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UniPRO
152810
User Guide
Page 15
Ethernet frame structure and Layers
Ethernet frames comprise a payload (the user data) inside a number of layers of control elements. The
higher the layer, the greater the frame’s inherent ability to enable the network to deliver frames to the
correct destination. Each layer adds to the complexity of the frames and to the amount of overhead. This
reduces the space available for the payload.
When the payload is tested with procedures such as BERT, it is recommended to use the lowest layer
frame type that will propagate through the transmission path under test. This makes sure the maximum
payload is available for the test data.
Layer 1
frames begin after a Preamble (PRE) and a Start of Frame Delimiter (SFD) which are used for
frame alignment. These frames are separated from one another by an Inter Frame Gap (IFG). Layer 1
Ethernet frames contain the maximum amount of BERT / Quality of Service (QoS) test data (the Payload
equals the Frame size) but they can only pass through a physical connection, and not through switches
or routers. Layer 1 frames do not have the ability to check frame errors.
Fig 13 Layer 1 Frame
Layer 2
frames have a Destination and Source MAC address added that follows the SFD. The MAC
addresses enable the frames to be sent to a specific destination through Layer 2 switches. Layer 2
frames may also have VLAN and/or MPLS tags added after the source MAC (sometimes this is called
“Layer 2.5”). They end with a Frame Check Sequence (FCS) used to check the frame for errors. The
Layer 2 Payload is everything between the end of the source MAC (or VLAN / MPLS tags if present) and
the start of the FCS.
Fig 14 Layer 2 Frame
Layer 3
frames add a Source and Destination IP address after the source MAC; this enables them to be
sent through Layer 3 switches and routers. The Layer 3 Payload is everything between the end of the IP
header and the start of the FCS.
Fig 15 Layer 3 Frame
Layer 4
frames add a Protocol header (PROT) after the IP header. This defines the transport layer
protocol (TCP / UDP) followed by Destination and Source Port numbers. This allows traffic to be routed to
a particular Port. The Layer 4 Payload is everything between the end of the Layer 4 header and the start
of the FCS.
Fig 16 Layer 4 Frame
Summary of Contents for UniPRO MGig1
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