ADTRAN Atlas 550 - Manual (Page 215)

Appendix B. OSI Model and Frame Relay Technology Overview
61200305L1-1 ATLAS 550 User Manual B-3
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.
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-
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
packet, specifies the IP
address of the far end
computer, and determines the
appropriate route.
Layer 3
The IP wrapper is
removed and the data is
then passed to the upper
layers.
The IP packet is placed inside
a frame relay packet with the
appropriate DLCI and placed
on the correct DS-1.
Layer 2
The frame packet is
unwrapped and the IP
packet is sent to layer 3.
The frame relay packet is
placed in the appropriate
DS0s. Layer 1
LEC and IXC
The frame relay packet is
removed and passed to
layer 2.