Basic Telephone Systems s1r14rq
Local loop: the telephone line that runs from the telephone company’s central office to your home or business.
Many wires
Central office: the building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone.
Many levels of offices
End office (central office class 5)
Toll office (Toll center office class 4)
Primary center Class 3
Sectional Center Class 2
Regional center class 1
Basic Telephone Systems
Trunk: a special telephone line that runs between central offices and other telephone company switching centers. usually digital, high speed, and carries multiple telephone circuits.
Many wires or fiber optic cable
Local access transport areas (LATAs)
IECs: Interexchange carriers: If your call goes from one LATA to another, it is a long distance call and is handled by a long distance telephone company.
LECs: Local exchange carriers: If your call stays within a LATA, it is a local call and is handled by a local telephone company.

Telephone Number
A telephone number consists of an area code, an exchange, and a subscriber extension.
The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services.
POTS
POTS: plain old telephone system
POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz.
A telephone conversation requires two channels, each occupying 4000 Hz.
If you want to send information faster, you need a signal with a higher frequency.
POTS cannot deliver faster signals.

Operations
When the handset is lifted off the base (off-hook), an off-hook signal is sent to the central office.
When the off-hook signal arrives at the central office, a dial tone is generated and returned to the telephone.
When the user hears the dial tone, they dial (or press) the number.
The central office equipment collects the dialed digits, and proceeds to place the appropriate call.

Breakup of AT&T
In 1984, the U.S. government broke up AT&T.
Before then, AT&T owned a large majority of all local telephone circuits and all the long distance service.
With the Modified Final Judgment of 1984, AT&T split off the local telephone companies from the long distance company.
1984: seven Regional Bell Operating Companies; Today, Bell South, SBC, Qwest (US West), and Verizon (Bell Atlantic).
LATA: (local access and transport area
LECs: local exchange carriers
IEC, or IXC: interexchange carriers
Before 1984: Class 1-5 offices;
Today: a collection of LECs, POPs, and IECs.

Telecommunications Act of 1996
This act opened up the local telephone market to competitors.
Now cable TV companies, long distance telephone companies, or anyone that wanted to start a local telephone company could incumbent local exchange carriers (ILEC)?competitive local exchange carriers (CLEC).

PBX
Private branch exchange: a common internal phone switching system for medium to large-sized businesses.
Handle all in-calls and out-calls
Many services: voice mail, call forwarding…and advanced intelligent features
Leased Line Services
A basic leased line, gives you a 56 Kbps data transfer rate.
Permanent direct connections between two specified points.
No dialup

T-1 Service
A digital, synchronous TDM stream used by businesses and telephone companies. used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world. always on and always transmitting
1.544 Mbps rate
One T-1 service can support up to 24 simultaneous channels.
These channels can be either voice or data
ISDN
Integrated Services Digital Network leased service that provides a digital telephone or data connection into a home or business.
With ISDN you can have a digital telephone line and a 64 Kbps data line, or one 128 Kbps data line.
B channel: basic user channel
D channel: data traffic channel
ISDN basic rate interface (BRI): two B channels and one D channel. primary rate interface (PRI): 23 B channels and one 64 Kbps D channel.
Frame Relay
Leased service that can provide a high-speed connection for data transfer between two points either locally or over long distances.
Packet switched service
Advantages:
Data link layer
Very high speed: 56K to 45M, Low price
High throughput, low error rate and high security (fiber)
No error control
Frame Relay
Permanent virtual circuit (PVC): a 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.
Switched Virtual Circuits (SVC): short-term connections.
An SVC can be created dynamically by the customer.

ATM
Asynchronous Transfer Mode
Unique features fixed-length packets of 53 bytes no error correction on the user data. a very different type of addressing from traditional data link layer protocols such as ethernet or token ring. prioritizes transmissions based on class of Service.
ATM switch is the key technology
Disadvantages potentially high costs (both equipment and support) a high level of complexity.

ATM vs. FR similarity
All data are packet-switched no error control at the intermediate computers within the network error control is the responsibility of the source and destination difference fixed packet lengths of 53 bytes(voice transmissions) extensive quality of service scaleable. easy to multiplex basic ATM circuits into much faster ATM circuits
622 Mbps vs. 45 Mbps.

ATM connections
Virtual Channel Connections (VCC).
A virtual channel connection (or virtual circuit) is the basic unit, which carries a single stream of cells, in order, from user to user.
Virtual Path Connections (VPC)
A collection of virtual circuits can be bundled together into a virtual path connection.
A virtual path connection can be created from end-to-end across an ATM network.
In this case, the ATM network does not route cells belonging to a particular virtual circuit. All cells belonging to a particular virtual path are routed the same way through the ATM network, thus resulting in faster recovery in case of major failures.
Virtual Circuit:
Permanent Virtual Circuit (PVC)
Switched Virtual Circuit (SVC)

ATM Virtual Circuits
ATM
Class of Service: a definition of a type of traffic and the underlying technology that will support that type of traffic.
Constant bit rate (CBR):
High speed, continuous data stream.
Variable bit rate (VBR):
A less demanding service can also support real time applications, as well as non-real time applications, but do not demand a constant bit stream.
Available bit rate (ABR): for bursty traffic that does not need to be transmitted immediately. ABR traffic may be held up until a transmission opening is available.
Unspecified bit rate (UBR) for lower rate traffic that may get held up, and may even be discarded part way through transmission if congestion occurs.
DSL
Digital Subscriber Line can provide very high data transfer rates over standard telephone lines: from 100Ks to less than 10M.
Use currently installed twisted pair cable, which can carry broader spectrum (1 MHz or more) less than half the telephone lines in the U.S. are incapable of supporting DSL. there has to be a DSL provider in your region, less than 5.5 kilometers (2-3 miles) from the DSL central office.
DSL Technologies
Symmetric vs. asymmetric
Greater capacity downstream than upstream
Frequency division multiplexing
Lowest 25kHz for voice
Use echo cancellation or FDM to give two bands
Use FDM within bands
DSL
Services:
Internet and regular telephone service (POTS).
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.

ADSL Modem
Inside Your PC: Your computer's ADSL modem connects to a standard analog phone line.
Voice and Data: A DSL modem has a chip called a POTS splitter, which divides the existing phone line into two bands: one for voice and one for data.
Split Again: Another chip in the modem, called a channel separator, divides the data channel into two parts: a larger one for downstream Internet data and a smaller one for upstream Internet data
ADSL
Over the Wire: another ADSL modem, located at the phone company's central office. This modem also has a POTS splitter, which separates the voice calls from the data.
Telephone Calls: Voice calls are routed to the phone company's public switched telephone network (PSTN).
Internet Requests: Data coming from your PC passes to DSLAM. The DSLAM links many ADSL lines to a single high-speed asynchronous transfer mode (ATM) line.
Back at You: The data retrieved from the Internet are routed back