Chapter 11: Voice and Date Delivery Networks

THE BASIC TELEPHONE SYSTEM

Plain old telephone service (POTS) was an analog system capable of supporting a voice conversation

Telephone lines, Trunks, and Numbers

• The local loop is the telephone line that leaves your house or business and consists of either four or eight wires and connects to the local telephone company's central office
• Central office – building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone
• As long as the phone call remains within LATA, the call is handled by a local telephone company
• Local access transport area (LATA) is a geographic area such as a large metropolitan area or part of a large state
• If you place a long-distance call, the central office passes your telephone call off to a long-distance provider

• Trunk: special telephone line that runs between central offices and other telephone switching centers
• Usually digital, high-speed, and carries multiple telephone circuits
• Typically a 4-wire circuit, while a telephone line is a 2-wire circuit
• Not associated with a single telephone number like a line is

The Telephone Network Before and After 1984

• In 1984, U.S. government broke up AT&T
• Before then, AT&T owned large majority of all local telephone circuits and all the long-distance service
• With Modified Final Judgment (MFJ) of 1984, AT&T had to split off local telephone companies from long-distance companyThe local telephone companies formed seven Regional Bell Operating CompaniesToday, there are only 3 left: AT&T (Southwestern Bell, Bell South, Ameritech, Pacific Telesis), CenturyLink (US West), and Verizon (Bell Atlantic, NYNEX)
• Another result of the Modified Judgment was creation of LATA (local access and transport area)
• Local telephone companies became known as local exchange carriers (LECs), and long distance telephone companies became known as interexchange carriers (IEC, or IXC)
• Centrex (central office exchange service) is a service from local telephone companies in which up-to-date telephone facilities at the telephone company's central office are offered to business users so they do not need to purchase their own facilities
• Private branch exchange (PBX) is a computerized, self-contained telephone system that sits in a telephone room on a company's premises
• Private line and Tie Lines are lease telephone line that require no dialing

Telephone Networks After 1996

• Another landmark ruling affecting the telephone industry was the Telecommunications Act of 1996
• Opened up local telephone market to competitors
• Now cable TV companies (cable telephony), long-distance telephone companies, or anyone that wants to start a local telephone company can offer local telephone service
• Local phone companies that existed before the Act are known as incumbent local exchange carriers (ILEC) while the new companies are competitive local exchange carriers (CLEC)
• ILECs must give CLECs access to their telephone lines, telephone numbers, operator services, and directory listings; access to poles, ducts, and rights-of-way; and physical co-location of equipment within ILEC buildings- and they must give these services at wholesale price

Limitation of Telephone Signals

• Telephone network was engineered to transmit signals of approximately 3100 Hz
• A telephone conversation requires two channels, each occupying 4000 Hz

• A 4000 Hz analog signal can only carry about 33,600 bits per second of information while a 4000 Hz digital signal can carry about 56,000 bits per second
• If you want to send information faster, you need a signal with a higher frequency or you need to incorporate more advanced modulation techniques

Dial Up Internet Service

• once upon a time, a majority of computer users accessed the internet via a dial-up internet service
• DSL and cable modem essentially have taken over the dial-up market, but can still find in rural areas where DSL or cable modem are available
• Current fastest dial-up modem is the 56k modem
• 56k modem stands for a data transfer rate of 56000 bits per second combining digital signaling with analog signaling
• Would actually achieve 64k except:
• Local loop is still analog, thus analog signaling
• Analog to digital conversion at the local modem introduces noise/error
• Combined, these shortcomings drop the speed to at best 56k
• Doesn’t even reach 56k due to line noise and other factors

• Based upon one of two standards:
• V.90: Upstream speed is maximum 33,600 bps
• V.92
• Newer standard
• Allows maximum upstream speed of 48 kbps (under ideal conditions)
• Can place a data connection on hold if the telephone service accepts call waiting and a voice telephone call arrives

DIGITAL SUBSCRIBER LINE

• DSL is a technology that allows existing twisted pair telephone lines to transmit multimedia materials and high-speed data
• Transfer speeds can range from hundreds of thousands bits per second up to several million bits per second

DSL Basic

• Transmission speed can be affected by one of the following:
• Carrier providing the service
• Distance of your house from the central office of the telephone company
• Asymmetric connection
• It is an "always on" connection
• uses permanent circuit instead of a switched circuit
• The DSL provider uses a DSL access multiplexer (DSLAM) to split off the individual DSL lines into homes and businesses
• A user then needs a splitter to separate the POTS line from the DSL line, and then a DSL modem to convert the DSL signals into a form recognized by the computer

DSL Formats

• A DSL service comes in many different forms:
• ADSL (Asymmetric DSL)
• DSL Lite
• Slower form than ADSL
• VDSL2 (Very high data rate DSL2)
• RADSL (Rate-adaptive DSL)
• Speed varies depending on noise level

CABLE MODEM

• Cable modem is a high-speed communications service that allows high-speed access to wide area networks such as the Internet via a cable television connection
• Most are external devices that connect to the personal computer through a common Ethernet card
• Can provide data transfer speeds between 500 kbps and 25 Mbps

T1 LEASED LINE SERVICE

• DSL and cable modems are great for home users and small commercial users.  But what else is there for commercial users?
• T-1 – digital service offered by the telephone companies that can transfer data as fast as 1.544 Mbps (both voice and computer data)
• To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection
• A T-1 service
• Is a digital, synchronous TDM stream used by businesses and telephone companies
• Is always on and always transmitting
• Can support up to 24 simultaneous channels
• These channels can be either voice or data (PBX support)
• Can also be provisioned as a single channel delivering 1.544 Mbps of data (LAN to ISP connection
• Requires 4 wires, as opposed to a 2-wire telephone line
• Can be either intra-LATA (local) which costs roughly $350-$400 per month, or inter-LATA (long distance) which can cost thousands of dollars per month (usually based on distance)
• A customer may also be able to order a 1/4 T-1 or a 1/2 T-1 (fractional T-1)

FRAME RELAY

• Frame relay is a packet-switched network that was designed for transmitting data over fixed lines
• Leased service that can provide a high-speed connection for data transfer between two points either locally or over long distances
• A business only has to connect itself to local frame relay port
• Once data reaches local frame relay port, the frame relay network, or cloud, transmits the data to the other side

• Permanent virtual circuit (PVC) – connection between two endpoints
• Created by the provider of the frame relay service
• The user uses a high-speed telephone line to connect its company to a port, which is the entryway to the frame relay network
• The high-speed line, the port, and the PVC should all be chosen to support a desired transmission speed

Committed Information Rate (CIR) or Service Level Agreements

• The user and frame relay service would agree upon a committed information rate (CIR)
• The CIR states that the carrier agrees to transfer the customer's data at the agreed rate, the customer agrees that it will not exceed the agreed rate, and the frame relay network becomes saturated, the carrier may drop any of the customer's frames that are in excess of the committed information rate.
• The burst rate allows customer to exceed the committed information rate by fixed amount for a brief moments of time

ASYNCHRONOUS TRANSFER MODE 

• asynchronous transfer mode (ATM) is a very high speed packet-switched service that is offered by a number of communications companies similar to frame relay
• transfer rates are as fast as 622 Mbps
• All data is sent in small 53-byte packages called Cells
• Cell size is kept small so that it will quickly pass through the node in an ATM network and continues its way to its destination
• Designed to simultaneously support voice, video, and data
• Similar to frame relay, data travels over a connection called a virtual channel connection (VCC)
• To better manage VCCs, a VCC must travel over a virtual path connection (VPC)
• One of ATM’s strengths (besides its high speeds) is its ability to offer various classes of service

ATM Classes of Service

• A Class of service is a definition of a type of traffic and the underlying technology that will support that type of traffic
• Four classes of service determine the type of traffic an ATM network can carry:
• Constant bit rate (CBR)
• Variable bit rate (VBR)
• Available bit rate (ABR)
• Unspecified bit rate (UBR)

Advantages and Disadvantages of ATM

• Advantages of ATM include very high speeds and the different classes of service
• Disadvantages include potentially higher costs (both equipment and support) and a higher level of complexity

MPLS AND VPN

• Frame relay and ATM are declining in popularity due to more people using the Internet
• But you can’t just send potentially important data over the Internet without doing something first
• One thing businesses are doing is applying MPLS labels to the IP packets
• The use of MPLS routes data packets quickly through the Internet
• And as we have also seen, VPNs (virtual private networks) create secure tunnels

SUMMARY OF THE DATA DELIVERY SERVICES

CONVERGENCE

• Big issue in the voice and data delivery industry
• Phone companies are buying other phone companies
• Older technologies are falling by the wayside as newer technologies take over a larger share of the market
• Newer devices are incorporating multiple applications
• Computer telephony integration is one large example of convergence

Computer-telephony Integration

• Computer-telephony integration (CTI) is a field that combines more traditional voice networks with modern computer network
• Has three advantages:
• creates new voice/data business application that can save time
• makes optimal use of current resources
• saves money
• CTI applications could include the following:
•  Unified messaging
•  Interactive voice response
•  Integrated voice recognition and response
•  Fax processing and fax-back
• Text-to-speech and speech-to-text conversions
• Third-party call control
• PBX graphic user interface
• Call filtering
• Customized menuing systems

Unified Communications

• Just as CTI is a convergence of multiple technologies and applications, unified communications is the convergence of real-time and non-real-time communications
• Unified communication os the convergence of real-time and non-real-time communication services such as telephony, instant messaging, video conferencing, speech recognition, voice mail, email and something called presence information into a unified interface