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Telco Basics, Convergence and
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Lesson 3
Some interesting numbers from CNN
What are our goals in Security?
 The “CIA” of security
Computer Security Operational Model
Protection = Prevention + (Detection + Response)
Access Controls
Intrusion Detection
Incident Handling
Textbook uses Prevention, Detection and Remediation
Is an ROI from Security Possible?
 Security as an ROI
 Improved Security ROI
 Security that provides savings in the budget
 Security that provides additional revenue
Switching Systems - Manual
Early telephone switchboards used flexible lines with plugs
on each end to connect two jacks.
 To make a connection:
The operator picked up a cord and plugged it into the jack for the
person making the call
The operator obtained the name or number from the caller for who
they wanted to connect to
The operator then plugged the other end of the cord into the
correct jack to complete the connection.
The plug had a couple parts referred to as the “tip” and
“ring”, terms later used to denote the different wires in a pair
of phone wires (the “tip” wire was connected to the tip of the
plug, the “ring” to the ring)
Early Switchboard
1884 Central Office
Early Telephone Switchboard
Early phone lines
A Toll Switchboard
Information Operators
Why are no men/young men working the
From “Information Warfare and Security” by Dorothy Denning, pg. 44:
“In 1878 – long before the invention of digital computers – AT&T hired teenage
boys to answer switchboards and handle office chores. It did not take long,
however, before the company realized that putting boys in charge of the phone
system was like putting a rabbit in charge of the lettuce. Bell’s chief engineer
characterized them as ‘Wild Indians.’ In addition to being rude to customers
and taking time off without permission, the boys played pranks with switchboard
plugs. They disconnected calls and crossed lines so that people found
themselves talking to strangers. A similar phenomenon took place in the United
Kingdom. A British commentator remarked, ‘No doubt boys in their teens found
the work not a little irksome, and it is also highly probable that under the early
conditions of employment the adventurous and inquisitive spirits of which the
average healthy boy of that age is possessed, were not always conducive to the
best attention being given to the wants of the telephone subscribers.’”
Tip and Ring
Newton’s Telecom Dictionary
Telephone terminology
 Old fashioned way of saying “plus” and “minus” or
ground and positive in electrical circuits
 Derive their names from the operator’s cordboard plug
The tip wire was connected to the tip of the plug
The ring wire was connected to the slip ring around the jack
Today, tip refers to the first wire in a pair of phone
wires, ring is the second wire. Together they constitute
the circuit that carries speech or data.
Tip and Ring
Switching Systems – Step-by-step
 The Step-by-step (or Strowger, the name of the
undertaker who invented the switch) switch
connects pairs of telephone wires by progressive
step-by-step operation of a series of switches.
 Replaced the manual switchboard
 Required frequent maintenance and generated
large amounts of electrical and mechanical noise
Almon B. Strowger – the legend
Strowger moved into telephony from the undertaking
business because, as the near-legend has it, he was convinced
that some local telephone operators, their power over him
having gone to their heads, were deliberately giving wrong
numbers and busy signal reports to his customers in order to
drive him out of business. Strowger determined to find a way
to rid the world of those pesky operators, once and for all.
The first Strowger office could serve only 99 telephones, used
buttons instead of a dial and each telephone needed a strong
battery and five wires to connect it to the central office.
During the next few years, however, these and other problems
were solved. In 1896 the first system, this time using a dial,
was built by the Automatic Electric Company of Chicago,
based on Strowger's patents. It went into operation at the
City Hall in Milwaukee, Wisconsin.
Switching Systems – Crossbar
Works on principle of Common Control
A method of switching in which the control equipment is
responsible for routing calls through the network (as opposed
to step device responsible only for the next step in the
Depends on a crossing or intersection of two points to
make a connection. The switching matrix, or crosspoint
array, depends on energizing a vertical line and a
horizontal line and the point where they intersect
represents the connection made.
Switching - Electronic
The next evolutionary step in switching technology was the
electronic switching system (ESS).
 Early electronic switches were still analog (the “reed
switch”), now replaced with digital switches.
 Use stored program control as the next step to common
control. Systems are much more fault tolerant.
 Tremendous increase in speed of switching with the new
digital switches.
Private Branch Exchange (PBX)
A privately owned (usually scaled-down) switching
system for a company.
A phone company central office was originally referred to as
a public exchange thus a PBX is just a small version of the
phone company’s larger central switching office.
May also be called a Private Automatic Branch
Exchange (PABX)
Original PBX’s were manual, then systems introduced without
the need for an operator – you would simply dial a ‘9’ for an
outside line. Thus the term automatic was added to PBX.
Today this distinction is obsolete.
Two broad categories of transmission media:
– Copper wire, coax, fiber optic
– Microwave, satellite
Numerous considerations when discussing transmission
Distance a signal will travel on a media, speed, requirement
of line of sight, delay, susceptibility to interference/noise,
cost, reliability, and of course, security
Transmission Media (cont.)
 Conducted Media
Copper Wire
Twisted pair
Coaxial Cable
Fiber Optics
 Radiated Media
Encoding and Decoding
Since voice is inherently analog, there is a conversion
process that must take place to change the signal from
analog to digital (and back).
 Pulse Code Modulation (PCM) is the most common method
of encoding an analog voice signal into a digital bit stream.
The amplitude is first sampled and then coded (quantized), and
then converted into a binary number.
Based on Nyquist theorem, sampling should be at a rate twice the
highest frequency on the channel to be effective.
– Thus, since highest frequency on voice channel is 4kHz, sampling should be
done 8,000 times per second.
Encoding and Decoding
Sampling – records the voltage level in time intervals
along an analog wave.
 Quantizing – rounding to the nearest discrete value
 Encoding – Converting the numeric amplitude voltage
levels into binary 8-bit code
Decoding – Converting the 8-bit code into the voltage
 Reconstruction – reproduces the original analog wave
from the voltage levels
 Filtering – strips noise out.
 The process of combining many signals into one
composite signal – thus several calls can be
transmitted at once over a single line.
 Three types of multiplexing in use
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Statistical Time Division Multiplexing (STDM)
Frequency Division Multiplexing
 The oldest method of multiplexing
 Limited to analog transmissions
 Possible when useful bandwidth exceeds the required
bandwidth of signals to be transmitted
 Splits bandwidth into multiple smaller pieces of
e.g. 14,400 Hz can be divided into 6 channels of 2,400 HZ
 Time Division Multiplexing
 Can be used to transmit digital signals
 Uses time not frequency to achieve greater
utilization of line
 Allocates a time slot for each device on the line
Similar to timesharing in an operating system
FDM –vs- TDM
Data and Computer Communications by Stallings, p. 186
Statistical Time division multiplexing
Also known as asynchronous TDM and intelligent TDM
Variation of TDM
 In TDM, if time slot not used, it is idle and wasted
 STDM assigns time slots dynamically, if time slot for
one device is idle it can be used for another
Requires address information to assure proper delivery
Some other ”phun phone sounds”
Call Trace
Please Deposit…
Quarter tone
2600 tone
2600 Hz tone
“Until the late 1960’s, America’s telephone network was run
100% by AT&T and used 100% in-band signaling, whereby
the circuit you talked over was the circuit used for signaling.
For in-band signaling to work there needs to be a way to
figure when a channel is NOT being used. You can’t have
nothing on the line, because that “nothing” might be a pause
in the conversation. So, in the old days, AT&T put a tone on
its vacant long distance lines, those between its switching
offices. That tone was 2600 Hertz. If its switching offices
heard a 2600 Hz, it knew that that line was not being used.”
From Newton’s Telecom Dictionary, 15th ed
Blue Boxes
“Blue boxes are nothing more then a device to generate pairs
of tones, and a single 2600 Hz tone. They had 12 keys, plus a
single button (or a key). Each key was numbered 0 - 9, and
had a "KP" key and "ST"key. The button emitted a pure
2600 Hz tone. A toll free number is dialed, and just as the
number is ringing, the 2600 Hz tone is sent to clear or "Blow
off" the call. A "Ker-chink" sound is heard, which is the
switch signaling back indicating its ready to receive the tones.
A "KP" is sent, followed by the 10 digit number, and ending
with an "ST" tone. Call goes through, and the only indication
was that an 800 number was dialed. This was how it was done
more than 15 years ago. Since then, all of the American and
Canadian phone companies have all but ditched this older
"in-band" signaling equipment.”
Voice Over Network
Newton’s Telecom Dictionary
Several potential benefits to moving voice over a data
You may save some money
You may achieve some benefits of managing a voice and data
network as one network
If you have IP phones, moves, adds, and changes will be
easier and cheaper
Added, and integrated, new services including
– Integrated messaging
– Bandwidth on demand
– Voice emails
IP Telephony Overview
H.323 Architecture
intranet, Internet, VPNs
Ethernet Phone
H.323 Terminal
IP Network
PSTN, ISDN, wireless
From: “Security Requirements and Constraints of VoIP” by Mika Marjalaakso
H.323 Components
Terminal – a terminal, or a client, is an endpoint where H.323
data streams and signaling originate and terminate. It may be a
multimedia PC with a H.323 compliant stack or a standalone
device such as a USB (universal serial bus) IP telephone. A
terminal must support audio communication; video and data
communication support is optional.
Gateway – a gateway is an optional component in a H.323enabled network. When communication is required between
different networks a gateway is needed at the interface. It
provides data format translation, control signaling translation,
audio and video codec translation, and call setup and termination
functionality on both sides of the network.
H.323 Components (cont.)
Gatekeeper – a gatekeeper is a very useful, but
optional, component of an H.323-enabled network.
Gatekeepers are needed to ensure reliable,
commercially feasible communications. When a
gatekeeper exists all endpoints (terminals, gateways,
and MCUs) must be registered with it.
A gatekeeper provides several services to all endpoints in its
zone. These services include:
Address translation
Admission and access control of endpoints
Bandwidth management
Routing capability
H.323 Components (cont.)
MCU – a multipoint control unit (MCU) enables
conferencing between three or more endpoints.
Although the MCU is a separate logical unit it may be
combined into a terminal, gateway, or gatekeeper. The
MCU is an optional component of an H.323-enabled
The multipoint controller provides a centralized location
for multipoint call setup. Call and control signaling are
routed through the MC so that endpoints capabilities
can be determined and communication parameters
Standards for IP Telephony
H.323 for IP Telephony
Terminal to
data transfer)
Unreliable Transport (UDP)
Reliable Transport (TCP)
From: IP Telephony, by Goralski & Kolon
H.225 and H.245
H.225 performs the signaling for call control
uses H.245 to establish and terminate individual logical channels
for communication
Five phases of signaling process
Call setup
Initial communications and capability exchange
Establishment of audiovisual communication
Call services
Call termination
Convergence & VoIP
April 03, 2000, Issue: 807,
Cisco Pushes VoIP To The Fore –
Merrill Lynch, TI seek cost savings in new convergence products
Cisco last week beefed up its voice and data convergence arsenal with new enterprise-oriented voice-over-IP products.
The new hardware and software, bundled under Cisco's Architecture for Voice, Video and Integrated Data (AVVID)
nameplate, is an outgrowth of Cisco's plans to mesh its voice and data products under a single architecture.
Both Merrill Lynch and Texas Instruments Inc. have been testing various components of AVVID for the past several
"There are clear advantages to be gained in deploying this platform," said Don McFarlane, system architect at Merrill
"We expect costs to be reduced as we deploy unified messaging and have a uniform troubleshooting capability" for
administering a single voice and data conduit, he said.
Texas Instruments is using VoIP to link its overseas offices to trim telecommunications costs. The company is also using
Cisco VoIP products as part of a trial with Expand Networks Inc. to push VoIP traffic over connections linking TI offices
in Texas with remote facilities maintained by the company in Mexico.
VoIP (cont.)
Among the products rolled out by Cisco were enhanced call processing management software, second-generation IP
phones, more advanced support of VoIP in its Catalyst 6000 line of switches, as well as a new media server
supporting converged voice/data networks.
The products will be available later this spring. Among the new products: the 7910 and 7960 IP phones are priced from
$145 to $495; the Cisco MCS server, an NT platform that supports transmission of voice, video and data across Cisco
switches and routers, is priced at $14,995.
CallManager software, which runs on the server, is offered free to existing customers and is preinstalled on the MCS
server. The latest version of the software, compatible with Windows 2000, is capable of handling up to 100,000
users in a cluster made up of up to five media servers.
"Cisco's move is a further endorsement of IP telephony within the enterprise,“ said analyst Tere Bracco of Current
"With a player like Cisco aiming products at large companies, it's telling IT managers that VoIP isn't a toy anymore; it's an
inducement for businesses to take a look at this technology.
What Will Drive VoIP?
Still, Bracco said convergence isn't necessarily what will drive enterprises to sample VoIP.
"It's the management that will drive deployment," she said. "Managing these IP devices is much simpler and can help a
company save a lot of money for moves, adds and changes."
Convergence & VoIP
(April 2000)
Why Converge?
$ Savings
Eliminate long distance toll charges
Eliminate duplicate infrastructures
Increased competition in the industry
Enhancement of current applications and development
of new applications
 Collaborative tools
 The industry has been heading there for a while now
Communication Networks
Switched Networks – data transferred through series of
intermediate nodes
Circuit-switched networks
Packet-switched networks
Broadcast networks – no intermediate switching nodes, each
station communicates over a shared medium
Packet radio networks
Satellite networks
Local networks
– E.g. bus or ring
Circuit Switching
Basic premise is that an uninterrupted connection exists
between the endpoints
Constant bandwidth dedicated to this session
Resources for this session reserved for the entire duration of
the call
Blocking is possible as a circuit may not be available
Initial connection requires considerable work but once
established, minimal to maintain it
The PSTN is a circuit switched network
“Addressing” is geographically based
Packet Switching
Data network is a packet switched network
Designed for “bursty” traffic
– Normal data traffic not as sensitive to delays
– Voice highly sensitive to delays
 In packet switched network, data is fragmented into discrete units
 Each packet contains information about its source and destination
 A complete message may consist of 1000’s of packets
 Packets may actually take different routes and may arrive out of order, or
not at all
 Packet switched networks do not reserve bandwidth for each connection
 Addressing is organizationally based
Circuit –vs– Packet Switching
Call request signal
Pkt 1
Pkt 2 Pkt 1
Time spent
Hunting for
An outgoing
Pkt 3 Pkt 2 Pkt 1
Pkt 3 Pkt 2
Pkt 3
trunk trunk trunk
Circuit –vs– Packet Switching
Dedicated Bandwidth
Circuit Switching
Packet Switching
Quality of Service
Voice Quality
Delay Latency
Utilization Level
Economics of Utilization
Call management features
Analog –vs– Digital Signaling
At one point, the entire voice session utilized analog signaling,
today it is only analog at the endpoints.
Analog voice signal now converted to digital for transmission
Digital transmissions preferred over Analog
Digital equipment cheaper to produce
Digital signals provide higher quality communication
– Digital less susceptible to ‘noise’
Digital signals easy to compress to reduce required bandwidth
Thus – digital transmission facilities developed for PSTN to
take advantage of these benefits: The
T-Carrier system is born!!
Voice Energy Frequency
Multiplexing Frequency Signals
The T1-Carrier
CH 1
8 bits
CH 2
CH 3
CH 4
CH 5
CH 6
CH 7
CH 24
The U.S. T1-Carrier system can handle 24 digitized voice
channels multiplexed together.
 A voice channel of 4-kHz must be sampled at an 8-kHz
rate to render clear representation with one sample thus
taken every 125 microseconds.
 With 193 bits (8*24channels plus 1 framing bit) taken
every 125 microseconds, the data rate is 1,544,000
bits/second, or 1.544 megabits/second.
 This can be used for voice, or…
Integrated Services Digital Network
Telephone industry has gathered statistics for years on average length of call,
average number of calls, etc. to be able to design a network that can handle the
With the introduction of calls made for computer connections, their statistics went
out the window – avg. length, for example, no longer applied.
In an attempt to provide large-scale digital services, ISDN was introduced but with
the 64Kbps rates, which at first seemed impressive, the need to use ISDN instead of
the normal PSTN diminished. Instead an even higher speed method was desired.
Digital Subscriber Lines (DSL) was an answer.
Uses the same twisted-pair telephone wires that currently exist but utilizes the higher
frequencies not used in voice band thus enabling both voice and data on the same
Asymmetric DSL (ADSL) takes advantage of the fact that the majority of traffic is
downstream not upstream and provides greater downstream data rates.
While all of the digital fun was going on in the PSTN, a new
element was introduced to the picture, Community Antenna
Television (CATV)
 Originally designed to carry one-way video signals, with the
addition of an upstream return channel voice and data
communication was possible.
Analog head-ends replaced with digital devices
Coax trunks replaced with fiber
While voice is not generally available (a connection to the
PSTN is necessary), data communication across cable has
become an increasingly popular option.
Computer Telephony Integration
The SW and HW elements that allow a computer to manage
telephone calls and integrate additional features beyond
those offered by the PBX, is known as CTI.
Popular in the SOHO environment
One of the earliest applications was hotel-motel hospitality
package (toll charge tracking, voice mail, housekeeping functions)
Unified messaging (a single GUI for fax, voice mail, and e-mail)
Additional and more sophisticated applications constantly being
 So, what factors have facilitated the move toward
converged networks?
Digitization of PSTN
Rise of digital networks
Competition from other industries such as CATV
Increased integration of computers and telephony
Technology advances which make things such as VoIP
economically feasible.
Sample network and Security
A Better Picture of the network
and the potential threats
Back Door
User Connected
And what else could possibly
happen next?
Back Door
User Connected
 What is the Importance and Significance of this
 How does this topic fit into the subject of “Voice
and Data Security”?