Introduction to Data Communications
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62c. X.25 Packet Formats

• Call Request - Call connection/disconnection
• Control Packet - Data control
• Data Packet - Information transfer

Call Request Frame

The Call Request Frame is used to initiate and setup the call from one X.25 service to another. After the call is established, the Call Request frame is not used.

Logical Channels

There are 4096 Logical Channels available on a single physical connection to an X.25 network. The Logical Channels are divided into Groups and Channels. There can be 16 groups (4 bits) of 256 channels (8 bits). 16 x 256 = 4096. The Logical Channel Numbers (LCN) are used to identify the connections to the Network.

Type Field:

For Call Request the Type Field is always equal to 0000 1011. Note the last bit is called the Control bit and is set to 1.

Length of Calling/Called Address:

The Length of Calling/Called Address allows different sizes of addresses for other protocols. The standard protocol is IP with an address length of 32 bits.

Calling/Called Address:

Only used during Call Connect until the Virtual Channel is established then the LCN (Logical Channel Numbers) are used to identify the connections.

Facilities Length

Indicates the length of the Facilities Field

Facilities Field

Indicates the types of facilities available. Facilities depend on the Network provider and can include information such as Charges or Call Forwarding.

Once a Call is established, The Calling/Called Addresses and Facilities Fields are not required and the Control Packet Format and Data Packet are used.

Control Frame

The Control Frame is used to control the communications during the call.

X.25 Type field table for all 3 packet types

The Data Frame is used to transfer data between destination and source.

Q

Optional - Q is used to distinguish between data and control information.

D

D controls the type of Acknowledgment:

• 0 - Network Control: DTE to DCE
• 1 - End to End Acknowledgement: DTE to DTE

Modulo

It is used by the sliding window. The modulo allows multiple unacknowledged packets on the network for faster response. The modulo can be Modulo 8 or Modulo 128 (127 packets out on the network)

Piggyback

The Piggyback field is used for Acknowledgement with the Modulus (sliding window)

Sequence

The Sequence field holds a unique packet number which identifies the packet.

More

The More field indicates that more data is coming.

Introduction to Data Communications
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