Tài liệu Lecture 25: The Telephone System doc

CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System The Telephone System: (Ref: Ch-4, Page-113 of your Text book) The telephone system remains as the backbone of low-cost telecommunications services in North America. An understanding of the telephone system is necessary when considering its use for data communications. Telephone: invented in 1874 by Alexander Graham Bell and patented in 1876 Single wire: In the beginning, the market was for telephones only. It is the customer who used to wire them between telephones using single wires and the earth as the ground or the other conductor to complete the circuit. Bell foreseen the problem of jumbling wires all over the city and formed the Bell Telephone Company, which opens its first switching office in Connecticut in 1878. The original problem soon returned: to connect every switching office became unmanageable, so second level switching offices were invented. Eventually, the hierarchy grew to five levels. Copper: At one time, 80% 0f AT&T's capital was the Copper in the local loops, the largest Copper mine. Now a days a variety of transmission media are used for telecommunication. Local loops consist of twisted pairs, although in the early days of telephony, uninsulated wires spaced 25cm apart on telephone poles were common. Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely used. Local loops still carry analog signals but all interexchange lines are carrying digital signal. In summary, the telephone system consists of three major components: 1. Local loops (twisted pair, analog signaling). 2. Trunks (fiber optics or microwaves, mostly digital). 3. Switching offices. We will next look at three different topics related to these three major telephone components: 4. How to send digital data over the local loops, quick answer is modem. 5. How to collect multiple calls together and send them together, the answer is multiplexing. 6. How to do switching, mechanical and digital switching. The telephone system (commonly called POTS for plain old telephone service) enables voice communication between any two telephones within its network. Houses and offices are connected to a telephone company end office (central office class 5) by a set of two twisted pair wires (called the local loop). Lecture25.doc Page 1 (6) CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System Local Loop: refers to 2-wire copper circuits brought to the end-user premises. In the local loop the 2 wires are referred to as TIP Positive (Green wire) RING Negative (Red) On-Hook: When the phone is "on-hook" it is not connected to the circuit. The open- circuit voltage presented to the phone is a 48 volts DC. Off-Hook: When the phone is lifted or connected to the local loop, about 5 volts DC is present at the phone and current flows in the local loop. This current is detected in the "end office" or local switch by a relay resulting in the presentation of a dial tone on the circuit. Ring: When the phone is ringing, a 20 Hz 85-volt RMS voltage is present on the line. Standard Rotary Telephone Handset (Type 500 Telephone) Dialing is done by interrupting the line current at a 10 Hz rate. 1 to 10 pulses are sent to dial digits 1 to 0. (Hayes modem command: ATDP) Touch-Tone Telephone: (DTMF or Dual Tone Multiple Frequency) Requires DTMF circuitry on the local telephone office switch. It uses pairs of frequencies (or "tones") to identify the digits of the number being dialed. Two tones are used instead of one to avoid dialling by unwanted single tones generated acidentally in front of the mouth piece. (Hayes modem command to tone dial is ATDT) 1209 1336 1477 1 2 3 697 Hz 4 5 6 770 Hz 7 8 9 852 Hz * 0 # 941 Hz 4 3 2 1 5 6 7 8 9 0 Pulse Dial Tone dial Lecture25.doc Page 2 (6) CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System Telephone Switching Facilities: • Switching machines (or "switches") are used to interconnect any pair of subscribers in the public telephone system. Within a radius of about 10 km from the "end office" (local switching office) up to 10,000 subscribers can be interconnected directly. They share a common "exchange" or switch. This method of making connections is called Circuit Switching. 759-XXXX 949-XXXX <====10,000 (= 9999) different phone numbers in each exchange 254-XXXX Switching Center Hierarchy: (Fig. 4.5, p.113) In North America there is a hierarchy of switching centers based on the number of subscribers they handle: Class of Office Name of Office Function CLASS 5 End Office (EO) Local Office Can connect up to 10,000 subscribers in each exchange. (759-, 254, 949-, 942-, etc.) CLASS 4 Toll Center Sault Ste. Marie Handles multiplexed calls between exchanges. CLASS 3 Primary Center Sudbury (increased capacity to handle inter-toll trunk lines) In 705-949-8384 for example, the "705" identifies the Primary center. CLASS 2 Sectional Centers Next level CLASS 1 Regional Centers Major switches between regions. There are only 2 in Canada: one in Regina and one in Montreal. The end office is connected to a central office class 4 by a trunk line. The central offices are arranged in a hierarchy; a class 4 is connected to a class 3 office which is connected to a class 2 etc. The telephone system was originally designed as an analog system, but today, most trunk lines are digital. Lecture25.doc Page 3 (6) CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System Types of Switches: There have been 3 major types of telephone switches developed over the past 100 years. 1. STEP-BY-STEP or Direct Progressive (used to be called Strowger Switch) The oldest style of electromechanical switches. They require a lot of space and maintenance. There are only a few of these switches left in service. • Generate line noise while switching calls, which causes errors in data communications. 2. CROSS-BAR or Common-Control Switches • They too are electromechanical in nature but have certain advantages over step-by-step switches. They consist of electrical contacts arranged in a grid. • They are faster, take up less space and are less expensive to maintain than step-by-step switches. • They are rapidly being replaced by digital switches. 3. DIGITAL SWITCHES (ESS or Electronic Switching Systems) • Digital switches are basically computers using stored-program control to handle the switching of calls. • They can handle 100,000s of subscribers at a time. • They are much faster, more reliable and flexible. • They don't generate electrical noise when switching making them more desirable for data communications. • Because they handle so many subscribers it may be disastrous if they fail. • Virtually all common carriers now have digital switching. Leased Lines vs. Dial-up Lines: Leased Lines • Leased phone lines (also called private circuits or dedicated circuits) are phone lines that are available permanently (without having to dial) from one site to another. • They are leased on a monthly basis and you pay for them whether you use them or not. Can be expensive unless usage is high. • You never get a busy signal, but you are always connected to the same site. • Ideal for permanent computer links between two sites that have significant traffic like air-line reservation terminals or Internet connections. • Lines can be conditioned for high quality data transmission by having noise and distortion minimized. Dial-up or Switched Circuits • Normal dial-up phone lines are inexpensive but have some disadvantages: • Calls may be blocked if there are no lines available or the line may be busy. • There is a dial-up delay. • Long-distance charges are based on usage so heavy use will be expensive. • May get poor quality connections since each time you dial a long-distance line you may get a different circuit. Lecture25.doc Page 4 (6) CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System Store-and-Forward Switching: If a called device is busy, the central switching office stores the incoming message and retransmits it to the destination when it becomes available. Commonly used for data communications because it provides the highest level of throughput on circuits. Transmission Impairments: Transmission lines suffer from three major problems: Attenuation - is the loss of energy as the signal propagates outward. The amount of energy lost depends on the frequency. Each Fourier component is attenuated by a different amount. Delay distortion - It is caused by the fact that different Fourier components travel at different speeds. For digital data, fast components from one bit may catch up and overtake slow components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception. Noise - unwanted energy from the sources other than the transmitter. Thermal noise, cross talk, impulsive noise. • These transmission impairments and the bandwidth limitation makes digital signaling (have a wide spectrum) unsuitable for transmission over the local loop. • To get around the problems associated with the dc signaling or digital signaling, especially on telephone lines, ac signaling is used. A continuous tone in the range 1000Hz to 2000Hz, called a sine wave carrier is used. Its amplitude, frequency, or phase can be modulated (changed with the information) to transmit information. Brief Summary of PBX’s, Cellular Technology and Fax Machines PBX: (Ref: Appendix-B, Page-426 of your Text book) PBX (Private Branch Exchanges) is a switch also called a switchboard. You can think it as a miniature version of the large scale switching exists in a Telephone company. PBXs (Private Branch Exchanges) are used by organizations to manage their voice (and sometimes their data) communications. They reduce costs by reducing the number of connections to the telephone company’s trunk lines. Essentially a PBX is like a “smart” switch placed on your premises instead of the phone company’s. There are digital PBXs and wireless PBXs (operates just above 900MHZ). Cellular Technology: (Ref: Appendix-C, Page-430 of your Text book) Commonly used to provide wireless mobile (high frequency radio) telephone service but is also used for data communications, pagers and personal digital assistants (PDAs). Regions are divided into cells, each with its own antenna and mobile phones transmit and receive using the nearest antenna. They are automatically switched from cell to cell as they travel. Using frequencies in the 870 to 890 MHz range Lecture25.doc Page 5 (6) CSN200 Introduction to Telecommunications, Winter 2000 Lecture-25 Telephone System they are accessible by scanners and should not be considered private. Cellular telephone fraud is becoming common as user’s security codes are discovered and used illegally. FAX (Facsimile Machines): A method or device for transmitting images (documents, drawings, photographs, or the like) by means of radio or telephone for exact reproduction elsewhere. The Basic operation of a FAX requires converting the image into binary bits for transmission. This is achieved by scanning the image: a photoelectric scanning device built into the facsimile system shines light onto the page, and a special sensor reads the reflection of the light on a line by line basis. The smallest individual position on a line is known as a picture element. A white area reflects light and sets the binary value of the picture element (PEL) to '1', while a black area absorbs light and sets the binary value of the PEL to '0' or vice-versa. A FAX has a horizontal resolution of 200 dots per inch (dpi) and the vertical resolution can be set to either 100 (standard) or 200 (fine) dpi. • Horizontal resolution - dot density in a line • Vertical resolution - line density in a page Evolved through 4 generations (ITU-T classifications): Group 1 - analog, carrier frequency is 1700Hz, uses FM plus and minus of 400Hz. For a letter size page it takes about 6 minutes to transmit. Data rate of 2400 bps. Group 2 - uses Vestigial Sideband Amplitude Modulation with suppressed carrier. Takes 3 minutes for transmitting a letter size paper. The carrier frequency is 2100Hz. Data rate of 9600 bps. Group 3 - uses 9600bps with Quadrature Amplitude Modulation. Takes 1 minute for transmitting a letter size paper. Group 4 - uses 14400bps with Trellis-coded Modulation. Takes 30 seconds for transmitting a letter size paper. FAX Class Designation: To standardize the Computer-to-FAX interface, the EIA developed a series of recommendations. Those recommendations are designed to standardize the commands used to control the Fax operation. • Class 1 • Class 2 Fax commands start with the prefix AT+F. e.g., AT+FCLASS? Means List the current mode setting. • A FAX machine scans an image and then transmits over the telephone line. • The call can be established by dialing manually or automatically. • A basic FAX machine is simply a modem with an added scanner and printer. There are 5 phases to a fax transmission: A. Call establishment. B. Negotiation of data rates and capabilities. C. Image transmission. D. Confirm page image received by the other party. E. Hang up. Lecture25.doc Page 6 (6) . Telecommunications, Winter 2000 Lecture- 25 Telephone System The Telephone System: (Ref: Ch-4, Page-113 of your Text book) The telephone system remains as the backbone of. In the beginning, the market was for telephones only. It is the customer who used to wire them between telephones using single wires and the earth as the