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Understanding ATM Concepts
300017-A Rev. 00
B-3
The fields in each ATM cell header provide all the information necessary for
networking. These fields include the following:
• Generic Flow Control (GFC): The first 4 bits of the cell header contain the
GFC. The GFC controls traffic flow onto the ATM network by controlling the
user-network interface (UNI).
• Virtual Path Identifier (VPI): The next 8 bits of the cell header (that is, the
last half of byte 1 and the first half of byte 2) contain the VPI. The VPI
specifies a virtual path on the physical ATM link. See the next section, “AT M
Data Transmission,” for additional information on virtual paths.
• Virtual Channel Identifier (VCI): The next 16 bits of the cell header (that is,
the last half of byte 2, all of byte 3, and the first half of byte 4) contain the
VCI. The VCI specifies a virtual channel within the virtual path on the
physical ATM link. See the next section, “ATM Data Transmission
,” for
additional information on virtual channels.
• Payload Type (PT): The next 3 bits (that is, bits 5 to 7 of byte 4) indicate the
type of information the cell is carrying (for example, user data or management
information).
• Cell Loss Priority (CLP): The last bit of byte 4 indicates the priority of the
cell and whether the network can discard it under heavy traffic conditions.
Setting the bit to 1 indicates that the network may discard the cell if necessary.
• Header Error Control (HEC): The last byte of the header field contains the
HEC. Its primary function is to guard against misdelivery of cells due to
header or single bit errors. However, the HEC does not gauge the quality of
the data in the information field.
ATM Data Transmission
Data transmission (also called cell switching) through the ATM network relies on
the establishment of logical connections between ATM entities. ATM is a
connection-oriented service. This means that an ATM entity cannot transmit
information until it establishes a connection with a receiving entity. These
connections consist of virtual channels, virtual paths, and transmission paths.
- Configuring ATM 1
- Half-Bridge Services 1
- Trademarks 2
- Restricted Rights Legend 2
- Statement of Conditions 2
- Contents 5
- 300017-A Rev. 00 11
- About This Guide 13
- Conventions 14
- Acronyms 15
- Bay Networks Customer Service 17
- How to Get Help 17
- ATM Half-Bridge Overview 19
- Chapter 1 19
- How the AHB Router Works 22
- DHCP server 24
- Using Source-Based Routing 28
- Chapter 2 31
- Starting AHB Services 31
- Caution: 32
- (continued) 34
- Configuring AHB Services 35
- Creating an ATM Interface 36
- Chapter 3 45
- Customizing AHB Services 45
- Specifying the Debug Level 50
- Modifying AHB Circuits 52
- Deleting AHB from the Router 55
- Appendix A 57
- AHB, BootP/DHCP, IP, and ATM 57
- Parameter Descriptions 57
- AHB Global Parameters 58
- AHB Circuit Parameters 62
- BootP Address Parameters 66
- IP Configuration Parameters 71
- Appendix B 81
- Understanding ATM Concepts 81
- Figure B-1. ATM Cell 82
- ATM Data Transmission 83
- ATM0006B 84
- Data Encapsulation 85
- PVC Access Methods 86
- Multiple PVCs 87
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