Metropolitan and Wide Area Networks

1
Chapter 9

• Metropolitan and Wide Area Networks

2
Outline

• Introduction
• Circuit Switched Networks
• Dedicated Circuit Networks
• Packet Switched Networks
• Virtual Private Networks
• Best practice MAN/WAN design
• Improving MAN and WAN Performance

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Introduction

• Metropolitan area networks (MANs)
• Span from 3 to 30 miles and connect backbone
  networks (BNs) and LANs
• Wide area networks (WANs)
• Connect BNs and MANs across longer distances,
  often hundreds of miles or more
• Typically built by using leased circuits from
  common carriers such as ATT
• Most organizations cannot afford to build their
  own MANs and WANs,

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Introduction (Cont.)

• Focus of the Chapter
• Examine MAN/WAN architectures and technologies
  from a network manager point of view
• Focus on services offered by common carriers (in
  North America), and how they can be used to build
  networks
• Regulation of services
• Federal Communications Commission (FCC) in the US
• Canadian Radio Television and Telecomm Commission
  (CRTC) in Canada
• Public Utilities Commission (PUC) in each state
• Common Carriers
• Local Exchange Carriers (Less) like Verizon, Bell
  South
• Interexchange Carriers (IXCs) like ATT

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Services Used by MANs/WANs

• Circuit Switched Network Services
• Dedicated Circuit Networks Services
• Packet Switched Networks Services
• Virtual Private Networks Services

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Circuit Switched Services

• Oldest and simplest MAN/WAN approach
• Uses the Public Switched Telephone Network (PSTN)
• i.e., telephone networks
• Provided by common carriers like ATT and
  Ameritech
• Basic types in use today
• POTS (Plain Old Telephone Service)
• Via use of modems to dial-up and connect to ISPs
• ISDN (Integrated Services Digital Network )

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POTS based Circuit Switched Services

• Use regular dial-up phone lines and a modem
• Modem used to call another modem
• Once a connection is made, data transfer begins
• Commonly used to connect to the Internet by
  calling an ISPs access point
• Wide Area Telephone Services (WATS)
• Wholesale long distance services used for both
  voice and data
• Users buy so many hours of call time per month
  (e.g., 100 hours per month) for one fixed rate

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ISDN based Circuit Switched Services

• Integrated Services Digital Network
• Combines voice, video, and data over the same
  digital circuit
• Sometimes called narrowband ISDN
• Provides digital dial-up lines (each requires)
• An ISDN modem which sends digital transmissions
  is used
• Also called Terminal Adapter (TA)
• An ISDN Network Terminator (NT-1 or NT-2)
• Each NT needs a unique Service Profile Identifier
  (SPID)
• Acceptance has been slow
• Lack of standardization, different
  interpretations. and relatively high cost

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Types of ISDN Services

• Basic rate interface (BRI)
• Basic access service or 2BD
• Two 64 Kbps bearer B channels (for voice or
  data)
• One 16 Kbps control signaling D channel
• Can be installed over existing telephones lines
  (if less than 3.5 miles)
• Requires BRI specific end connections
• Primary rate interface (PRI)
• Primary access service or 23BD
• Twenty three 64 Kbps B channels
• One 64 Kbps D channel (basically T-1 service)
• Requires T1 like special circuit

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Broadband ISDN

• A circuit-switched service but it uses ATM to
  move data
• Backwardly compatible with ISDN.
• B-ISDN services offered
• Full duplex channel at 155.2 Mbps
• Full duplex channel at 622.08 Mbps
• Asymmetrical service with two simplex channels
  (Upstream 155.2 Mbps, downstream 622.08 Mbps)

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Circuit Switched Services

• Simple, flexible, and inexpensive
• When not used intensively
• Main problems
• Varying quality
• Each connection goes through the regular
  telephone network on a different circuit,
• Low Data transmission rates
• Up to 56 Kbps for POTS, and up to 1.5 Mbps for
  ISDN
• An alternative
• Use a private dedicated circuit
• Leased from a common carrier for the users
  exclusive use 24 hrs/day, 7 days/week

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Dedicated Circuits

• Leased full duplex circuits from common carriers
• Used to create point to point links between
  organizational locations
• Routers and switches used to connect these
  locations together to form a network
• Billed at a flat fee per month (with unlimited
  use of the circuit)
• Require more care in network design
• Basic dedicated circuit architectures
• Ring, star, and mesh
• Dedicated Circuit Services
• T carrier services
• Synchronous Optical Network (SONET) services

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T-Carrier Services

• Most commonly used dedicated digital circuits in
  North America
• Units of the T-hierarchy
• DS-0 (64 Kbps) Basic unit
• T-1 (a.k.a. DS-1) (1.544 Mbps)
• Allows 24 simultaneous 64 Kbps channels which
  transport data or voice messages using PCM
• T-2 (6.312 Mbps) multiplexes 4 T-1 circuits
• T-3 (44.376 Mbps) 28 T-1 capacity
• T-4 (274.176 Mbps) 178 T-1 capacity (672 DS-0
  channels)
• Fractional T-1, (FT-1) offers a portion of a T-1

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T-Carrier Digital Hierarchy

• T-Carrier Designation DS Designation Data
  Rate

T-1 T-2 T-3 T-4
DS-0 DS-1 DS-2 DS-3 DS-4
64 kbps 1.544 Mbps 6.312 Mbps 33.375
Mbps 274.176 Mbps
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Synchronous Optical Network (SONET)

• ANSI standard for optical fiber transmission in
  Gbps range
• Similar to ITU-T-based, synchronous digital
  hierarchy (SDH)
• SDH and SONET can be easily interconnected
• SONET hierarchy
• Begins with OC-1 (optical carrier level 1) at
  51.84 Mbps
• Each succeeding SONET hierarchy rate is defined
  as a multiple of OC-1

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SONET Digital Hierarchy

• SONET Designation SDH Designation Data
  Rate

OC-1 OC-3 OC-9 OC-12 OC-18 OC24 OC-36 OC-48 OC-192
STM-1 STM-3 STM-4 STM-6 STM-8 STM-12 STM-16
51.84 Mbps 155.52 Mbps 466.56 Mbps 622.08
Mbps 933.12 Mbps 1.244 Gbps 1.866 Gbps 2.488
Gbps 9.952 Gbps
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Packet Switched Services

• Remember - In both circuit switched and dedicated
  services
• A circuit established between two computers
• Solely assigned for use only between these two
  computers
• Data transmission provided only between these two
  computers
• No other transmission possible until the circuit
  is closed
• Now, Packet switched services
• Enable multiple connections to exist
  simultaneously between computers over the same
  physical circuits
• User pays a fixed fee for the connection to the
  network plus charges for packets transmitted

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Packet Switching

• Interleave packets from separate messages for
  transmission
• Most data communications consists of short burst
  of data

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Packet Routing Methods

• Describe which intermediate devices the data is
  routed through
• Connectionless (Datagram)
• Adds a destination and sequence number to each
  packet
• Individual packets can follow different routes
• Packets reassembled at destination (by using
  their sequence numbers)
• Connection Oriented (Virtual Circuit (VC))
• Establishes an end-to-end circuit between the
  sender and receiver (before the packets sent)
• All packets for that transmission take the same
  route over the virtual circuit established
• Same physical circuit can carry many VCs

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Types of Virtual Circuits

• Permanent Virtual Circuit (PVCs)
• Established for long duration (days or weeks)
• Changed only by the network manager
• More commonly used
• Packet switched networks using PVCs behave like a
  dedicated circuit networks
• Switched Virtual Circuit (SVC)
• Established dynamically on a per call basis
• Disconnected when the call ends

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Packet Switched Service Protocols

• X.25
• Asynchronous Transfer Mode (ATM)
• Frame Relay
• Switched Multimegabit Data Service (SMDS)
• Ethernet/IP packet networks

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X.25

• Oldest packet switched service
• A standard developed by ITU-T
• Offers SVC and PVC services
• Uses LAPB and PLP protocols at the data link and
  network layers, respectively
• Requires protocol translations at PADs (for those
  users who use different protocols at their LANs)
• A reliable protocol (it performs error control
  and retransmits bad packets)
• Widely used in Europe
• Not in widespread use in North America
• Low data rates (64 Kbps) (available now at 2.048
  Mbps)

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Asynchronous Transfer Mode (ATM)

• Newer than X.25 also standardized
• ATM in MAN/WAN similar to ATM technology
  discussed for BNs
• Similar to X.25
• Provides packet switching service
• Different than X.25 Operating characteristics
• Performs encapsulation (no translation) of
  packets
• Provides no error control (an unreliable
  protocol)
• Provides extensive QoS information
• Scaleable (easy to multiplex ATM circuits onto
  much faster ones)

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Error Control in X.25 vs. ATM
Error control in ATM is handled typically the
transport layer (providing end-to-end
communications)
ACKs sent immediately by each node
ACKs sent by final destination
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ATM Features

• Uses fixed length, 53 byte cells
• 5 bytes of overhead and 48 bytes of user data
• More suitable for real time transmissions.
• Provides extensive QoS information
• Enables setting of precise priorities among
  different types of transmissions (i.e. voice,
  video e-mail)
• Data Rates
• Same rates as SONET 51.8, 466.5, 622.08 Mpbs
• New versions T1 ATM (1.5 Mbps), T3 ATM (45 Mbps)

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Frame Relay

• Another standardized technology
• Faster than X.25 but slower than ATM
• Encapsulates packets
• Packets delivered unchanged through the network
• Unreliable, like ATM
• Up to the end-points to control the errors
• NO QoS support (under development)
• Common CIR speeds
• 56, 128, 256, 384 Kbps, 1.5, 2, and 45 Mbps

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Ethernet/IP Packet Networks

• Offer Ethernet/IP packet services for building
  MAN/WAN networks
• Gigabit Ethernet fiber optic networks (bypassing
  common carrier network)
• Currently offer CIR speeds from 1 Mbps to 1 Gbps
  at 1/4 the cost of more traditional services
• No need to translate LAN protocol (Ethernet/IP)
  to the protocol used in MAN/WAN services
• X.25, ATM, Frame Relay and SMDS use different
  protocols requiring translation from/to LAN
  protocols
• Emerging technology expect changes

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Virtual Private Networks

• Provides equivalent of a private packet switched
  network over public Internet
• Use PVCs (tunnels) that run over the Internet
• Appear to the user as private networks
• Encapsulate the packets sent over these tunnels
• Using special protocols that also encrypt the IP
  packets they enclose
• Provides low cost and flexibility
• Uses Internet Can be setup quickly
• Disadvantages of VPNs
• Unpredictability of Internet traffic
• Lack of standards for Internet-based VPNs, so
  that not all vendor equipment and services are
  compatible

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VPN Architecture
ISP
Access Server
VPN Device
leased circuits
Telephone Line
Office
VPN Device
Employees Home
Internet
VPN Tunnel
Backbone
VPN Tunnel
Office
VPN Device

• VPN is transparent to the users, ISP, and the
  Internet as a whole
• It appears to be simply a stream of packets
  moving across the Internet

Backbone
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VPN Encapsulation of Packets
Packet from the client computer
Packet in transmission through the Internet
PPP
IP
TCP
SMTP
ATM
IP
L2TP
PPP
IP
TCP
SMTP
ISP
L2TP Layer 2 Tunneling Protocol (An emerging VPN
Layer-2 access protocol)
Telephone Line
VPN Device
Access Server
Employees Home
Packet from the VPN
VPN Tunnel
PPP
IP
TCP
SMTP
Outgoing packets from the VPN are sent through
specially designed routers or switches.
Internet
VPN Device
Access Server
Backbone
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VPN Types

• Intranet VPN
• Provides virtual circuits between organization
  offices over the Internet
• Extranet VPN
• Same as an intranet VPN except that the VPN
  connects several different organizations, e.g.,
  customers and suppliers, over the Internet
• Access VPN
• Enables employees to access an organization's
  networks from remote locations

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MAN/WAN Design Practices

• Difficult to recommend best practices
• Services, not products, being bought
• Fast changing environment with introduction of
  new technologies and services from
  non-traditional companies
• Factors used
• Effective data rates and cost
• Reliability
• Network integration
• Design Practices
• Start with flexible packet switched service
• Move to dedicated circuit services, once
  stabilized
• May use both packet switched services as backup

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MAN/WAN Services
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Recommendations
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Improving MAN/WAN Performance

• Handled in the same way as improving LAN
  performance
• By checking the devices in the network,
• By upgrading the circuits between computers
• By changing the demand placed on the network

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Improving Device Performance

• Upgrade the devices (routers) and computers that
  connect backbones to the WAN
• Select devices with lower latency
• Time it takes in converting input packets to
  output packets
• Examine the routing protocol (static or dynamic)
• Dynamic routing
• Increases performance in networks with many
  possible routes from one computer to another
• Better suited for bursty traffic
• Imposes an overhead cost (additional traffic)
• Reduces overall network capacity
• Should not exceed 20

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Improving Circuit Capacity

• Analyze the traffic to find the circuits
  approaching capacity
• Upgrade overused circuits
• Downgrade underused circuits to save cost
• Examine why circuits are overused
• Caused by traffic between certain locations
• Add additional circuits between these locations
• Capacity okay generally, but not meeting peak
  demand
• Add a circuit switched or packet switched service
  that is only used when demand exceeds capacity
• Caused by a faulty circuit somewhere in the
  network
• Replace and/or repair the circuit
• Make sure that circuits are operating properly

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Reducing Network Demand

• Determine impact on network
• Require a network impact statement for all new
  application software
• Use data compression of all data in the network
• Shift network usage
• From peak or high cost times to lower demand or
  lower cost times
• e.g., transmit reports from retail stores to
  headquarters after the stores close
• Redesign the network
• Move data closer to applications and people who
  use them
• Use distributed databases to spread traffic across

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Implications for Management

• Changing role of networking and telecom managers
• Increased and mostly digitized data transmission
  causing the merger of these positions
• Changing technology
• Increasing dominance of VPNs, Frame Relay and
  Ethernet/IP
• Decreasing cots of setting up MANs/WANs
• Changing vendor profiles
• From telecom vendors to vendors with Ethernet and
  Internet experiences