e. Data Unit
Interface block, Controller block, Pay Load Data Storage block, MAC Header Register block, Data Processing block. In this paper, we
are considering only two blocks i.e. Payload Data Storage block & Data Processing block.
Introduction
Due to technology advancement in the 21st century, wireless
communication had been most popular choice of
communication. More and more people are turning to
wireless due to the convenience of mobility.
An 802.11 LAN is based on a cellular architecture where the
system is subdivided in to cells, where each cell [called
basic service set or BSS] is controlled by a base station
[called access point, or in short ap]. Even Though that a
wireless LAN may be formed by a single Cell, with a single
access point [can also work without an access point], most
installations will be formed by several cells, where the
access points are connected through some kind of back bone
[called distribution system or ds], typically Ethernet, and in
some cases wireless itself. The whole interconnected
wireless LAN Including the different cells, their respective
access points and the distribution system, is seen to the
upper layers of the OSI model, as a single 802 network and
is called in the standard as extended service set [ESS]. The
standard also defines the concept of a portal, a PORTAL is a
device that interconnects an 802.11 and another 802 LAN
.However, all is not prefect in the WLAN world. Offering
nominal bit rates of 11mbps [802.11b] and 54mbps (802.11a
and 802.11g) the effective throughputs are actually much
lower-owing to packet collisions, protocol overhead, and
interference in the Increasingly congested unlicensed bands
at 2.4ghz and 5ghz. Furthermore, operation in these bands
entails a strict regulatory transmit power constraint, thus
limiting range and even bit rates beyond a certain distance.
A wireless LAN (WLAN) is a data transmission system
designed to provide location-independent network access
between computing devices by using radio waves rather than
a cable infrastructure. WLAN utilizes spread-spectrum
technology based on radio waves to enable communication
between devices in a limited area, also known as the basic
service set. This gives users the mobility to move around within a broad coverage area and still be connected to the
network. For the home user, wireless has become popular
due to ease of installation, and location freedom with the
gaining popularity of laptops. The majority of future
wireless LAN growth is expected in healthcare facilities,
educational institutions, and corporate enterprise office
spaces.
1.1 Project Overview
Wi-Fi stands for Wireless Fidelity. Wi-Fi is based on the
IEEE 802.11 family of standards and is primarily a local
area networking (LAN) technology designed to provide inbuilding
broadband coverage. Wi-Fi is a universal wireless
networking technology that utilizes radio frequencies to
transfer data. Wi-Fi is more commonly used in point-tomultipoint
(PMP) environments to allow extended network
connectivity of multiple portable devices such as laptops,
telephones, or PDAs. Wi-Fi also allows connectivity in
point-to-point (P2P) mode, which enables devices to directly
connect and communicate to each other. Wi-Fi networks
operate in the unlicensed 2.4 radio bands, with an 11 Mbps
(802.11b) or 54 Mbps (802.11a) data rate, respectively.
There are three most important items which makes Wi-Fi
working in your laptop or desktop. These are:
Radio Signals
Wi-Fi Card which fits in your laptop or computer.
Hotspots which create Wi-Fi Network.
Wi-Fi also can be used to create a wireless mesh network,
which is a decentralized, reliable, resilient, and relatively
inexpensive solution that can support areas of lacking or
destroyed network infrastructure (e.g., connectivity for a
field office or emergency command center).
History
VHDL was originally developed at the behest of the U.S
Department of Defense in order to document the behavior of
the ASICs that supplier companies were including in
equipment. That is to say, VHDL was developed as an
alternative to huge, complex manuals which were subject to
implementation-specific details. The idea of being able to
simulate this documentation was so obviously attractive
those logic simulators were developed that could read the
VHDL files. The next step was the development of logic
synthesis tools that read the VHDL, and output a definition
of the physical implementation of the circuit. Due to the
Department of Defense requiring as much of the syntax as
possible to be based on Ada, in order to avoid re-inventing
concepts that had already been thoroughly tested in the
development of Ada, VHDL borrows heavily from the Ada
programming language in both concepts and syntax. The
initial version of VHDL, designed to IEEE standard 1076-
1987, included a wide range of data types, including
numerical(integer and real),logical(bit and boolean),
character and time plus arrays of bit called bit vector and of
character called string. A problem not solved by this edition,
however, was "multi-valued logic", where a signal's drive
strength (none, weak or strong) and unknown values are also
considered. This required IEEE standard 1164, which
defined the 9-value logic types: scalar std_ulogic and its
vector version std_ulogic_vector. The updated IEEE 1076,
in 1993, made the syntax more consistent, allowed more
flexibility in naming, extended the character type to allow
ISO-8859-1 printable characters, added the xnor operator,
etc.
Minor changes in the standard (2000 and 2002) added the
idea of protected types (similar to the concept of class in
C++) and removed some restrictions from port mapping
rules. In addition to IEEE standard 1164, several child
standards were introduced to extend functionality of the
language. IEEE standard 1076.2 added better handling of
real and complex data types. IEEE standard 1076.3
introduced signed and unsigned types to facilitate
arithmetical operations on vectors. IEEE standard 1076.1
(known as VHDL-AMS) provided analog and mixed-signal
circuit design extensions. Some other standards support
wider use of VHDL, notably VITAL (VHDL Initiative
Towards ASIC Libraries) and microwave circuit design
extensions. In June 2006, the VHDL Technical Committee
of Accellera (delegated by IEEE to work on the next update
of the standard) approved so called Draft 3.0 of VHDL-
2006. While maintaining full compatibility with older
versions, this proposed standard provides numerous
extensions that make writing and managing VHDL code
easier. Key changes include incorporation of child standards
(1164, 1076.2, 1076.3) into the main 1076 standard, an
extended set of operators, more flexible syntax of case and
generate statements, incorporation of VHPI (interface to
C/C++ languages) and a subset of PSL (Property
Specification Language). These changes should improve
quality of synthesizable VHDL code, make testbenches
more flexible, and allow wider use of VHDL for systemlevel
descriptions.
In February 2008, Accellera approved VHDL 4.0 also
informally known as VHDL 2008, which addressed more than 90 issues discovered during the trial period for version
3.0 and includes enhanced generic types. In 2008, Accellera
released VHDL 4.0 to the IEEE for balloting for inclusion in
IEEE 1076-2008. The VHDL standard IEEE 1076-2008 was
published in January 2009.
2.1 Benefits of Wireless Fidelity
The popularity of wireless LANs is a testament primarily to
their convenience, cost efficiency, and ease of integration
with other networks and network components.
The benefits of Wi-Fi include:
1) Convenience: The wireless nature of such networks
allows users to access network resources from nearly any
convenient location within their primary networking
environment (home or office).
2) Mobility: With the emergence of public wireless
networks, users can access the internet even outside their
normal work environment.
3) Deployment: Initial setup of an infrastructure-based
wireless network requires little more than a single access
point
4) Expandability: Wireless networks can serve a suddenlyincreased
number of clients with the existing equipment.
In a wired network, additional clients would require
additional wiring.
5) Cost: Wi-Fi chipset pricing continues to come down,
making Wi-Fi a very economical networking option and
driving inclusion of Wi-Fi in an ever-widening array of
devices.
2.2 Disadvantages of Wireless Fidelity
For a given networking situation, wireless LANs have
following limitations.
1) Security - Security concerns have held back Wi-Fi
adoption in the corporate world. It can be easy to crack
the current security technology, known as wired
equivalent privacy (WEP), used in most Wi-Fi
connections. A hacker can break into a Wi-Fi network
using readily available materials and software.
2) Range - The typical range of a common 802.11b Wi-Fi
network is on the order of tens of meters. It will be
insufficient in a larger structure.
3) Reliability- Like any other radio frequency transmission,
wireless networking signals are subject to a wide variety
of interference, as well as complex propagation effects
such as multipath.
4) Speed- The speed on most wireless networks (typically
1-108 Mbps) is reasonably slow compared to the slowest
common wired networks.
5) Energy - Power consumption is fairly high compared to
some other standards, making battery life and heat a
concern. To overcome these disadvantages of Wi-Fi we
are designing controller which controls the accessing of
medium at the transmitter of Wi-Fi using IEEE 802.11
standards. For computer networks, a seven layer –layer
ISO (International Standards Organization) OSI (Open
Systems Interconnection) reference model is widely
used. The communication sub network can be described
by the lower three layers (i.e., physical, data link, and
network layers). Existing LAN’s, MANs (metropolitan area networks) do utilize broadcast channels rather than
point-to-point channels for information transmission.
Therefore, a simple modification of OSI model is done
by adding the so called MAC (medium access control)
sub layer in data link layer. The MAC sub layer
protocols, usually known as the multiple access
protocols, are primarily a set of rules that communicating
terminals need to follow, and these are assumed to be
agreed upon a priori.
Exponential Back Off Algorithm
Backoff is a well known method to resolve contention
between different stations willing to access the medium, the
method requires each station to choose a Random Number
(n) between 0 and a given number, and wait for this number
of Slots before accessing the medium, always checking
whether a different station has accessed the medium before.
The Slot Time is defined in such a way that a station will
always be capable of determining if other station has
accessed the medium at the beginning of the previous slot.
This reduces the collision probability by half. Exponential
Backoff means that each time the station chooses a slot and
happens to collide; it will increase the maximum number for
the random selection exponentially. The 802.11 standard
defines an Exponential Backoff Algorithm, which must be
executed in the following cases:
(a)If when the station senses the medium before the first
transmission of a packet and the medium is busy,
(b) After each retransmission, and
© After a successful transmission
(d) The only case when this mechanism is not used is when
the station decides to transmit
We are dealing with only 802.11 MAC layer for this paper
using 802.11b standard due to its ease of operating
standards. The following figure shows a schematic of the
access mechanism: