| MTP
level 2 frames |
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| MSU
(Message Signal Unit): |
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| Contains
signalling messages (User Part
SIO) |
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| The received
frame is MSU if LI > 2 (number of |
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| octets) |
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| LSSU
(Link Status Signal Unit): |
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| Contains
signalling messages for link supervision |
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| The received
frame is LSSU if LI = 1 or 2 |
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| FISU
(Fill-In Signal Unit): |
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| Can be used to
monitor quality of signalling link |
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| The received
frame is FISU if LI = 0 |
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