The Background Of Wireless Networking

Published Date: 02 Nov 2017

In networking wireless connectivity has become a great advantage in that it provides you with a mobile network connection. Your centrally stored data can be accessed from anywhere in range of your wireless router, which increases flexibility and productivity, rather than being tied to one position.

In terms of flexibility, Wireless networking has no cabling. If you need to use your laptop in a meeting you can retain your network connection in another room without having the time or potential cost of getting someone to re-cable your network connection to another position.

Wireless networking is also advantage in terms of cost, if you need to add someone to your network it is simple a case of connecting their machine to the wireless router saving you the cost of having someone repeatedly visit to create a new network point.

IEEE 802.11 Architecture

IEEE 802.11 is the networking standards that our laptop is using to connect to connect to our network. It’s the set of protocols that allow machines with wireless network cards or other means of wireless networking to communicate without a wired connection.

On the OSI model 802.11 performs on both the Physical (layer 1) and Data Link (layer 2). Shown below you see the physical layer and the data link layer in relation to the description below.

At the physical layer there is a split into the sub layers Physical Layer Convergence Protocol (PLCP) and Physical Medium Dependent (PMD).

At the data link layer, there is a split into the sub layers. These layers are the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

Physical Layer - PLCP/PMD Sub Layers

The PLCP controls the preparation of the data units to be transmitted and received.

The PLCP procedure prepares the MAC frame for transmission across a wireless network. The PLCP check the signal, service length and Cyclic Redundancy Check. PLCP is used for frequency hopping protocols and direct sequence modulation.

The PMD performs modulation/demodulation, controls reception and takes care of the actual transmission of data.

Data Link Layer – MAC/LLC Sub Layers

MAC

The first sub link of the Data Link Layer is the MAC layer. The MAC is a series of rules that allows data to be sent and also allows access to your medium. The MAC layer is responsible for taking data packets and moving them from one networked machine to another across a network.

The MAC performs the following main functions;

Access control

Receiving and transmission of frames

Retransmission and back off functions

Checking the frame check sequence

Dropping malformed frames

Making changes to MAC address

The diagram below indicates what a MAC header looks like.

MAC Header

Frame Control

Duration/ID

Address 1

Address 2

Address 3

Sequence Control

Address 3

Network Data

FCS

2 bytes

2 bytes

6 bytes

6 bytes

6 bytes

2 bytes

2 bytes

0-2312 bytes

2 bytes

The Frame Control field contains information that allows the other fields to be controlled.

The Duration/ID field tells you how long you have until you can receive the next MAC frame.

The Address fields hold addresses dependant on the type of MAC field. It can hold the Destination Address, Source Address, Receiver Address and Transmitter Address.

The Sequence Control field contains the sequence and fragment numbers.

The Network Data contains the actual data that is being transmitted, hence the larger amount of bytes required.

The FCS contains information on whether or not any errors occurred during transmission.

LLC

The other Data Link sub layer is the LLC.

The LLC performs the following main functions;

Frame synchronisation

Flow Control

Error Checking

In relation to Frame synchronization, the LLC multiplexes. This is when messages are transmitted over the MAC and the messages become split. The LLC puts the messages back together. The data is arranged into frames when the messages are put back in order.

In error checking the LLC uses Cyclic Redundancy Check (CRC), there is a trailer on each frame with a few bits that show how the frame should be put together. When the frame is received these bits are checked to see if the order of the frame is the same as when it was sent.

The LLC can detect errors, but errors cannot be repaired at this layer.

Wireless Security - WPA2

Wireless networking uses WPA2 for authentication. There are three WPA2 features – Authentication, Key Management and Advanced Encryption.

Authentication

WPA2 authenticates in two phases, open system authentication and 802.1X with EAP. EAP is an Extensible Authentication Protocol. Authentication uses remote authentication dial-in with a pre-shared key.

Key Management

Key Management is a security function that uses a four way handshake.

The four way handshake is an authentication process that uses keys and nonce (abbreviated term for Number Used Once) values to authenticate your access point with your client.

The initial authentication process creates a Pairwise Master Key (PMK). Another key is established by the four way handshake called the Pairwise Transient Key (PTK). Then finally the process creates a Group Temporal Key (GTK).

The Access Point (AP) sends a nonce-value to the client (STA), this value is known as ANonce. The client can now create the Pairwise Transient Key from all the necessary attributes.

The client sends its own nonce-value, known as Snonce, to the Access Point together with an authentication code.

The Access Point sends the Group Temporal Key and a sequence number together with another authentication code. The receiving client can perform basic replay detection using the sequence number from the previous frame.

The client sends confirmation to the Access Point.

Advanced Encryption

WPA2 advanced encryption uses advanced encryption standard counter mode. This is a function that encrypts your data in 128-bit blocks with a 128-bit encryption key.