IEEE 802.11 MAC Enhancement

IEEE 802.11 Architecture, more popularly known as WiFi is widely used to provide anywhere-anytime networking access. New enhancements keep getting introduced as global connectivity demands evolution and improvement. The IEEE 802.11 amendment was introduced to enhance the traditional 802.11 for higher throughput and improve wireless technologies.

The primary objective of this amendment is to bolster the MAC (Medium Access Control) layer’s Quality of Service (QoS). By refining QoS, the IEEE aspires to cater more efficiently to the escalating demands for bandwidth. This is increasingly crucial in an era where high-definition streaming, online gaming, remote work, and the proliferation of Internet of Things (IoT) devices are driving unprecedented levels of data traffic.

In this article, we will delve deeper into the enhancements introduced in this amendment. This will encompass improvements in both the physical layer communications design and the MAC layer. We will discuss how these enhancements not only facilitate higher throughput but also offer a more resilient and efficient wireless communication system.

How IEEE 802.11 Works?

Step 1: Data Frame Preparation:

When a station wants to transmit data, it creates a MAC Service Data Unit (MSDU). This is essentially the raw data the station wishes to send. To ensure successful transmission and proper communication, the station adds specific headers to the MSDU. With these headers added, the data unit is now termed a MAC Protocol Data Unit (MPDU).

Step 2: Wait Before Transmission:

Before rushing to transmit the MPDU, the station first waits for a time duration called DCF Interframe Space (DIFS). This is a predetermined time that ensures the medium is not accessed immediately and reduces the chances of collision.

Step 3: Channel Assessment:

Using the CSMA/CA protocol, the station checks if the transmission medium (typically air in the case of Wi-Fi) is currently being used. If the channel is detected as busy, the station doesn’t transmit immediately and moves to the back-off procedure. If it’s idle, it proceeds to transmit.

Step 4: Back-off Procedure:

Instead of all stations trying again immediately after the medium becomes free, each station chooses a random time period from the Contention Window (CW) to wait. This randomness reduces the chance of collisions when multiple stations want to transmit. After the back-off time elapses, the station senses the medium again to check if it’s free.

Step 5: Data Transmission:

If after the back-off (or immediately after DIFS if the channel was initially free), the channel is sensed as idle, the station transmits its MPDU.

Step 6: Acknowledgement Process:

Once the receiving station gets the MPDU and determines it’s for them and that it’s error-free, it waits for a short duration called Short Interframe Space (SIFS) before acknowledging. Post-SIFS, the receiving station sends an ACK frame to the sender, signaling the successful reception of the MPDU.

Step 7: Successful Transmission:

The sending station, upon receiving the ACK frame, knows that its data has been successfully transmitted and received. This marks the end of the current DCF cycle for that data unit.

Legacy IEEE 802.11 Operation

Limitations

During the entire procedure, the transmission time is divided into a DCF Interframe Space (DIFS), a Contention Window back-off time, the Physical Layer Protocol Data Unit (PPDU) transmission time, a Short Interframe Space (SIFS) and the ACK frame transmission time. The overhead and waiting time involved in this mechanism results in the inefficiency of the channel utilization and limited data throughput.

Enhancements in Physical Layer

In its physical layer, 802.11n employs MIMO technology, leveraging multiple antennas to either boost data speeds or extend the signal range.

Additionally, it incorporates channel bonding, merging two 20 MHz channels from the older 802.11 standard into one 40 MHz channel, thereby enhancing the data transmission rate.

Multiple Input Multiple Output (MIMO)

This technique provides higher data rates upto 600 Mbit/s and higher range. MIMO technology provides the ability to receiver and/or transmit simultaneously through multiple antennas. The more antennas a 802.11n device uses simultaneously, the higher its maximum data rate.

Spatial Division Multiplexing

In the SDM technique, a single outgoing signal is split into multiple streams. These streams are then simultaneously sent out through various antennas but remain within a single frequency channel.

Space Time Block Coding

In the STBC method, several duplicates of the same data are sent using multiple antennas. By analyzing these diverse data streams when they arrive, the receiver can more accurately identify the original data, even when there’s interference or signal distortion.

Channel Bonding

Traditional 802.11 devices operate on 20MHz channels. On the other hand, 802.11n based products support both 20MHz and 40MHz channels.

The 20MHz channels are used where spectrum availability is limited. Meanwhile, 40 MHz channels are the combination of two adjacent channels. This is the process of channel bonding, in which two adjacent channels within a given frequency band are combined. It provides higher data rates and double peak rate.

Enhancements in MAC Layer

There are three notable MAC layer enhancements introduced by the amendment:

Frame Aggregation

This method consists of combining multiple data frames into an aggregate frame. New 802.11n devices have the option of bundling frames together for transmission. Frame aggregation reduces MAC layer overhead caused by inter-frame spacing and preamble. It avoids the time wasted due to backoff and collisions of 802.11 MAC protocol.

There are two different forms of aggregation supported by 802.11n:

MAC Service Data Unit Aggregation (A-MSDU)

MSDU comprises of an LLC header (Logical Link Control), IP header and the IP packet payload. This technique combines multiple MSDUs with the same 802.11 QoS into a single MAC frame (MPDU).

MAC Service Data Unit Aggregation

MAC Protocol Data Unit Aggregation (A-MPDU)

This aggregation occurs later, after MAC headers are added to each MSDU. It groups multiple MPDUs frames as a single frame. Unlike A-MSDU, all MSDUs need not be destined to the same MAC address.

MAC Protocol Data Unit Aggregation

Block Acknowledgement

In the traditional 802.11 MAC protocol, every piece of data sent to a specific address gets an immediate acknowledgement from the receiver.

To reduce the overhead involved, the 802.11n amendment introduced the Block Acknowledgment (BACK) technique. Instead of acknowledging each piece of data individually, it groups several ACKs together into one response. This way, one BACK frame can confirm the receipt of multiple MPDUs.

Reverse Direction

Reverse direction is a tool that makes exchanging data in wireless networks faster and more efficient, especially when data needs to move back and forth, like in Voice over IP or TCP traffic because of backward TCPAck flow.

Here’s how it works:

• Normally, devices take turns sending and receiving data, which can slow things down.
• With reverse direction, whenever there is an opportunity to send data (called a TXOP), the device that’s usually just receiving can also send its data back at the same time.
• Imagine a two-way street instead of a one-way lane: it lets traffic flow both ways, making the process more efficient and quicker.

Frequently Asked Questions

1. What is the main objective of the IEEE 802.11n amendment?

The primary objective of the IEEE 802.11n amendment is to enhance the MAC (Medium Access Control) layer’s Quality of Service (QoS). The amendment seeks to efficiently cater to the rising demands for bandwidth, especially considering the increase in high-definition streaming, online gaming, and IoT devices.

2. What is MIMO and how does it benefit wireless communication in the 802.11n standard?

MIMO stands for Multiple Input Multiple Output. In the context of the 802.11n standard, MIMO technology leverages multiple antennas to either boost data speeds or extend the signal range. It provides the ability to receive and/or transmit simultaneously through multiple antennas, allowing for higher data rates of up to 600 Mbit/s and a broader range.

3. What is the difference between A-MSDU and A-MPDU aggregation techniques in the 802.11n MAC layer enhancements?

Both A-MSDU and A-MPDU are aggregation techniques introduced in the 802.11n amendment. A-MSDU (MAC Service Data Unit Aggregation) combines multiple MSDUs with the same 802.11 QoS into a single MAC frame (MPDU). The MSDU typically comprises an LLC header, IP header, and the IP packet payload. On the other hand, A-MPDU (MAC Protocol Data Unit Aggregation) groups multiple MPDU frames as a single frame after MAC headers have been added to each MSDU. Unlike A-MSDU, in A-MPDU, all MSDUs need not be destined to the same MAC address.

4. How does channel bonding enhance data transmission in the 802.11n standard?

Channel bonding in 802.11n involves merging two adjacent 20 MHz channels from the older 802.11 standard into a single 40 MHz channel. This enhancement effectively doubles the data transmission rate. Traditional 802.11 devices operate on 20MHz channels, but 802.11n products can support both 20MHz and the wider 40MHz channels. The 40MHz channels provide higher PHY data rates and a double peak rate.

5. What is the Block Acknowledgement (BACK) technique introduced in the 802.11n amendment and how does it improve efficiency?

In traditional 802.11 MAC protocols, every data piece sent to a specific address receives an immediate acknowledgement from the receiver. The 802.11n amendment introduced the Block Acknowledgment (BACK) technique to reduce this overhead. Instead of individually acknowledging each piece of data, the BACK technique groups several acknowledgments together into one response. As a result, a single BACK frame can confirm the receipt of multiple MPDUs, making the process more efficient.