HT (Header Type)

Last updated on 17 Apr 2023

In computer networking, a header is a part of a packet or frame that contains information about the packet or frame itself. Headers are added to the beginning of the packet or frame by the sending device and are used by the receiving device to interpret and process the packet or frame. There are many different types of headers used in computer networking, each with their own unique format and purpose. One such header is the HT header, which is used in the context of wireless networking. In this article, we will discuss the HT header in detail, including its format, fields, and function.

What is the HT Header?

The HT header is a type of header that is used in the context of wireless networking. Specifically, it is used in conjunction with the IEEE 802.11n standard, which is a wireless networking standard that was developed to improve the speed and reliability of wireless networks. The HT header is an extension to the existing 802.11 header format and is used to support the additional features and capabilities provided by the 802.11n standard.

The HT header is used to identify and manage packets that are transmitted using the 802.11n standard. It contains a number of fields that provide information about the packet, such as the packet type, length, and transmission parameters. The HT header is added to the beginning of the packet by the sender and is removed by the receiver before the packet is processed.

HT Header Format

The HT header has a specific format that is used to ensure that it can be processed correctly by wireless devices. The format of the HT header is as follows:

Let's take a closer look at each field in the HT header:

Frame Control: The Frame Control field is a 2-byte field that contains a number of subfields that specify various parameters related to the packet. These subfields include the packet type (e.g., data, management, or control), the duration of the transmission, and the addressing mode used.

Duration/ID: The Duration/ID field is a 2-byte field that is used to specify the duration of the transmission or the identifier of the packet. The value of this field depends on the value of the Frame Control field.

Address 1: The Address 1 field is a 6-byte field that contains the MAC address of the destination device.

Address 2: The Address 2 field is a 6-byte field that contains the MAC address of the source device.

Address 3: The Address 3 field is a 6-byte field that contains the MAC address of a third device, which is typically used for multi-hop transmissions.

Sequence Control: The Sequence Control field is a 2-byte field that is used to manage the order of packet transmission and to detect missing or duplicate packets.

Address 4: The Address 4 field is a 6-byte field that is used to support a variety of wireless networking functions, including mesh networking and virtual access points.

QoS Control: The QoS Control field is a 2-byte field that is used to specify the priority of the packet and to manage Quality of Service (QoS) parameters.

HT Control: The HT Control field is a 4-byte field that is used to manage the additional features and capabilities provided by the 802.11n standard, including channel bandwidth, spatial streams, and modulation schemes.

FCS: The FCS field is a 4-byte field that contains a checksum value that is used to ensure the integrity of the packet during transmission.

HT Header Fields

Now that we have discussed the format of the HT header, let's take a closer look at some of the individual fields and their functions:

Frame Control Field:

The Frame Control field is a 2-byte field that is used to specify various parameters related to the packet. It is divided into subfields that indicate the packet type, duration, and addressing mode. The Frame Control field also contains flags that indicate whether the packet is part of a sequence, whether it is encrypted, and whether it requires acknowledgement from the receiver.

Duration/ID Field:

The Duration/ID field is a 2-byte field that is used to specify the duration of the transmission or the identifier of the packet. The value of this field depends on the value of the Frame Control field. When the Frame Control field indicates that the packet is a data packet, the Duration/ID field specifies the duration of the transmission. When the Frame Control field indicates that the packet is a control packet, the Duration/ID field specifies the type of control packet.

Address Fields:

The Address fields (Address 1, Address 2, Address 3, and Address 4) contain the MAC addresses of the devices involved in the transmission. Address 1 specifies the destination device, Address 2 specifies the source device, and Address 3 is used for multi-hop transmissions. Address 4 is used for various functions, such as supporting mesh networking and virtual access points.

Sequence Control Field:

The Sequence Control field is a 2-byte field that is used to manage the order of packet transmission and to detect missing or duplicate packets. It contains a sequence number and a fragment number, which are used to ensure that packets are transmitted and received in the correct order.

QoS Control Field:

The QoS Control field is a 2-byte field that is used to specify the priority of the packet and to manage Quality of Service (QoS) parameters. It contains subfields that indicate the priority of the packet, the type of QoS used, and the number of retries that are allowed.

HT Control Field:

The HT Control field is a 4-byte field that is used to manage the additional features and capabilities provided by the 802.11n standard. It contains subfields that specify the channel bandwidth, the number of spatial streams, the modulation scheme, and other features such as frame aggregation and block acknowledgment.

FCS Field:

The FCS field is a 4-byte field that contains a checksum value that is used to ensure the integrity of the packet during transmission. The checksum value is calculated by the sender and verified by the receiver to ensure that the packet has not been corrupted during transmission.

HT Header Function

The HT header is used to support the additional features and capabilities provided by the 802.11n standard. These features include:

Multiple antennas: The 802.11n standard allows for the use of multiple antennas to improve the speed and reliability of wireless transmissions. The HT Control field is used to specify the number of spatial streams that are being used for the transmission.
Channel bonding: The 802.11n standard allows for the use of wider channels (up to 40 MHz) to increase the amount of data that can be transmitted at once. The HT Control field is used to specify the channel bandwidth that is being used.
Advanced modulation schemes: The 802.11n standard supports advanced modulation schemes such as 64-QAM and 256-QAM to increase the amount of data that can be transmitted over the wireless channel. The HT Control field is used to specify the modulation scheme that is being used.
Frame aggregation: The 802.11n standard allows for the aggregation of multiple frames into a single transmission to reduce the overhead associated with transmitting individual frames. The HT Control field is used to specify the type of frame aggregation that is being used.

Overall, the HT header is an essential component of the 802.11n wireless networking standard, as it enables the use of advanced features that can significantly improve the speed and reliability of wireless transmissions. By providing support for multiple antennas, wider channels, advanced modulation schemes, and frame aggregation, the HT header enables wireless devices to transmit and receive larger amounts of data at faster speeds than was possible with previous Wi-Fi standards.