WLAN IP and Frame
- 1. IP – Internet Protocol The Internet Protocol is essential in the global network as it is used to simplify communication and packet exchange between different hosts over the Internet. Each host has a unique IP that determines its address inside the network, not the physical one (MAC address). Technically, WLAN is very similar to Ethernet and Internet when it comes to addressing. Structure of the Internet Protocol: In version 4 (IPv4), 4 sequences of 1 byte each are used to represent the 32 bits IP address of any user. Actually, this protocol is divided into two parts: Host ID and Network ID. Therefore from any host IP, the Internet can define which network this user belongs to. Figure: IP representation As shown in the example above, each of the 4 numbers has a significant 8 bits sequence that make the IP, with two major parts of 16 bits each: Network and Host. These integer numbers are used to simplify the representation of this protocol for technical and implementation reasons. Classes and Subnets: Due to the huge increase in the number of Internet users worldwide, reaching over 3 billion users, smaller networks, called subnets, are a must to manage, serve and divide these hosts by geographical areas. Virtually created using the IP, the network and user numbers are defined by classes. Classes are based on the modification of the IP, as by decreasing the number of bits in the Network ID as shown in the figure below, more bits will be available for the Host ID. Therefore, the number of subnets that can be created will decrease while the number of users in each subnet will increase and reach up to 1 million. This is the case of class A, as shown in the following table, and is used in overpopulated countries like China and India. Class B is the most balanced one and is used for smaller countries. While Class C focusses on the increase in the number of subnets, reaching up to 1 million with 254 users available in each one.
- 2. Figure: Classes Representation Class # of Subnets # of hosts in each Subnet A Hundreds One Million B Thousands Thousands C One Million 254 Table: Variation in numbers of subnets and users per class Implementation: Routers are the physical devices used to create and manage networks. They connect the internal network of hosts, connected through WLAN or Ethernet cable, with the external network, for example Internet. Only these devices have the ability to create and manage subnets by reaching the Network layer of any system. Routers also act as gates between the internal and external networks, with store and forward technique to manage packet exchange between the hosts. Therefore, only internal hosts can share their IPs with each other, while external hosts can only reach the router’s IP. Virtual IP: Due to the increase in the number of devices using the Internet Protocol to access the global network, and while the IP cannot serve all these devices with unique sequences, virtual IPs are being used to extend the scale of connected devices. Unlike the MAC protocol where the address cannot be changed, the virtual IP address can vary every time the host reconnects to the router. Changes are done to provide sufficient addresses for the new online devices while other devices are disconnected. By assigning only the main router an original IP address, the other devices and sub routers will have virtual IPs as they cannot be seen by the external network. Therefore, knowing the destination of each device, the main router will receive the external packets and forward to the destination through its virtual network.
- 3. Frame Similarly to Ethernet, Wireless LAN is half duplex where hosts can either send or receive packets at a time. With the increased number of applications in a single network, hosts in the same network always communicate with each other to exchange information using frames. Known as the unit of transmission, frames are essential to manage network communication and exchange data between users. Used in the Link layer, they contain all necessary information to deliver the message to its destination. Therefore, frames are not only essential in exchanging data, but also in initiating and controlling the communication. Three control frames are widely used in WLAN to provide efficient communication between the two users: 1. Request to Send – (20 Bytes) Sent by the host to its desired destination, requesting the possibility of transmitting data over the network. 2. Clear to send – (14 Bytes) Reply by the destination to the host, allowing it to begin transmission. These two frames are used to initialize the data exchange, with request-allow process between the two hosts. 3. Acknowledgment – (14 Bytes) Mainly used after receiving the data frame, it informs the sender that the packet is received and allows him to send the rest of the frames, if available. The format of these control frames are shown in the figure below, with the receiver address of 6 Bytes mainly included in these frames: Figure: Control Frame Format
- 4. As the WLAN is half duplex, Network Data Frames are used to limit the data transmitted by one user continously, allowing the other user to send its frame if available, like Acknowlegment for the one received. The WLAN data frame is limited to 2346 Bytes, with a maximum 2312 Bytes of data. As shown in the figure below, 34 Bytes are used for frame control and information. When the user’s data file exceeds the maximum allowed network data , more than one frame will be required to deliver the file while taking into consideration the correct order of the frames. Therefore, the frames will be numbered from the lowest to highest, for destination synchronization purpose. Figure: Network Data Frame format Frame Control includes all necessary information for the network, like the protocol version (IPv4 for example). Duration ID, known as TTL (Time to Live), is used to discard the frame if the time is exceeded and the frame didn’t reach the destination. This sequence is essential to avoid the overflown of the network with useless frames. Destination and Sender addresses are always available. Sequence Control shows the number of frames sent to the destination, with the number of each one if more than one frame is to be delivered. FCS, Check Sum, is used for error detection and correction. The sum of all the bits in the frame is summed and included in this sequence, and therefore when the frame reaches the destination, the FCS should be equal to the summed bits of the frame.