IDEAL INDUSTRIES UniPRO MGig1, Руководство пользователя

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152810 UniPRO
Page 16 User Guide
Loopback and Layers
When traffic is looped, the type of loop may need to be determined by the type of traffic and the structure
of the transmission path to be tested. Usually, the Loop type is the same as the Layer type of the traffic –
e.g. Layer 3 traffic is looped at Layer 3.
When (a) the loopback device is connected to a dedicated port, or, (b) when the network segment to
which the loopback unit is connected carries only test traffic, the device can be set to loop all traffic
received. When the network segment carries both test traffic, and non-test traffic that should not be
looped, the loopback unit can be set to only loop traffic that is specifically addressed to it.
Loop types are as follows:
Layer 1 (Physical) Loop. Every bit received is immediately re-transmitted to its source with minimal
delay and exactly as received even when it contains errors. This loop type can only be used over a
physical medium with no active network elements.
Layer 1 (Regenerate) Loop. Errored frames are dropped. Good frames are retransmitted exactly as
received. To determine if a frame contains an error it is necessary to receive and check the complete
frame before it is retransmitted. This introduces at least one frame of delay in the retransmission.
Therefore the loopback operation introduces some latency that must be allowed for in latency
measurements. Furthermore, if the frame size varies (e.g. if the traffic contains multiple streams with
different frame lengths) the latency will vary and this will appear as jitter, which must be allowed for in jitter
measurements. (This also applies to all the higher layer loops.)
Layer 2 (MAC). Good frames are retransmitted, as in L1 (Regenerate), but are altered so that the Source
and Destination MAC addresses are swapped. This makes sure that frames sent to the loopback device’s
MAC address (Destination MAC in the received frame) are returned to the sender’s MAC address (Source
MAC in the received frame). This loop type can be used over a physical connection or over a switched
network using Layer 2 switching.
Fig 17 Layer 2 Loop