Indicates the frame format of the interface.
crc4: indicates that the frame format is CRC4.
no-crc4: indicates that the frame format is no-CRC4.
sf: indicates that the frame format is Super Frame.
esf: indicates that the frame format is Extended Super Frame.
By default, the frame format of an E1 interface is no-crc4.
By default, the frame format of a T1 interface is esf.