ADTRAN 550 user manual Frame Relay, Virtual Circuits

Appendix B. OSI Model and Frame Relay Technology Overview

Example 2: OSI Model Related to Process of Moving Data Packet

A more typical example of the OSI model involves moving a data packet across an IP network.

Upper Layers E-mail message

Layer 3 Network - IP/IPX

Layer 2 Data Link - Frame Relay/PPP

Layer 1 Physical - T1/DDS

Send Process		Receive Process
Creates a data packet.	Upper Layers	Data packet is processed.
Wraps the data in an IP		The IP wrapper is
packet, specifies the IP		removed and the data is
address of the far end	Layer 3	then passed to the upper
computer, and determines the		layers.
appropriate route.
The IP packet is placed inside		The frame packet is
a frame relay packet with the	Layer 2	unwrapped and the IP
appropriate DLCI and placed		packet is sent to layer 3.
on the correct DS-1.
The frame relay packet is	Layer 1	The frame relay packet is
placed in the appropriate		removed and passed to
DS0s.	LEC and IXC	layer 2.

FRAME RELAY

Frame relay is one of several layer 2 (data link) protocols that transport data across a serial data network. These protocols also include Point-to-Point Pro- tocol (PPP) and High-level Data Link Connection Protocol (HDLC). Frame relay networks are composed of virtual circuits that connect customer loca- tions. To reduce a customer’s overall monthly connection, multiple virtual circuits could be delivered to the customer’s location over a single physical connection.

Virtual Circuits

Virtual circuits can be either permanent (PVC) or switched (SVC). PVC bandwidths are determined when the circuit is ordered from the frame relay provider. PVCs are always active, even when no data is being transmitted. SVC bandwidths are created and used only when needed and allow for ne-

61200305L1-1 ATLAS 550 User Manual B-3