The core aspects of Frame Relay function at the lower two layers of the OSI reference model. The same physical serial connections that support point-to-point environments also support the Frame Relay connection to the service provider. Cisco routers support the following serial connections:
EIA/TIA-232
EIA/TIA-449
V.35
X.21
EIA/TIA-530
Working at the data link layer, Frame Relay encapsulates information from the upper layers of the OSI model. For example, IP traffic would be encapsulated into a frame format that can be transmitted over a Frame Relay link.
A Frame Relay frame contains the following fields:
Opening flag (0x7E).
Address: The address field is two bytes in length and consists of 10 bits representing the
actual circuit identifier and 6 bits of fields related to congestion management.
Data: The data field contains encapsulated upper-layer data.
Frame check sequence (FCS).
Closing flag (0x7E)