Signaling CAS & CCS

        Signaling 

In today’s world Two types of signaling is used:-

1.        CAS – Channel
   Associated Signaling
2.        CCS – Common
   Channel Signaling

 

In CAS,signaling bits are associated inside the voice channel i.e.
no dedicated channel for signaling also called in band signaling.

T – Carrier CAS

 

T1 in CAS à
24 voice channel (1.544Mbps)

 

Basic single channel rate is 64 kbps also called DS0. (as per
sampling theorem sampling rate should be twice the maximum frequency of the
signal. We know that human voice range is 4Hz – 4kHz of frequency so to
reproduce the signal we must sample it twice i.e. 2*4KHz=8KHz=8000  sample/sec
& each channel is of 8 bits so channel rate is 8000*8=64Kbps)

So, T1 contains 24 DS0 voice channel in CAS. but all channel
is used for voice so  how can we obtain
the singling. Here is the concept CAS in T1 use every last bit (Least Significant
Digit)of each channel of every 6^th frame for signaling (not total
channel but only a bit).this is known as robbed bit signaling or RBS.

fig1 T1 CAS

Note one thing here ,we know that T1 is of 1.544 Mbps.

lets calculate 24ch*8bits=192bits

192bits*8000samples = 1536 Kbps=1.536 Mbps so how come its 1.544 Mbps.

here, is the conclusion we use 1 bit for framing at the end of each frame so, its 192+1=193bits and 193bits*8000samples=1544 Kbps=1.544 Mbps which is our T1 Line.

8000 samples means each frame is repleted 8000 times to obtain 1.544Mbps T1 line.T1 CAS uses two types of framing :-

Super Frame sends 12 frame at a time.in SF we get 2 bits A,B for signaling

Extended Super Frame 24 frame at a time.in ESF we get 4 nits A,B,C,D for signaling.

The meaning of these bits depends on what type of signaling is used on the channel.The most common types of signaling are loop start,ground start and E&M.

Loop Start

Ground Start

E&M

E – Carrier CAS

 

E1 in CAS à  30 voice channel + 2  dedicated signalling channel (2.048 Mbps)E1 CAS has :-

• CH1 dedicated channel for framing
• CH17 for signaling
• CH2-16 & CH18-32 are dedicated for voice

why is it called CAS even if it uses dedicated channel for signaling ? answer is E1 CAS uses same signaling type as that of T1 CAS as shown in below fig.

so, E1 CAS is compatible with T1 CAS.one can able to convert E1 to T1 or vice-versa.

T – Carrier CCS & E – Carrier CCS

T1 in CCS à  23 voice channel + 1 signaling channel (1.544Mbps)

T1 in CCS uses 1 dedicated channel for signaling which operates at 64Kbps.

E1 in CCS à  30 voice channel + 2  dedicated signalling channel (2.048 Mbps)

E1 in CCS uses 1 dedicated channel for signaling(ch 17 in our diagram above) & 1 dedicated for sync purpose ch 1 (which is neither bearer(voice) ch nor data ch(signaling))

so, what is new in CCS ? answer is instead of using bits (A,B,C,D) CCS uses packets for exchanging short messages for signaling i.e it uses protocol for communication on signaling channel (64kbps)

some of the available protocols are ISDN & SS7(signaling system 7).

note here applications/protocols such as SS7 have the flexibility to define any of channel as signaling channel.