Principles of Voice & Data
Lesson 5:
Analog and Digital Signaling
Bellevue Community College
Bob Young, Instructor
Signaling Categories
• Supervision
• Addressing
• Alerting
Let's look at each of these three
categories in more detail...
Signaling Categories:
Supervision
• Detects changes to the line
–On hook
–Off hook
–Call transfer
–et cetera
Signaling Categories:
Addressing
• Responsible for routing functions
– Passes analog or digital routing
information to the correct location to
establish the connection
– What are some analog routing signals?
– What are some digital routing signals?
Signaling Categories:
Alerting
• Signals intended for the user
– often audible
– ring
– busy
– call waiting
– caller ID (inaudible)
– et cetera
In-Band & Out-of-Band
Signaling
• In-band signaling shares the
same circuit or path as the
voice or data
• Out-of-band signaling uses a
separate path
In-Band Signaling
• Also called "Channel
Associated Signaling," or CAS
In-Band Signaling
• Standard for POTS lines
• Standard for T1 circuits
• Several inherent problems:
–slow
–susceptible to line noise
–security issues
Out-of-Band Signaling
• Also called "Common Channel
Signaling," or CCS
Out-of-Band Signaling
• Standard between Central Offices
• Standard for ISDN
• Several advantages:
– faster
– more reliable
– more secure
E & M Signaling
• E & M Signaling is (was) used between
Central Offices on trunks
• It is (was) also used to connect DID
trunks to PBX's
E & M Signaling
To prevent glare on E & M
trunks:
Use wink-start signaling!
Wink-Start Signaling
• A momentary change in line status
sent as an acknowledgement.
– Momentary loop current ("off-hook")
– Momentary battery reversal
– Momentary ground
Glare
• "Glare" is the condition that
occurs when both ends of the
same trunk are seized by two
different switches for two
different calls.
2-Wire E&M
Voice
Voice
Voice
Voice
E-Lead
E-Lead
M-Lead
M-Lead
4-Wire E&M
TX Voice
TX Voice
RX Voice
RX Voice
RX Voice
RX Voice
TX Voice
TX Voice
E-Lead
E-Lead
M-Lead
M-Lead
E&M Signaling Types
Signal E Lead
E Lead M Lead M Lead OffType On-Hook Off-Hook On-Hook
Hook
I
Open
Ground
Ground
Battery
II
Open
Ground
Open
Battery
III
Open
Ground
Ground Loop Current
IV
Open
Ground
Open
Ground
V
Open
Ground
Open
Ground
SSDC5
Open
Ground
Open
Ground
Ground Start Signaling
Like E & M Signaling,
• Ground Start Signaling is (was)
used between Central Offices
on trunks
• It is (was) also used to connect
DID trunks to PBX's
Ground Start Signaling
Primary advantage/reason for
Ground Start Signaling:
Prevents glare!
Ground Start Signaling
• To initiate a call, a ground is applied to
the ring wire momentarily
• The far end responds by grounding the
tip momentarily
• Start loop current, send DTMF or MF
T1's and In-Band Signaling
• Channelized T1
–24 DS0's
–64 kbps channels
–Minus 8 kbps for signaling
–Leaves 56 kbps per DS0
T1's and In-Band Signaling
• Unchannelized T1
–One complete data "pipe"
–24 x 64 kbps = 1.536 Mbps
T1's and In-Band Signaling
• Bit robbing
• Superframe (SF) robs 2 bits
–A&B
• Extended Superframe (ESF) robs 4 bits
– A, B, C, & D
• Indicate on-hook and off-hook for both
ends of the circuit
Uses for A & B Bits
• Used for supervision in:
–Digital E&M circuits
–Digital loop start circuits
–Digital ground start circuits
–Digital OPX circuits
ISDN
• Integrated Services Digital Network
• "I Smell Dollars Now!"
• "It Still Does Nothing!"
ISDN
• All digital, from subscriber
premise to Central Office
• Uses out-of-band signaling
• Also called "common channel
signaling (CCS)
Channel Structure
Bearer Service -- 64 Kbs
Data Channel -- 16 Kbs
Overhead -- 48 Kbs
Service Levels
• Basic Rate Interface (BRI)
– 2 Bearer Services plus 1 Data Channel
– (2B+D)
– Linking (Bonding) of Bearer Services
• Primary Rate Interface (PRI)
– 23 Bearer Services plus 1 Data Channel
– (23B+D)
– Linking (Bonding) of Bearer Services
Let's Do the Math…
• 2B+D = (2 x 64) + 16 = 144 Kbs
• With overhead:
2B+D+O = (2 x 64) + 16 + 48 =
192 Kbs
The Data Channel
At the PRI level, a single
Data Channel can support
multiple "T1's" with 24
Bearer Services each.
PRI Bearer Services
• For example:
• 3 "T1's"
24 + 24 +23 B channels
1 D channel
"Parts" and Service
• ISUP
– Integrated Services User Part
• SCCP
– Signaling Connection Control Part
• TCAP
–Transaction Capability
H Channels
• Whenever ISDN B
channels are
combined, they are
called H Channels
ISDN H Channels
Channel
Name
H0
H10
H11
H12
Speed
(Kbps)
384
1,472
1,536
1,920
B
Channels
6
23
24
30
ISDN in the UK
• UK: ISDN 2e equivalent to US BRI
–Bearer channels are called echannels
–Multiple Subscriber Numbering
(MSN) service is used to provide
voice and data signaling services
ISDN in the UK
• ISDN 30 equivalent to US PRI
• ISDN 30 (ADS 2) provides up to 30
bearer channels
– Not compatible with Europe
• ISDN 30 (I.421) provides up to 30
bearer channels
– Compatible with Europe
ISDN Protocols
• I.430
• Q.921
• Q.931
• Q.932
• Let's examine these...
ISDN Protocols: I.430
• Defines the reference points
between the CO and the
terminal equipment (user's
equipment)
ISDN Protocols: I.430
Reference Points
• U Interface: 2 wire connection,
terminated by an RJ-11
• S/T Interface: 4 wire connection,
terminated by an RJ-45
• Network Termination Equipment (NTE)
– Converts U Interface to S/T Interface
• Terminal Adaptor: connects multiple
devices to the S/T Interface
ISDN Protocols: Q.921
• This protocol is specific to ISDN
• Also known as LAP-D
LAP-D (Q.921)
• Stands for, "Link Access
Procedure - D Channel"
• Part of the Q.921 Protocol
• Used to control connections
LAP-D (Q.921)
• Operates solely in the ISDN
D-channel
• Provides flow control
• Provides data reliability
ISDN Protocols: Q.931
• This protocol is used to establish and
end sessions (voice or data)
in other words, it is for:
• Establishing connections
• Terminating connections
Q.931 Message Types
Message
Setup
Call
Proceeding
Alerting
Name
SETUP
Description
Begins data transmission.
Message returned to call
CALL PROC originator.
Equivalent to "ringback."
Sent from call receiver to
ALERT
caller.
A connection between a
Terminal Adaptor and a CO
CONNECT has been established.
Connect
Connect
Acknowledge CONNECT
ment
ACK
Far-end acknowledgement
to a CONNECT message
Q.931Session Termination
Caller B
Caller A
Step 1
On hook - send DISCONNECT message
Step 2
Receive DISCONNECT message
Send RELEASE message
Step 3
Send RELEASE COMP message
Send RELEASE COMP message
ISDN Protocols: Q.932
• Used to modify a call (session)
in progress
–Transfer a call
–Place a call on hold
–etc.
PISN
Private Integrated Services
Network
Connects standard telco services
AND
LAN and data services
PINX
• Private Integrated Services Network
Exchange
• Also called an Integrated Services PBX
• Core element in a PISN
PISN Advantages
• Multiple vendors can be mixed
• Multiple communications protocols
• Multiple topologies
– star, token ring, etc.
• Multiple technologies
– Ethernet, wireless, satellite
• VPN capabilities on public networks
PISN: Two Main Elements
• End Nodes
– VoIP Phones
– Computer Work Stations
– Servers
• Transport Nodes
– PBX
– Router
PISN Signaling Methods
• Digital Private Network Signaling
Scheme (DPNSS)
– Primarily in the UK
– Being replaced by QSIG
Private Integrated
Services Network
Q&C
Reference Points
Q Reference Point
• The Q Reference Point is
the LOGICAL interface
between the local PINX
and the remote PINX.
C Reference Point
• The C Reference Point is
the PHYSICAL interface
between the local PINX
and the remote PINX.
Mapping
• Q and C reference points must
be mapped to each other
• This mapping may be static or
dynamic
On-Demand/Dynamic
Mapping
• The connection is established
on a per-call (per-session) basis
• Generally done with TCP
(Transmission Control Protocol)
Permanent Mapping
• Establishes a continuous
connection between the logical
Q Reference Point and the
physical C Reference Point
• Example: a CCS (Common
Channel Signaling) data circuit
C Reference Point
• Physical line testing
• Network measuring and
monitoring
SS7
•
•
•
•
Signaling System 7
A kind of common channel signaling
The backbone of the PSTN
The reason wireless phones can roam
– IS-41, now called TIA-41
SS7 and ISDN
•
•
•
•
•
They are not the same thing
SS7 is a background protocol
ISDN is used by the customer directly
Both use common channel signaling
ISUP (ISDN User Part) of SS7 allows
ISDN to use SS7 for call setup and
routing
Connecting to the PBX
• Automatic Route Selection
– Uses a static routing table
• Least Cost Routing
– Looks at the available options and chooses
the least expensive routing option
PBX Trunks
• AC15A: long distance, diverse power
sources
• AC15B: four-wire trunk
• AC15C: OPX trunk
• AC15D: for international trunks
PBX Architecture
•
•
•
•
CPU
Operating System
Line and Trunk Interface Cards
Power Supply
CTI
• Computer Telephony Integration
– The voice network and the data
network are starting to become one
and the same network
The unPBX
A PC functioning as a PBX
Special Feature:
API
Application Programming
Interface
ACD
• Automatic Call Distribution
–The ability to route calls to the
next available operator, or sales
rep, etc.
Why Packet-Switched Voice
and Data?
• More "bulletproof" than circuitswitched
• Can be routed across various
media - cable, fiber, copper,
satellite...
• Greater versatility and functionality
• More efficient use of the media
REN
• Ringer Equivalency Number
• Add up all the REN's that are connected
to the same circuit
• The number shouldn't be greater than 5.
The End
• Have a great day!
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