EXFO ETS-1000 - Manual (Page 103)

Configuring and Starting a Test
Ethernet Tester Analyzer 95
BERT

At the Network layer (Level 3), all data is encapsulated into IP packet,
and then into Ethernet-frame thus allowing to transmit test packets
through a network with both OSI’s second- and third-layer equipment
(for example, network switch, network router).

At the Transport layer (Level 4), test data is encapsulated into an UDP
header, then into IP header and an Ethernet frame thus allowing to
transmit test pattern using transport protocols.