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					Business Data Communications,
        Fourth Edition
    Chapter 13: Communications
             Services
                       Learning Objectives
     • Differentiate between narrowband and
       wideband communications services
     • Describe the switching offices in the public
       switched telephone network and how the
       network is connected to a customer’s site
     • List the types of exchange carriers
     • Define what is meant by a virtual private
       network (VPN)


Chapter 13: Communications Services              2
                       Learning Objectives
     • Explain the concept of telephone number
       portability
     • List the differences between narrowband
       ISDN and broadband ISDN
     • Differentiate between voice-grade and
       wideband analog circuits
     • Describe the types of digital subscriber
       lines

Chapter 13: Communications Services               3
                       Learning Objectives
     • Explain how a synchronous optical
       network (SONET) operates
     • Define fast packet services including X.25,
       frame relay, Asynchronous Transfer Mode
       (ATM), and Switched Multimegabit Digital
       Service (SMDS)
     • Define the Advanced Intelligent Network
       and some of its services
     • Describe multiprotocol label switching
       (MPLS)
Chapter 13: Communications Services             4
                                  Introduction
     • Long distance carrier services are very
       important in today’s digital environment
     • Enterprise networks use these services
     • Topics covered:
          – Switched and dedicated circuits
          – Integrated Services Digital Network (ISDN)
          – Fast Packet Services
          – Other services


Chapter 13: Communications Services                      5
       Categories of Communications
                 Services
     • Narrowband Services
          – 64 Kbps or less channels
     • Wideband Services
          – 1.5 Mbps to 45 Mbps (North American)
          – 2 Mbps to 34 Mbps (International)
     • Broadband Services
          – 45 Mbps or greater (North American)
          – 34 Mbps or greater (International)


Chapter 13: Communications Services                6
                           Switched Circuits
     • Public Switched Telephone Network
       (PSTN)
          – Central office – serves local customers
          – Tandem toll center – connects customers to
            toll network
          – Primary toll center – intrastate toll calling
          – Sectional toll center – interstate calling within
            a region
          – Regional toll center – interregional toll calling


Chapter 13: Communications Services                         7
Chapter 13: Communications Services   8
                           Switched Circuits
     • Demarcation Point (DEMARC)
          – Point of connection between a customer’s
            telephone equipment and the common carrier
            network
          – Network Interface Unit (Residential customer)
          – Minimum Point of Entry (Business customer)




Chapter 13: Communications Services                     9
Chapter 13: Communications Services   10
                           Switched Circuits
     • Local Exchange Carriers (LECs)
          – Incumbent LECs (ILECS)
               • Baby Bells
               • Originally provided service in a limited area
               • Now can compete for interLATA services
          – Competitive Local Exchange Carriers
            (CLECS)
               • Not limited in area of coverage
               • Competitive Access Provider (CAP) connects a
                 user directly to interexchange carrier.


Chapter 13: Communications Services                              11
                           Switched Circuits
     • Virtual Private Network (VPN)
          – Has components of both a private and public
            network
          – Allows a private enterprise network to use a
            public network, yet maintain privacy
          – Packets only travel to nodes connected to the
            VPN, although they use the PSTN




Chapter 13: Communications Services                     12
Chapter 13: Communications Services   13
                           Switched Circuits
     • Telephone Number Portability
          – Local number portability – if you move within the
            same city, you can keep the same telephone number
               • Number is looked up in a database
          – Interim number portability – a telephone call can be
            forwarded to another number
               • Makes it appear that a number follows a user
               • Also implemented by Personal Communication Services
                 (PCS)




Chapter 13: Communications Services                                    14
                           Switched Circuits
     • Integrated Services Digital Network (ISDN)
          – Digital form of switched circuits
          – Combines voice, video, data on a single line
          – Narrowband ISDN
               • Speeds of 1.5 Mbps or lower
               • Made up of B channels (data) and D channels
                 (signalling)
               • Basic Rate Interface (BRI) – 2B + D
               • Primary Rate Interface (PRI) – 23 B + D



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Chapter 13: Communications Services   16
Chapter 13: Communications Services   17
                           Switched Circuits
     • Integrated Services Digital Network (ISDN)
          – Broadband ISDN (BISDN)
               •   Speeds above 1.5 Mbps
               •   Common speeds of 25 Mbps up to Gigibit speeds
               •   Newer technology than ISDN
               •   Used with frame relay, ATM and SMDS (fast
                   services described later)




Chapter 13: Communications Services                            18
                         Dedicated Circuits
     • Leased circuit from a common carrier
     • Data travels over the same line to its
       destination (as opposed to dial-up line)
     • Line is available 24 hours a day, 7 days a
       week




Chapter 13: Communications Services             19
                         Dedicated Circuits
     • Voice-Grade Circuits
          – Intended for voice conversations
          – Operates at 4000 Hz
          – Category 3 twisted pair cable used
          – Formerly analog, now primarily digital lines
     • Wideband Analog Circuits
          – Multiples of voice-grade circuits
          – 48,000 Hz can handle 12 voice-grade
            channels

Chapter 13: Communications Services                        20
                         Dedicated Circuits
     • T-Carrier Circuits
          – Dedicated digital lines
          – Switched 56 service used prior to T-1
               • Maximum transmission was 56 Kbps
               • Uses a digital service unit (DSU) instead of a
                 modem
          – Digital Signal (DS) service
               • Hierarchy of digital signals
               • Gives a name to each level of service



Chapter 13: Communications Services                               21
Chapter 13: Communications Services   22
                         Dedicated Circuits
     • T-Carrier Circuits
          – DS levels are associated with T-Carrier lines
          – T-1 transmit at 1.5 Mbps
          – Fractional T1
               • For those who don’t need the entire capacity of T1
               • Transmit at some fraction of 1.5 Mbps
                     – 128 Kbps, 256 Kbps, 384 Kbps, 512 Kbps, 768 Kbps




Chapter 13: Communications Services                                       23
                         Dedicated Circuits
     • Digital Subscriber Line
          – Fast transmission over dedicated circuit
          – Used for residential and small business
            customers
          – No dial up is needed
          – Special modem (splitter) is used




Chapter 13: Communications Services                    24
                         Dedicated Circuits
     • Digital Subscriber Line
          – Types
               • Asymmetric DSL (ADSL)
                     – Upstream and downstream speeds are different
               • ADSL Lite
                     – Slower version of ADSL
               • High Bit-Rate DSL (HDSL)
                     – Oldest DSL
                     – Transmission speed 1.5 Mbps
                     – HDSL2 transmits up to 3Mbps




Chapter 13: Communications Services                                   25
                         Dedicated Circuits
     • Digital Subscriber Line
          – Types
               • ISDN DSL (IDSL)
                     – Transmits digital data on twisted pair lines
                     – Bypasses central office switches
                     – Transmits 128 Kbps in each direction
               • Rate Adaptive DSL (RADSL)
                     – Modems adjust transmission rate based on network
                       conditions
                     – 40 Kbps to 7 Mbps (downstream), 768 Kbps (upstream)



Chapter 13: Communications Services                                      26
                         Dedicated Circuits
     • Digital Subscriber Line
          – Types
               • Very High-Rate DSL (VDSL)
                     – Must be 1,000 feet or less from central office
                     – Used by the telephone company to connect parts of the
                       telephone network
                     – 12.96 Mbps to 55 Mbps (downstream), 3 Mbps
                       (upstream)
               • Voice Over DSL (VoDSL)
                     – Voice and data on the same copper line
                     – Special version of Voice over IP (VoIP)


Chapter 13: Communications Services                                        27
                         Dedicated Circuits
     • Synchronous Optical Network (SONET)
          – Used in fiber-optic networks
          – Standard set by the Exchange Carriers
            Standards Association (ECSA)
          – Subset of Synchronous Digital Hierarchy
            (SDH)
          – Optical Carrier levels defined at the physical
            layer of the OSI model
          – Speeds from 51.8 Mbps to 2.48 Gbps


Chapter 13: Communications Services                          28
Chapter 13: Communications Services   29
                     Fast Packet Services
     • Faster, more reliable transmission
       services
     • X.25
          – First fast packet service
          – Uses store-and-forward on packet switching
            networks
          – Servers and clients do not have to transmit at
            the same speed
          – Ensures data integrity
Chapter 13: Communications Services                      30
                     Fast Packet Services
     • Frame Relay
          – Faster than X.25 at a lower cost
          – No data integrity
          – Packets switched to destination without store-and-
            forward
          – Little or no error checking done
          – Digital networks have few errors, so frame relay
            works well
          – Upper layer software must catch any errors



Chapter 13: Communications Services                              31
Chapter 13: Communications Services   32
                     Fast Packet Services
     • Frame Relay
          – No flow control, so some frames may be
            discarded if not accepted by receiving device
          – Users specify lines speed and Committed
            Information Rate (CIR)
               • CIR is maximum rate for delivery of packets
               • If more transmitted than the CIR, some will be
                 discarded
               • Notifications can be sent if packet transmission is
                 slowing due to network conditions


Chapter 13: Communications Services                                33
                     Fast Packet Services
     • Asynchronous Transfer Mode (ATM)
          – Very fast form of packet switching
          – Packets are called “cells”
               • 53 bytes long
               • Transmitted on a virtual circuit
          – Transmits video, audio, data
          – Used to implement BISDN
          – 1.5 Mbps to 622.1 Mbps


Chapter 13: Communications Services                 34
                     Fast Packet Services
     • Asynchronous Transfer Mode (ATM)
          – No error detection done, which makes
            transmission much faster
          – Validity of data checked at upper layers
          – Connection-oriented network
               • Permanent virtual circuit (PVC)
               • Switched virtual circuit (SVC)
               • Quality of Service (QoS)



Chapter 13: Communications Services                    35
Chapter 13: Communications Services   36
                     Fast Packet Services
     • Switched Multimegabit Data Service
       (SMDS)
          – Used on packet switching networks
          – Used for intermittent data transmission
          – One use is to connect LANs via a switched
            WAN circuit for large data transfers
          – Speeds between 1 Mbps and 34 Mbps
          – Connectionless service


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Chapter 13: Communications Services   39
                     Fast Packet Services
     • Advanced Intelligent Networks
          – Signaling System 7 (SS7)
               • Used in 1970s to implement packet switching on
                 the telephone network
               • It is a protocol
               • Allowed services such as caller ID
               • Services controlled by signaling transfer points
                 (STPs)




Chapter 13: Communications Services                                 40
Chapter 13: Communications Services   41
                     Fast Packet Services
     • Advanced Intelligent Networks
          – Intelligent Network 1 (IN/1)
             • Developed to provide logic for new services
             • Service control points (SCPs)
                     – Database to store special telephone services
                     – 800 service, calling card were introduced




Chapter 13: Communications Services                                   42
                     Fast Packet Services
     • Advanced Intelligent Network (AIN)
          – Advanced Intelligent Network (AIN)
               • Improvement on IN/1
               • Many new services
                     –   866, 877, 888 as well as 800
                     –   Disaster recovery service
                     –   Area number calling service
                     –   Do not disturb service
                     –   Alternate destination on busy




Chapter 13: Communications Services                      43
                     Fast Packet Services
     • Multiprotocol Label Switching (MPLS)
          – Allows connection of many different types of networks
          – Combines layer 2 switching with layer 3 routing
          – Defines a set of protocols to allow fast packet
            services to operate over IP networks
          – Label switching routers (LSRs) used to route packets
          – Label edge routers (LERs) labels packets and route
            them on the MPLS network




Chapter 13: Communications Services                            44
Chapter 13: Communications Services   45
                                      Summary
     • Standards such as narrowband, wideband,
       and broadband have been developed for
       communications services
     • Many types of switching centers in the
       telephone system
     • Point of connection is used for business
       and residential customers
     • Local exchange carriers provide service


Chapter 13: Communications Services             46
                                      Summary
     • Virtual Private Networks use components
       of private and public networks
     • ISDN was the first totally digital network
     • Several types of dedicated circuits exist,
       from voice-grade circuits to DSL
     • SONET is used on fiber-optic networks




Chapter 13: Communications Services                 47
                                      Summary
     • X.25, frame relay, ATM, and SMDS are
       packet-switching services
     • Telephone system has evolved into a very
       sophisticated intelligent system
     • MPLS allows the interconnection of many
       of the fast switching systems in today’s
       networks


Chapter 13: Communications Services             48
Chapter 13 Complete

				
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