Week 3

Things you really want to know

Things to know

• Describe functions/services of a central office• Describe trunk• Describe local loop• Describe the NANP, how does the CO fit into this plan.• Describe pulse dial and DTMF• Difference between in-band & out of band signaling• Describe the concept of grade of service• What is the busiest telephone day of the year? Day

with the most collect calls?

An Overview of the Public Network

• Public Switched Telephone Network (PSTN) - consists of all the facilities and connections maintained by all local and long distance providers.

• In telecommunications, line is used frequently to refer to one of two things: – the physical connection between a subscriber and the telephone

company’s facilities – a single communications channel between a subscriber and the

central office

An Overview of the Public Network

• Termination - the place where a wire is connected to another part of the public telephone network (for example, a switch or a customer’s home).

• Point of presence (POP) - refers to a carrier’s facilities that allow it or its customers access to the public network.

Common Carriers

• Common carriers - entities directly involved in supplying regulated telecommunications services to the public.

• Reseller - a common carrier, or a company that leases another company’s facilities, and then sells services over those facilities under its own name.

Local Exchange Carriers (LECs)

• Currently, two types of common carriers provide local phone service: – Incumbent local exchange carriers (ILECs): companies that have been

providing local phone service since before competition was allowed for intraLATA traffic

– Competitive local exchange carriers (CLECs): companies that began offering local phone service after the Telecommunications Act of 1996 introduced competition.

• Facilities-based - CLECs that build their own facilities in addition to leasing and using ILEC facilities to provide service under their name.

Interexchange Carriers (IXCs)

Interexchange Carriers (IXCs)

Demarcation Point

Demarcation Point

Demarcation Point

Local Loop

The portion of a business or residential telephone network that connects the demarcation point to the local phone company’s nearest central office is called the local loop.

• Local loop (last mile) portion of a connection is the most expensive for a carrier to provide because separate lines must be installed for each individual subscriber.

• The local loop is the part of a connection most likely to have the lowest throughput and, further, be the most susceptible to damage or noise.

Local Loop

Local Loop

http://upload.wikimedia.org/wikipedia/en/0/00/Central_Office_Locations.png

Serving Area Concepts (SAC)

• Drop wire - the cable that runs from a subscriber’s demarcation point to a telephone pole or underground conduit. – The drop wire connects the subscriber’s home or business line to a

distribution cable, which gathers multiple drop wires from a neighborhood.

• Conduit - the thick tube (usually made of PVC plastic) that surrounds a distribution cable.– The conduit protects the wires within the cable from environmental

damage.

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Cable Vaults

Cable Vaults

Distributing Frames

• Main distributing frame (MDF) - a piece of equipment where incoming wires terminate and their circuits are connected to another set of wires that lead to central office equipment.

• Punch-down block - a row of metallic clips (or receptors) that accept a wire termination.

• Jumper wires - used to connect incoming lines’ punch-down blocks with the outgoing lines’ punch downblocks.

• Cross-connect - wires terminating at two sets of punch-down blocks are interconnected.

Distributing Frames

Distributing Frames

Distributing Frames

Switching Equipment

Switching Equipment

• Major functions of switching equipment at a central office:• Dial tone • Customer and phone number identification• Call setup• Call routing• Call supervision• Line testing and maintenance• Billing

Figure 18–35 The three sources of power available in a central office.

Power Equipment

Central Office Hierarchy

• Serving area (of a local office) - the geographical boundary that includes all its subscribers. It extends roughly three miles in all directions from the central office (CO).

• Trunk - a transmission route between switches that typically has a great deal more capacity than a feeder.

• Regional offices - Class 1 central offices.

Central Office Hierarchy

LEC-to-LEC Connections

LEC-to-LEC Connections

LEC-to-LEC Connections

LEC-to-IXC Connections

LEC-to-IXC Connections

Billing Between Carriers

The fees charged by ILECs are based on the leasing carrier’s:– Grade of service received– Number of trunks used– Amount of traffic transmitted– Placement of equipment in ILEC’s facility, also called collocation– Facilities and circuit installation– Maintenance and support agreement

North American Numbering Plan

A scheme for assigning unique phone numbers to every line in the country.

• For numbering purposes, North America is divided into several smaller geographic regions called Numbering Plan Areas (NPAs), more commonly known as area codes.

North American Numbering Plan

• NPAs reserved for special purposes include:

– Easily Recognizable Codes (ERCs) - When the second and third digit of the NPA are identical (for example, the numbers 888 or 411), the NPA is known as an Easily Recognizable Code (ERC).

– N9b - these 80 NPA codes have been reserved for use when the current NANP numbering scheme undergoes further expansion.

– 37b and 96b - these 20 NPA codes have been reserved in case a previously unanticipated need for blocks of 10 contiguous NPAs arises.

Summary

• The Public Switched Telephone Network (PSTN) is defined as the collection of local and long distance providers’ facilities and connections that are available for public voice (and more recently, data) communications.

• Common carriers are entities directly involved in supplying regulated telecommunications services to the public.

• The local loop, or "last mile," is the connection between a subscriber and the nearest central office.

42

Key Terms in Switching Systems• Common control systems– Translation of the telephone number, automatic call

routing, digit conversions, and trunk signaling • Direct control systems– Lack alternate routing and digit translation capabilities

• Virtually non-blocking– Not totally non-blocking but provides enough paths so

users are rarely blocked• Busy Hour Call Attempts (BHCA)– The number of calls the system can handle during peak

hour• Concentration or line-to-trunk ratio– Determines the probability that a call will be completed

43

Telecom Infrastructure Hierarchy

44

Telephone Cable Architecture

• Telephone Cable Hierarchy– Trunks (in North America, that are same as

“Junctions” in Europe)• High-speed digital carriers that interconnect nodes

– Feeders– Branch Feeders– Station Drops (local loops, subscriber lines)• One pair of UTP wire that is usually analog

45

T-Carriers and theirTransmission Capacity

46

Optical Carriers and their Transmission Capacity

47

Line Conditioning

• Line Conditioning – Is used to tighten telephone company parameters so

that they can transfer data at higher speed with reduced errors

• Propagation delay– Time taken by a signal to travel from source to

destination and “envelope delay distortion” measures the variance in propagation delay within the voice band

• Attenuation distortion– Gain fluctuations with frequency

48

The Telephone

• Telephony– Science of translating sound into

electrical signals

• Tip and Ring– Transmit and Receive wire that

connect the instrument to a plug in the wall using RJ-11 jack

49

The Telephone: Tip and Ring

50

Outgoing Call• Pulse Dial– In general, pulse repetition rate is between 8

and 11 pulses per second (pps)• Dual Tone Multiple Frequency (DTMF)– Most commonly used signaling system today– More reliable and faster than “pulse dial”– Transmission rate is 7 digits per second– Consists of a frequency matrix

• Multi-Frequency (MF)– Used on trunk circuits– Transmission rate is 7 digits per second

51

Incoming Call

• Ringer Equivalence Number (REN) – Is used to ensure that the local exchange can

provide the correct amount of power required to ring the telephone

• The Ring voltage is about 90 to 105 volts AC with a frequency of 20 Hz

• The –48 volts DC that is always on the line operates the telephone when it is being used

52

Line Signaling: Loop Start• Current flows only when the phone is off-hook• Local exchange senses that and provides a dial tone• No need for accurate ground references between

the local exchange (remote end) and the telephone (local end)

• Tip and Ring wires may be reverse• Problem of “glare” (when both the local end and the

remote end attempt to access the circuit at the same time)

53

Line Signaling: Ground Start

• Usually used only on trunks and PBXs• Minimizes the possibility of “glare” • Tip and Ring wires cannot be reversed• Local end and remote end must be at the

same potential

54

Trunk Signaling

• Out-of-band– Separate network to pass call setup, charging, and

supervision information• In-band– Carries call setup, charging, and supervision information

over the same circuit• Advantages of out-of-band over in-band– Lower susceptibility to fraud– Lower setup time– Capable of supporting virtual networks

55

In-band Signaling Methods

• Single Frequency– Most common in-band analog signaling system– Idle or busy status indicated by the presence or absence of

a 2600 Hz tone in the U.S.

• E&M Signaling (recEive and transMit)– Used on digital four-wire circuits– Type I: Common in North America– Type II: Usually on Centrex circuits– Type V: Most popular outside North America

56

Out-of-band Signaling Method• Common Channel Signaling– Most common out-of-band signaling system

• Signaling System Seven (SS7) Standard– HDLC-based protocol developed by CCITT– Uses layered protocol that resembles the OSI model– Message Transfer Part of SS7 (bottom three layers of

OSI) Telephony User Part (top four layers of OSI) – Components:

• Service Switching Point (SSP) or Action Control Point (ACP)• Signal Transfer Point (STP)• Service Control Point (SCP) or Network Control Point (NCP)

57

Network Design Parameters• Grade of Service (GoS)– Ratio of the number of lost calls to the total

number of attempted calls, same as the probability of blockage.

– The lower the number the better the system (A GoS of 0.01 is better than a GoS of

0.05)

Grade of Service = Number of lost calls Number of attempted calls

58

Network Design Parameters continued…

• Estimated Traffic– Traffic is the term that quantifies usage. Usage

or total traffic intensity is measured in centi-call seconds (CCS) = 100 call seconds of traffic in one hour. 36 CCS = 100% utilization

• Network Design– Trade-off between cost and quality of service– Optimum designs: cost-savings while

maintaining quality