|  | 2.4 | WAN Encapsulation Formats
| 2.4.1 | Serial line frame fields
The two most common point-to-point WAN encapsulations are HDLC and PPP. All the serial line encapsulations share a common frame format, which has the following fields, as shown in the Figure: Flag-Indicates the beginning of the frame and is set to the hexadecimal (base 16) pattern 7E. Address-A 1- or 2-byte field to address the end station in multidrop environments. Control-Indicates whether the frame is an information, a supervisory, or an unnumbered type frame. It also contains specific function codes. Data-The encapsulated data. FCS-The frame check sequence (FCS). Flag-The trailing 7E flag identifier. Each WAN connection type uses a Layer 2 protocol to encapsulate traffic while it is crossing the WAN link. To ensure that the correct encapsulation protocol is used, you need to configure the Layer 2 encapsulation type to use for each serial interface on a router. The choice of encapsulation protocol depends on the WAN technology and the communicating equipment. Encapsulation protocols that can be used with the WAN connection types covered in this chapter are PPP and HDLC.