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Introduction to the main functions of the mac layer
**Introduction to MAC**
The Media Access Control (MAC) layer is a sublayer of the Data Link Layer in the OSI model. It is responsible for defining how data frames are transmitted over a shared communication medium. In shared bandwidth environments, access to the medium follows a "first come, first served" approach. The MAC layer handles physical addressing and defines the logical topology, which describes the path signals take through the physical network. Additionally, it manages line control, error notification (without correction), frame ordering, and optional flow control.
**Types of MAC Protocols**
The main challenge in multiple-user access is determining how to coordinate users when they compete for the same channel. There are three primary categories of MAC protocols:
1. **Fixed Channel Allocation**: This method assigns specific channels to users using techniques like FDMA, TDMA, CDMA, or SDMA. It ensures high real-time performance but can be inefficient, mainly used for voice communications.
2. **Random Channel Allocation**: Users transmit without prior coordination, such as in ALOHA or CSMA protocols. This is common in data transmission where efficiency and flexibility are more important than strict timing.
3. **On-Demand Channel Allocation**: Channels are allocated based on user demand, often seen in systems like 802.16 that support multimedia traffic. This allows dynamic resource allocation and better performance in variable load conditions.
**MAC Frame Format**
A typical MAC frame consists of several fields that define its structure and content. Below is a breakdown of the key components:
**1. Frame Control Field**
This field contains various flags that determine the type and behavior of the frame. Key elements include:
- **Protocol Version**: Currently set to 0.
- **Frame Type**: Can be Management, Control, or Data.
- **Subtype**: Further categorizes the frame within each type.
- **ToDS/FromDS**: Indicates whether the frame is being sent to or from a Distribution System (e.g., an Access Point).
- **More Fragments**: Indicates if more fragments follow.
- **Retry**: Specifies whether the frame is a retransmission.
- **Power Management**: Indicates if the station is in power-saving mode.
- **Order**: Indicates whether the frame should be strictly ordered.
**2. Other Fields**
- **Duration/ID**: Used to set the Network Allocation Vector (NAV) or manage power-saving modes.
- **Address Fields**: Includes source, destination, BSSID, and other addresses depending on the frame type.
- **Sequence Control**: Contains a 4-bit fragment number and a 12-bit sequence number for tracking frame order.
- **Frame Body**: Carries the actual payload from upper layers, such as IP packets.
- **FCS (Frame Check Sequence)**: Ensures data integrity using CRC checks.
**3. Frame Body Data**
Unlike Ethernet, the 802.11 frame body encapsulates higher-layer protocols using the 802.2 Logical Link Control (LLC). Common encapsulation methods include RFC1042 and 802.1H, with examples showing how IP packets are embedded in the frame body.
**Modifying the MAC Address**
The MAC address is a unique identifier assigned by IEEE and is essential for network identification. If a duplicate occurs, it may cause network conflicts. To modify the MAC address:
- On Unix/Linux systems, you can use commands like `ifconfig`, `ip`, or edit configuration files such as `/etc/rc.d/init.d/network` or `/etc/rc.local`.
- Some systems require firmware tools provided by the manufacturer, especially for hardware-specific MAC chips.
Always ensure that changes are made carefully to avoid network instability or device malfunction.