Ernie's
CompTIA Convergence+ /
Avaya ACA Notes
Erlang
- A unit in telephony used to measure load.
Examples include the number of call center agents and the number of telephone
lines required given certain parameters such as average calls per hour and
average length of each call.
Erlang
is named after
Agner
Krarup Erlang who invented
Teletraffic Engineering and
Queueing
Theory.
A bit
of humor (funny to Ernie anyway): Erlang proved in his 1909 publication: "The
Theory of Probabilities and Telephone Conversations (Pages131-137)" that the
"Poisson
distribution" (part of
probability theory)
applies to random telephone traffic.
The funny part is: Examples that are well-modeled as
Poisson distribution processes include the
radioactive decay of atoms, telephone calls arriving at a switchboard, page
view requests to a website, and rainfall!
James Burke would be proud of the "Connections!"
Traditional Switched Circuit Voice Operation
Switch Topology
In the USA 1974 saw an anti-trust suit brought against AT&T due to its very
large user base of telephone switches giving it an overwhelming monopoly
within the voice provision market. The Modified Final Judgement in 1982 by
Judge Greene (referred to as the
Bell System Divestiture) resulted in AT&T having its local call access
given to seven Regional Holding Companies which were nick-named 'Baby
Bells'. AT&T kept its manufacturing businesses and its long distance
services.
The structure for telephone provides now means that local telephony services
are provided by a Local Exchange Carrier (LEC). The LEC is however
restricted in its operation to within its Local Access Transport Area
(LATA). Calls between LATAs have to be handled by an Inter Exchange
Carrier (IEC or IXC). There are 200 LATAs within the US.
The services that are offered by telephone companies include Plain Old
Telephone Service (POTS) and Custom Local Area Signalling Services
(CLASS) which enhances POTS by providing call screening, security and
display features. Also available is Advanced Intelligent Networking
which brings the CLASS-type features back into a centralized database,
examples include Centrex that provides a virtual PBX with most of the
features of a PBX being supplied by the CO.
Tone Dialing - Now more commonly used is the Dual Tone
Multi-Frequency (DTMF) method that uses the concept of the keypad where each
key position is represented by two tones.
Each row is assigned a different low frequency whilst each column is assigned a
different high frequency.
- Informational Signaling - The following tones are used to
describe the call progress:
- Dial Tone - (Continuous 350Hz + 440Hz) indicates that the the
switch is ready to receive digits.
- Busy Tone - (480Hz + 620Hz, 0.5s on and 0.5s off) indicates
that the other end is busy.
- Line Ring Back - (440Hz + 480Hz, 2s on and 4s off) means that
the telephone company is in the process of completing a call on behalf
of the caller.
- PBX Ring Back - (440Hz + 480Hz, 1s on and 3s off) means that
the switch is in the process of completing a call on behalf of the
caller.
- Congestion - (480Hz + 620Hz, 0.2s on and 0.3s off) means that
there is congestion in the network along the path so that the call
cannot be set up.
- Reorder - (480Hz + 620Hz, 0.3s on and 0.2s off) means that
all the circuits are busy on the local switch.
- Receiver Off Hook - (1400Hz + 2060Hz + 2450Hz + 2600Hz, 0.1s
on and 0.1s off) means that the other end has left the receiver off the
hook.
- No Such Number - (Continuous 200Hz + 400Hz) means that the
dialed number does not exist.
- Confirmation Tone - (Noise at a frequency of 1Hz sounds like
a slow rasping noise) means that the call setup is being attempted.
Analogue Signaling
Loop-Start
Domestic and small office telephones are connected to the PSTN CO switch via
a pair of wires called the Local Loop. The signaling used in this
situation is called Loop-Start and Loop-Disconnect Signaling.
Loop-Start is the most common form of signaling in the analogue environment
and it provides the following services:
- Public Telephone Service (PTS)
- Manual or Automatic data service
- Message Telecommunications Service (MTS)
- Attendant call service on a manual PBX.
- One-way incoming service to an attendant or Automatic Call Distribution
(ACD) service
One wire of the local loop is called the Tip which is connected to
ground. and the other wire is called Ring which is connected to the
negative side of the 48v DC Battery. Picking up the telephone handset
takes it Off Hook and makes a connection on these wires thereby
allowing current to flow. The switch sends a dial tone to the receiver of
the phone that has gone off hook, thereby informing the caller that the
switch is ready to receive dial digits. The digits are either sent via
pulses or via DTMF dial tones. The bell is always connected to the switch
however a capacitor prevents the DC current flow from the battery in the
switch. When dialing occurs the remote end is notified by the AC ringing
voltage applied at between 20 - 47Hz (The traditional operation of the
telephone was described earlier).
Telephone Call Operation
Using Loop-Start signalling, traditional telephone systems operate more or
less as follows:
- The telephone starts off by being On Hook (Idle).
- The caller lifts the handset, this is called going Off Hook and
tells the switch that you wish to make a call, the telephone 'seizes' the
line.
- The initial electrical circuit is set up because the by going Off Hook
the circuit is made and the battery can send current. The CO switch now
knows that a call is being requested and acknowledge the seizure of the line
by 'Winking' the circuit.
- The telephone switch either public or a Private Branch Exchange (PBX)
returns a dial tone (2500Hz in the UK) which informs the caller that the
switch is ready to receive dialed digits.
- The number to be called is dialed.
- In a private organization if it is an external call then the PBX makes a
routing decision and using network signalling setup messages, requests a
64Kbps slot in the trunk link to the Central Office (CO) e.g. E1 or
T1.
- The CO sets up a path based on the number, it does this by 'seizing' a
circuit and sending a request to the destination PBX.
- The PBX at the other end learns of the call.
- The PBX at the other end sets up an AC voltage (20 - 47Hz) for the
ringing of the remote telephone.
- The local PBX sends a ringback tone to the caller to inform them that
the phone is ringing at the other end.
- The telephone handset is picked up and the loop is established local to
the called party.
- The ringing voltage and ringback tones are removed from the circuit.
- Acoustic couplers in the phones convert the speech into modulating
current that is transmitted end-to-end.
- Part of the signal is fed back into the talking person's earpiece. This
is called Sidetone and is a comfort signal.
Release signals can vary from switch manufacturer to switch manufacturer.
Some switches are able to measure the time from going off hook until the
first digit is dialed. If this exceeds a pre-defined time limit then the
loop may be connected to an announcement and/or a Receiver Off Hook (ROH)
tone.
The signalling between the subscriber switches and the telephony service
providers can be identified as follows:
- Supervisory Signalling - electrical voltages and tones that can
be heard are used to signify call status as follows:
- On-hook - produces an open circuit which does not allow any
signalling, only the ringer can operate.
- Off-hook - lifting the handset closes the circuit and allows
the telephone switch to send an audible dial tone to the receiver.
- Ringing - the switch sends a ringing voltage to the
destination telephone as notification of an incoming call. Also an
audible ringing tone is sent to the caller telephone to indicate that
the call is progressing. This tone takes the form of a pattern called
Cadence In Europe this Cadence takes the form of a double ring
(duration of 0.4s separated by 0.2s) followed by two seconds of silence,
whereas in the US it takes the form of two seconds of ring followed by
four seconds of silence.
- Address Signalling - there are two types of dialing:
- Pulse Dialing - this is the original form of dialing a
number. The telephone has a rotary dial mounted on to a spring that
returns the dial to its original position when it is turn. Each number
is identified by the switch by how many makes and breaks are made of the
local loop. The ratio of make to break must be 40% : 60%. The number of
make/break cycles corresponds to the number being dialed. Each position
on the rotary dial corresponds to a different number. Typically the cam
that causes the makes and breaks will give 10-20 pulses a second.
Ground-Start
One problem associated with Loop-Start Signaling, particularly where there
are a large number of calls, is that you can experience a situation where
the trunk is seized from both ends at the same time so that you end up with
someone already at the other end. This is called Glare. This is due
to a lack of recognition for the time interval between the seizure of a
trunk at one end and the subsequent making busy the trunk at the other.
Originally, a method where the user had to wait for a long timeout (up to 40
seconds) was used. After the timeout a particular tone would be heard which
encouraged the user to replace the handset and try again. Ground-Start
Signaling (also called 'Earth Start') is a modified form of Loop-Start
Signaling whereby there is current detection at both ends which is used to
request and then confirm that the trunk is available before it is seized.
When a local PBX seizes the trunk it grounds one of the wires which informs
the other end. This limits the possibility of glare at least outside of
100ms. Electronic switches can detect glare by timing the wink start or
delay-dial signal, maybe even switching the call to another trunk.
The ground-start line conductors transmit common battery loop supervision,
loop dial pulses/DTMF dial tones, alerts and the voice signal. The lines can
send a 'Start to Dial' signal rather than wait for a dial tone, they can
send a message indicating a new call and they can detect call disconnects
and unauthorized calls.
When in the Idle state, the phone has an open circuit Tip (T) to Ring
(R). The phone also has a 10-20,000 ohm Ground Detector that links it
to ground and detects an Off-hook from the network.
When in Call Initiated state, the phone closes contact S which
causes current to flow on the Ring side. The Network sees this and responds
by closing contact N. This results in the Tip being grounded and the
Ground Detector in the phone sees this.
If the Network makes the disconnection by opening N and removing
ground from the Tip, then the current stops flowing. The phone waits 350ms
to determine that this is an actual disconnect as opposed to an Open
Switching Interval (OSI). If the phone makes the disconnection, then it
opens the loop so that the line appears busy until the network removes
ground from the tip and the line can return to Idle. An OSI is where both
Ground and Battery are removed for a maximum period of 350ms in between
state changes. There are never less than 100ms between OSIs.
Ethernet
Cross Over Cable
Only one end of a crossover cable is switched, and is in the
following order:
| Crossover Cable Wiring Order [1 + 3 and 2 + 6
switched from patch cable order] |
| Wire Color |
green/white |
green |
orange/white |
blue |
blue/white |
orange |
brown/white |
brown |
| Wire Number |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
- 802.3 - Ethernet LAN Specifications
- 802.3u - Fast Ethernet
- 802.3z - Gigabit Ethernet
Links:
Cisco's Guide to Ethernet:
http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Ethernet.html
Traditional Switched Circuit Voice Operation:
http://www.rhyshaden.com/voice.htm
Debugging and troubleshooting VOIP problems:
http://www.voip-info.org/wiki/view/How+To+Debug+and+Troubleshoot+VOIP
