Specialized Network Services

1
Lecture 8

• Specialized Network Services

2
Overview

• This chapter explores
• high-speed services
• digital network services
• Will discuss
• how they work
• speed and capacity
• applications in which these services are used

3
Services to be Discussed

• T-1 and T-3
• ISDN
• ATM
• DSL
• Sonet

4
What in common?

• All technologies to be discussed today are
  digital services
• Meet requirements for high speed and accurate
  transmission

5
T-1 -- Voice or Data

• Developed in 1960s to save on cabling
• Scheme designed to carry 24 voice channels over
  one telephone circuit
• In order to reduce number of cables wired between
  telephone company switches
• Available to user locations in 1983

6
Circuit

• A path for electrical transmissions
• between two points

7
Initial Users and Applications

• Large organizations, universities, financial
  institutions
• Used to connect locations in support of
  applications such as payroll, inventory
• Cost was high
• Better than hand carrying huge data tapes from
  location to location

8
T-1 in 80s

• High rates
• Service took months to install
• Needed long lead times to plan for installation
• T-1 digital, needed to be connected to analog
  system by using channel banks

9
Channel Banks

• Multiplexing device used to connect T-1 circuits
  to analog PBX and CO systems
• samples signals from PBX systems
• digitizes these signals
• sends down the T-1 channel
• channel bank on other end decodes digital signal
  back to analog signal
• PCM pulse code modulation (page 183)

10
Channel Banks

• Also used to split up the use of T-1 line into
  voice and data applications
• Controls the communication to the devices to
  which it connects, PBX or computer
• Can be used to break out each of the 24 T-1
  channels

11
Sampling

• Converting continuous signals, like voice or
  video, into discrete values, e.g. digital signals
• Sampling rate
• The number of times per second that an analog
  signal is measured and converted to a binary
  number, most commonly used algorithm is 8,000 per
  second

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PCM

• Pulse Code Modulation
• most common method of encoding an analog voice
  signal into a digital bit stream

13
Digital Signal Levels

• DS-O 64,000BPS
• DS-1 1,544,000BPS
• speed at which T-1 and T-2 lines run

14
DS levels

• DS-1
• refers to entire T-1 line
• entire bandwidth of the circuit
• 1,544Mbps

• DS-O
• refers to each speed of each channel in T-1
• 64Kbps

15
European, Japanese and US T-standards
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Time Division Multiplexing

• All T carrier signals are based on division
  multiplexing
• each device which communicates over a T-line is
  assigned a time slot
• 8 devices communicating, 8 time slots assigned
• assigned for duration of the communication, even
  when no information being transmitted
• TDM not efficient in handling a WAN

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Idle Time Slots

• Pauses in data transmission result in idle time
  slots
• In a network with millions of time slots, empty
  time slots represent waste or inefficient use of
  transmission capacity
• ATM and IP do not assign specific time slots to
  each device

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T-1 Configurations

• T-1 circuit has 24 lines available for voice,
  data, or video
• voice usually connected to a PBX
• data usually bypasses the PBX
• video can bypass or be routed through the PBX,
  depending on weather it can transmit at high
  speeds

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Digital Cross Connects

• Used to connect multiple sites with dedicated
  lines
• Digital cross connects provided by telcos
• Used to divide a T-1 connection over several users

20
Digital Cross Connectivity
6 lines

T-1 Mux
6 lines
12 lines
Digital Cross Connect
21
T-3

• Equivalent to 28 T-1 lines or 672 channels
• 28 X 24 672
• Total speed 44,736, includes bits for overhead
  and 28 X 1.544Mbs
• Used for businesses with large calling volume
• Fractional T-3 service, partial use of a T-3
  circuit

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ISDN

• Integrated Services Digital Network
• BRI ISDN basic rate
• PRI ISDN primary rate
• Worldwide public standard for sending voice,
  video, and data or packets over public switched
  network

23
Characteristics ISDN

• Digital connectivity consistent, high quality
  service
• Out of channel signaling call set up fast, total
  bandwidth of each channel dedicated to transfer
  of user data
• Switched service fees based on usage time
• Standard interface all ISDN users can interface
  with each other

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ISDN in the Real World

• Works on copper wiring
• Availability an issue
• Takes long time to install
• Poor support from providers
• 1 of US access lines equipped in US
• Must be within 18,000 feet from central office

25
Basic Rate Interface

• Two bearer channels (voice, data, video)
• 64Kps
• One signaling channel
• 16Kps

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Uses Basic Rate Interface

• Internet access
• desktop video conferencing
• Centrex ISDN multi-line telephones
• use of D signal for credit card verification
• downloading software
• work-at-home
• connecting remote local area networks

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ISDN for work at home

• To share line for voice and other
• need a terminal adapter with an NT-1 (network
  terminating device)
• used to connect non-ISDN equipment to the ISDN
  line

28
ISDN and Analog Lines

• ISDN is a digitally based technology
• Can not transmit to an analog line
• Can only communicate with ISDN-equipped services
• Must have matching modems on each end of the
  connection

29
Pricing BRI

• Ranges considerably
• Installation fees 150-300
• Monthly fees 10-40 more than charge for analog
  line
• Per minute, sometimes flat rate for 140-300
  monthly hours

30
Primary Rate Interface ISDN

• PRI has 24 channels
• 23 channels are bearer channels
• 1 channel used for signaling
• Similar to a T-1 both have 24 channels
• T-1 in-band signaling
• PRI out-of-band signaling on 24th channel
• Trunk connection

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Trunk Connection

• A communication line between
• two
• switching systems.

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Uses PRI ISDN

• Video conferencing (128 to 384 Kbps)
• Receiving the calling party numbers at large call
  centers
• backing up LAN-to-LAN connections
• backing up dedicated lines
• Internet service providers for dial in from BRI
  ISDN customers
• Corporate sites for remote access

33
PBXs with PRI Trunks

• Calls and signals sent separately
• PBX may send the signal to a database to match
  the number and then pull the data
• call centers
• call screening

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PRI Bandwidth on Demand

• PRI use of channels dynamic always open for
  voice or data
• setup signals notifies network how to route the
  call
• voice public network
• data data network
• T-1 channels permanently set aside for voice or
  data purposes

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Digital Subscriber Line

• Initially used to transmit video and TV signals
  from CO to end-users over copper
• Main application has changed to Internet access
• ADSL asymmetric digital subscriber line
• line has different speeds from and to customer
• upstream slower than downstream connection

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Digital Subscriber Line

• Many different flavors
• speeds may vary
• distance from CO varies
• Has potential to relieve network congestion
  caused by modem traffic on PSTN
• DSL traffic routed onto a data network when it
  hits the CO switch

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DSL Flavors

• ADSL asymmetric, upstream slower than downstream
• DSL Lite Universal DSL, ITU-based standard for
  residential consumers, offered by local exchange
  providers
• HDSL high-bit-rate, two competing flavors
• VDSL very-high-bit-rate for TV and Internet usage

38
DSL Applications

• Alternative to cable modem for Internet
• CLEC to serve data only communications
• IXC to provide high speed local voice, Internet
  access, and support telecommuting
• Internet service providers

39
Obstacles DSL

• Availability been slow, now popular
• Cost high to install for providers
• loading coils and bridge caps must be removed by
  providers
• Not easy to install for end users

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Technical Explanation DSL

• Uses unused frequencies available in the
  telephone wire for data transmission
• Voice carried between 300-3300 Hrz
• DSL uses rest of frequencies available for data
  communications
• DMT discrete multi-tone modulation used to send
  data in frequencies over 4,000Hz

41
Technical Explanation DSL

• DMT used compression algorithms to encode and
  compress signals
• Different frequencies are used for the upstream
  and downstream portions of the channel
• Routing functionality enables multiple users to
  share one DSL line

42
DSL Lite

• Developed to lower the cost of providing DSL
  service
• also referred to as Universal DSL
• known as a splitterless DSL
• save telcos from having to dispatch a technician
  to install splitter
• Works consistently on longer telephone company
  loops, within 3 miles of telco CO
• no internal wiring changes required on part of
  user

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Splitter

• Used with full rate ADSL
• sits between the customer demarcation point and
  the ADSL line and another at telco
• separates the low frequencies voice signals from
  the high frequency data signals
• small in size

44
Concatenation

• Puts streams of data into one fat or one high
  bandwidth contiguous stream
• Joining several fibers together end-to-end
• To link together in a series or chain

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DSLAMs

• Digital subscriber lines access multiplexers
• Located at network providers site
• Take traffic from multiple DSL lines and combine
  into higher speeds before sending it out to
  Internet or data network
• Can stream video together rather than separating
  it with data

46
DSL for Internet Access

• Competes against cable modems, T-1, ISDN and high
  speed modems
• Suitable for
• Internet access
• Telecommuters to corporate databases
• High bandwidth applications
• Telecom Act notable impetus to this service

47
Frame Relay

• Network access method offered by local and long
  distance providers
• Shared by multiple users
• Acts like a private, dedicated network
• Leasing dedicated lines not necessary
• Alternative to those building their network

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Frame Relay

• First implemented in 1992
• Allows customers to transmit data between
  multiple locations
• Used for
• LAN connectivity
• Internet access
• connections for remote users

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Frame Relay

• Advantages
• network managed by provider
• less hardware required at each location
• capacity on frame relay flexible
• has internal backup routes

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Frame Relay

• Shared network
• Fast, due to low error checking
• Customer must check data upon arrival
• X.25 packet network, oldest most widely used,
  slower due to extensive error checking into every
  packet transmitted
• Excels at LAN-to-LAN communications

51
ATM

• Asynchronous transfer mode
• High speed, carrier voice, data, video, and
  multimedia images (13.22 Gbps)
• Carries multiple types of traffic over a single
  connection
• Higher cost, higher speed, more flexible service
  than frame relay

52
ATM

• Packages data that it switches into discrete
  cells
• Requires less handling than variable-sized
  packages
• Each cell
• 53 bytes
• 5 bytes of these 53 are header cells

53
ATM Header Cells

• Identifies type of information contained in the
  cell (voice, data, video)
• Prioritization of cells based on content
• voice and video need constant transmission rate
• prioritization done to avoid delays
• Other header content used for
• routing
• putting the cells in the correct sequence
• error checking

54
ATM Switching Hardware

• ATM is fast because cells are switched in the
  hardware
• ATM switch does not look up each cells address
  in software
• Route set up upon investigation of first cell
• All cells with same header routed the same

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SONET

• Equipment and software enable network providers
  to carry traffic from many types of customer
  equipment in a uniform way on backbone fiber
  optic cabling
• High speed portion of SONET network called
  backbone or core
• Sonet can handle traffic coming from T-1 lines,
  ATM, T-3 lines, and other sources

56
SONET

• Is a standard way to multiplex high speed traffic
  from various vendors onto fiber
• Advantages
• lower bandwidth pipes can be fed into SONET
  multiplexers and carried at high speeds
• High reliability due to ring configuration