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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)
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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
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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)
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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
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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)
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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
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Chapter 13: Communications Services
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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)
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Chapter 13: Communications Services
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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.
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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
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Chapter 13: Communications Services
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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)
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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
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Chapter 13: Communications Services
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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)
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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
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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
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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
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Chapter 13: Communications Services
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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
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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
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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
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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)
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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)
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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
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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
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Fast Packet Services
• Frame Relay
– Faster than X.25 at a lower cost
– No data integrity
– Packets switched to destination without store-andforward
– Little or no error checking done
– Digital networks have few errors, so frame relay
works well
– Upper layer software must catch any errors
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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
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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
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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)
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Chapter 13: Communications Services
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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
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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)
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Chapter 13: Communications Services
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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
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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
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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
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Chapter 13: Communications Services
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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
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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
• X.25, frame relay, ATM,
and SMDS are packetswitching 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
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