SS7 is also known as Common Channel Signaling (CCS). It runs on a separate network that consists of Service Switching Point (SSP), Signal Transfer Point (STP) and Service Control Point (SCP).

While CAS and Out-of-Band Signaling such as PRI are used on the edge of the PSTN network (between the CO switch and the PBX), SS7 is used on the core.

SONET is a layer 2 protocol, similar to ATM, but more efficient. IP over SONET needs to encapsulate IP into PPP first.

ATM network can be built either upon fiber optic directly, or on top of SONET, which allows service providers to provide ATM service through a traditional SONET network.

SONET is purely TDM based. Currently a fiber supports up to OC192 (10Gbps) data transmission rate with a single wavelength. With WDM/DWDM we essentially create multiple SONET rings on a single fiber (up to 128 wavelengths), and therefore multiple ADM is needed. With proper traffic grooming, we can reduce the number of ADMs therefore reduce the cost.

Voice Signaling: Loop-Start; Ground-Start; E&M

Loop-Start; Ground-Start and E&M can be either analog or digital. When they are used as digital signaling, the different state is represented by the A, B, C, D bits of CAS.

The main difference between ground-start and loop-start signaling is that ground-start requires ground detection to occur in both ends of a connection before the tip and ring loop can be closed. This ensures that both ends are ready before the call can start.

E&M does not creat a loop, it uses two separate lines - Ear Lead and Mouth Lead. And the voltage change on the E-Lead or M-Lead indicates the change of state.

More information can be found at:

Voice Signaling and Control

Understanding One-stage and Two-stage Dialing

Understanding DID

SONET/WDM Taffic Grooming

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