Frame Relay Polling, Error Handling, and Specification Enhancements
We continue our comprehensive look at Frame Relay from Cisco Press' Network Consultants Handbook, complete with illustrations, charts and tables you'll want to keep as a reference.
Network Consultants Handbook - Frame Relay
by Matthew Castelli
SVC X.121/E.164 Addressing
X.121 is a hierarchical addressing scheme that was originally designed to number X.25 nodes. X.121 addresses are up to 14 digits in length and are structured as follows:
- Country Code: 3 digits
The first digit is a zone number that identifies a part of the world. For example, Zone 2 covers Europe and Zone 3 includes North America). The zone numbers can be found in Appendix C, List of ITU-TX.121 Data Country or Geographical Codes. These codes can also be found in ITU-T Recommendation X.121.
- Service Provider: 1 digit
- Terminal Number: Up to 10 digits
- E.164 is a hierarchical global telecommunications numbering plan, similar to the North American Number Plan (NANP). E.164 addresses are up to 15 digits in length and are structured as follows:
- Country Code: 1, 2, or 3 digits
This code is based on the international telephony numbering plan and can be found in Appendix D, International Country Codes. These codes can also be found in any phone book.
- National Destination Code and Subscriber Number: Up to 14 digits in length (maximum length is dependent on the length of the Country Code).
Subaddress: Up to 40 digits
Frame Relay Status Polling
The Frame Relay Customer Premises Equipment (CPE) polls the switch at set intervals to determine the status of both the network and DLCI connections. A Link Integrity Verification (LIV) packet exchange takes place about every 10 seconds, verifying that the connection is still good. The LIV also provides information to the network that the CPE is active, and this status is exported at the other end. Approximately every minute, a Full Status (FS) exchange occurs, passing information regarding which DLCIs are configured and active. Until the first FS exchange occurs, the CPE does not know which DLCIs are active, and as such, no data transfer can take place.