Chapter 11: Signaling Systems
Overview
This chapter covers signaling in circuit-switched networks. It explains how signals are used as an encoding method required to supervise the connection and to relay addressing signals.
Signals travel between controllers either over the talking path or over separate date networks. Telecommunications networks have a variety of methods for setting up and
taking down circuit connections,
such as signaling. They are not carried through to the LECs and IXCs. Can be
classified by the method of exchange signals:
·
Direct current
(dc)- used in local loops and tie lines
·
Tone- local
loops and tie lines
·
Bit-robbed-
hybrid in-band system, that is neither tone nor direct current signaling
·
Common channel.
There
are 2 types of signaling:
In-Band:
signals are carried over the same circuit that carries formation during the
session.
·
Has drawbacks,
such as fraud; toll thieves are able to defeat automatic message accounting
systems by using devices that emulate signaling tones.
Out-of-Band: uses a
separate network to carry the signals. Two examples:
·
D channel of
ISDN (see Chapter 32 for elaboration)
·
Signaling System
7 (SS7)- separate common channel signaling network; this eliminates toll fraud.
Signals can be grouped into four functions:
·
Supervising
- monitoring the status of a line or circuit to determine if it is busy, idle,
or is
requesting service.
·
Alerting
- indicates to the addressee the arrival of an incoming call; audible bells
and tones or visual lights.
·
Call Progress Tones -
informs the user of the status of the call setup process; eg.: busy signal.
·
Addressing - the process
of transmitting route and destination signals over the network; such as dial
pulses, tone pulses, or data pulses over loops, trunks, and signaling networks.
In-Band Signaling
·
Line busy or
idle; Central Office (CO) monitors constantly the status
·
When you lift
receiver, it signals CO intent to make a call
·
CO marks the
line busy and sends back a dial tone
·
Caller dials
the number and CO transmits the dual-tone multi-frequency (DTMF) or dial pulses
·
System translates
digits and sends them to the terminating end thus producing a ring
·
The process
of setting up the talking path determines whether the path is operative since
audible signals are passed over the channel that will be used for talking.
·
Are limited
to transferring call setup and supervision information, and are therefore incapable
of supporting virtual voice networks.
·
Call setup
consumes circuit time (takes several seconds to setup), which is important because
it represents a direct expense in the access charges paid to the LECs.
Common Channel Signaling (Out of Band)
Replaces the In-Band Signaling
and uses a separate data communications network to exchange signals and route
the calls.
·
SS7 is the
international common channel signaling system.
·
Service switching
points (SSPs): Software applications running in the central offices, and are
linked via data circuits to
·
Signal transfer
points (STPs): Network nodes that act as hubs for signaling messages.
·
STPs are linked
to service control points (SCPs), which contain a database of network information
that can be accessed by the network nodes.
·
Each stage
of the call, ringing, connect, and disconnect, is signaled with a data message.
·
It is so fast,
the circuit connection can wait until the called party answers, further increasing
circuit utilization.
·
Important to
the LECs’ market strategies; enables them to receive more revenue from CLASS.
·
Custom Local
Area Signaling Services (CLASS): Uses a separate packet-switched network to
pass call setup, charging, and supervision information.
·
Can access
the carrier’s database to obtain account information, such as features and points
served, on a virtual network.
·
Is required
for local number portability - take your previous phone number to your new location.
Signaling on interoffice circuits uses two leads that are both used to communicate status to attached central office equipment:
Signaling equipment converts the binary state of line signals:
Can be employed on metallic
facilities (most subscriber loops and voice frequency interoffice trunks). Not
Mandatory.
·
Loop Start=
when the caller takes the telephone receiver off the hook, closing a direct
current path between tip and ring and allowing loop current to flow; all subscriber
loops that terminate in station sets use loop start signaling.
·
Glare=When
a party picks up the phone to find that there is someone there without it ringing.
·
No problem
for the callers, but a problem in the trunks, so use one-way signaling
·
Most PBXs have
two-way trunks so they are subject to glare.
·
Cannot use
loop-start two-way trunks because the only indication the PBX has of a call
incoming from the central office is the ringing signals.
Single-Frequency Signaling (SF)
2600-Hz Single-Frequency
is the most common analog trunk signaling system.
·
Voice frequency
leads from a carrier channel connect directly to the SF set.
·
Contains circuitry
to change the state of the E & M leads in response to the presence of absence
of the SF tone and to turn the signaling tone on and off when the switching
system or other central office equipment changes the status of its leads.
·
Talk-Off= hazard
of SF; can occur when the user’s voice contains enough 2600-Hz energy to actuate
the tone-detecting circuits in the SF set.
·
Voice filters
minimizes this, but not for people with high pitched voices.
·
Travel over
the voice path
·
Transmitted
over trunks as DTMF, dial-pulse, or multi-frequency signals, or over a common
channel as a data signal.
Uses:
·
Dial pulse
= requires from 1-10 pulses plus an inter-digital interval.
·
DTMF signals
= requires a DTMF receiver in the central office to convert the tones to the
addressing signals.
Improves call management;
handles call setup and disconnect; handles end-to-end call supervision, and
call timing and billing.
·
Enhances network
management
·
Separates network
control from hardware; a control system independent of the circuits and transmission
equipment that make up the network.
·
Supports Integrated
Service Digital network (ISDN)
·
Supports user
database; in the form of virtual networks
·
Handles addressing
and supervision
·
Supports 800
number portability; when dialed, a SS7 message to a central database returns
with the identity of the IXC.
·
Supports local
number portability.
Protocol
Resembles the OSI model,
but has 4 layers instead of 7.
·
Message Transfer
Part (MTP)= first of three layers and is a data-gram service, which means it
relays unacknowledged packets.
·
Signaling connection
control part (SCCP)= the fourth layer and it is responsible for addressing requests
to the appropriate application and for determining the status of the application.
Functions
·
The signaling
data link is the physical layer; provides physical links between network nodes.
·
The signaling
link layer is a data link that has three functions: flow control, error correction,
and delivering packets in the proper sequence.
·
Signaling network
layer routes messages from source to destination and from the lower levels to
the user part of the protocol; enables it to handle link failures and to route
messages based on their logical address.
Uses all in-band signaling
and Selective Signaling, which is an in-band system for operation of certain
private line switching systems. Special dedicated circuits require signaling
arrangements that use the same techniques and equipment as the telephone network,
but have no direct counterparts in switched systems, ex.:
·
No signaling-
some private lines require no signaling.
·
Ring down circuits-
generator rings the bell of a distant station.
·
Selective signaling-
some private line networks use 4-wire selective signaling system to route calls
without the use of switching systems.
Many private networks use tie trunks without-of-band signaling links.
Coin telephones are owned by the LECs and are generated in the telephone to enable the operator to distinguish between coin denominations. They are privatly owned are not connected to coin control. They use the dialing and ringing signals of ordinary telephones plus direct current signals that operate apparatus within the telephone to collect and return coins.
Few published standards exist
on the carrier single-frequency signaling system. Internationally, most carriers
use the ITU nos. 6 and 7 standards for common channel signaling and ITU nos.
4 and 5 line-signaling systems.
·
These standards
are of little concern to users because the IXCs administer them.
Most economical configuration
in digital systems is the built-in equipment of the T carrier channel. Circuits
either have:
·
End-to-End
signaling= the easiest to test
·
Link-by-Link
signaling= the signals are extracted at intermediate points and connected by
a pulse link repeater, such as PBX equipment furnished by the user.
Signaling compatibility is
rarely a problem and testing capability is important.
·
Circuit design
is essential before selecting signaling equipment.
·
Where built-in
signaling is not included as part of the channel unit, external signaling converters
are required.
Signaling compatibility is an important consideration in acquiring signaling equipment.
The timing of signals, which is controlled by switching and transmission equipment connected to signaling sets, is a frequent cause of incompatibility, but this has little to do with the signaling equipment itself.
Key Points
Switched connections over the telecommunications network involve some analog signaling; even though most of the connections is digital, the subscriber loop remains analog and requires analog signals.
Today’s system works like this: If a station is busy, the signal stops at the end office, but if you have call waiting, the call in progress is interrupted. ISDN provides a separate out-of-bank signaling channel that allows the network to send alerting messages to the user, where they can display on the phone.
· When local networks are converted to ISDN, analog signaling is eliminated.
· With analog trunks the tone operates auxiliary single frequency (SF) signaling sets that show the line status by changing the status of direct current voltages on their signaling leads.
· SF signaling have been replace in nearly all circuits because of toll fraud and wasted circuit time during call setup.
· Signals pass among IXC, LEC, and users as data messages.