An ATM Cell & What is its Structure

An ATM (Asynchronous Transfer Mode) cell is a fixed-size packet used in ATM networks to carry different types of data, including voice, video, and computer data. Each cell is 53 bytes long, consisting of a 5-byte header and a 48-byte payload.

 

 

Structure of an ATM Cell

1. Header (5 bytes):

The header contains several fields used for routing and managing the cell through the ATM network. The exact fields can vary slightly depending on whether the cell is traveling through a User-Network Interface (UNI) or a Network-Network Interface (NNI).

• Generic Flow Control (GFC) (4 bits): Used for flow control at the user-network interface. This field is typically set to zero in the core network.

• Virtual Path Identifier (VPI) (8 bits for UNI / 12 bits for NNI): Identifies the virtual path the cell belongs to. The VPI is used for routing purposes.

• Virtual Channel Identifier (VCI) (16 bits): Identifies the specific virtual channel within the virtual path. Combined with the VPI, it uniquely identifies the connection.

• Payload Type (PT) (3 bits): Indicates the type of payload. For example, it can differentiate between user data, management information, or indicate whether the cell is the last in a sequence of cells.

• Cell Loss Priority (CLP) (1 bit): Indicates the cell's priority in case of network congestion. A CLP of 0 means the cell has a higher priority, while a CLP of 1 means it can be discarded more readily if necessary.

• Header Error Control (HEC) (8 bits): Provides error checking for the header using a cyclic redundancy check (CRC). It helps detect and correct single-bit errors in the header.

 

2. Payload (48 bytes):

The payload is the part of the cell that carries the actual user data. It can be data from different types of applications, such as voice, video, or computer data.

 

Detailed Breakdown of the Header Fields

1. Generic Flow Control (GFC):

   • 4 bits used for local flow control to manage traffic between user equipment and the ATM network. Often set to zero in the core network.

2. Virtual Path Identifier (VPI):

   • 8 bits in UNI, 12 bits in NNI. Helps in the routing of cells through the ATM network by identifying the virtual path.

3. Virtual Channel Identifier (VCI):

   • 16 bits. Identifies the virtual channel within the virtual path for the connection. Used along with the VPI for unique identification of a connection.

4. Payload Type (PT):

   • 3 bits. Indicates the type of information in the payload (e.g., user data, management data, congestion control).

5. Cell Loss Priority (CLP):

   • 1 bit. Indicates the priority of the cell. CLP=0 means higher priority, while CLP=1 means lower priority, allowing for possible discarding under congestion.

6. Header Error Control (HEC):

   • 8 bits. Used for error detection and correction of the header. Employs a polynomial code for error checking.

 

Summary of ATM Cell Structure

Functionality of ATM Cells

• Routing and Switching: The VPI/VCI fields are critical for the routing and switching of cells through the ATM network. They ensure that cells reach their intended destination by following the correct virtual path and channel.

• Error Handling: The HEC field allows for error detection and correction, ensuring reliable data transmission.

• Priority Management: The CLP field helps manage network congestion by prioritizing which cells to drop first if necessary.

• Payload Identification: The PT field helps differentiate between different types of payloads and control information, aiding in the appropriate handling of the data.

 

 

In essence, the fixed size and standardized structure of ATM cells contribute to the efficiency and predictability of ATM networks, supporting a variety of applications with different requirements for bandwidth and quality of service.

 

 

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