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SS7 - Message Transfer Part Layers
The Message Transfer Part (MTP) is divided into three levels:
MTP Level 1
MTP Level 1 defines the physical, electrical, and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).
Physical, defines the physical interface (bearer channel, HDLC framing)
Difference between E1 and Link(or SS7 Link)
Above Diagram shows E1 Framing. Most of new users are confused between difference between E1 and Link. An "E1" is a physical connection between two Signalling Nodes which has 32 channels of 64kps each(making it total 2 Mbps line). All of these Channels either carry Data (Call Control Information for Calls or Data payload like SMS,USSD etc.) or Voice. Channels which Carry data are called Links. For example,There can be 4 E1s between MSC and an IVR service node but single Link on First E1.
Following are some quick points about E1 :
a. E1 carrys both VOICE Channels(called Trunks) and Signalling
b. Each E1 can have maximum 30 Voice trunks
c. Signalling Channel for Call Control can be shared between E1s. Each Signalling Channel can handle upto 1024 Voice Trunks
d. Each E1 can theoratically carry maximum 30 Signalling Links though practically maximum of 16 links can be carried over(as SLS field is of 4 bits, covered in detail in coming sections)
e. Default timeslot(or channel) which carry Call Control Information is 16th Timeslot.
f. Timeslot 0 on an E1 is reserved for Framing/Synchronization
E1 Cable Layout Following diagram shows cable layout of an E1 table. Two wires are used for Rx and two are used for Tx
