Frame Relay Service Architecture

The Frame Relay service architecture is defined as two planes: the C-plane (control) and U-plane (user). Both planes are governed by standards, such as ANSI T1.601.^[2] Figure 14-1 shows the relationship between the two planes. The Frame Relay and frame switching concepts are shown in Figure 14-2 and Figure 14-3.

Figure 14-1. C-Plane and U-Plane in Frame Relay Using the ANSI UNI Protocol

Figure 14-2. Frame Relay Concept

Figure 14-3. Frame Switching Concept

The C-plane uses the Q.921 and Q.931 protocols, and the U-plane deals with the 64-kbps B channel, the 16- or 64-kbps D channel, or the H channel (384, 1472, or 1536 kbps). The main interfaces used today are the following:

• ITU-T V.35 A full duplex interface that typically operates at 56 kbps

• ANSI T1.403 A metallic interface between carriers and the customer premises equipment (CPE), operating at DS1 or 1.544 Mbps rates

The current version of FRF.1.1^[3] adds the following high-speed interfaces:

• ANSI/EIA/TIA-530-A-1992 An electrically balanced interface, operating at data rates below 2.1 Mbps, using a 25-pin connector

• ANSI/EIA/TIA-613-A-1993 Cisco's High-Speed Serial Interface (HSSI), operating at up to 53 Mbps, using a 50-pin connector

• ANSI t1.107a The DS3 interface, operating at 33,736 Mbps

• ITU G.703 The E3 interface, operating at 34.368 Mbps

• ITUV36/V.37 An interface operating at 2-10 Mbps, using a 25-pin connector

From the point of view of a troubleshooting engineer, it is good to know that the most commonly available Frame Relay service is the PVC service with typically 56 kbps, or incremented fractions of 64 kbps, such as 64, 128, 384, up to 1544 kbps. Frame Relay supports switched virtual circuit (SVC) and PVC, but SVC is not commonly deployed in Frame Relay.