| MTP
level 2 frame formats |
| Level 3
signalling message |
| MSU (Message Signal Unit) |
| F |
| CK |
| SIF |
| SIO |
| LI |
| Control |
| F |
|
|
|
|
|
|
|
|
| Network: |
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| National |
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| International |
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| User part: |
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| TUP |
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| ISUP |
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| SCCP |
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| Network |
|||||||
| management |
|||||||
| LSSU (Link Status Signal
Unit) |
| F |
| CK |
| SF |
| LI |
| Control |
| F |
| FISU (Fill-In Signal
Unit) |
| F |
| CK |
| LI |
| Control |
| F |