Chapter 12

Wide Area & Large Scale Networks

WAN

• Wide Area Network
• Spans large geographical area
• Distance poses problems of:
  • Maintaining
  • Administering
  • Troubleshooting

Links for WANs

• Packet-switching networks
• Fiber-optic cable
• Microwave transmitters
• Satellite links
• Cable television coaxial links

Communication Technologies

• Communicate between LANs across WANs:
  • Analog
  • Digital
  • Packet switching

Analog Connectivity

PSTN Line Types

Digital Connectivity

Digital Data Service (DDS)

• Dedicated @ 2.4, 4.8, 9.6, or 56Kbps
• Line types:
  • ISDN
  • T1
  • T3
  • Switched 56K

CSU/DSU

• Channel Service Unit/Data Service Unit
• Digital "modem"
• Bridge or router sends data to CSU/DSU
• Sends out to receiving CSU/DSU
• Receiving sends to bridge or router

T1

• Two pair wire up to 1.544 Mbps
• One pair transmits, one receives
• 24 channels each with 64 Kbps
• Channel subscription is Fractional
• T1Data, voice, & narrow band video

Multiplexing

• "Muxing" is common terminology
• Several data streams encoded
• Transmitted on same cable segment
• T1 uses Muxing to mix data sources on a single cable
• Upon reception signal is decoded

Channel Divisions

• T1 comprised of 24 separate channels
• Each channel samples data 8,000/second
• Each sample comprises 8 bits
• Each data sample = 64Kbps= DS-0
• DS-1 = 24 DS-0 channels = one T1 line
• DS specifications categorize DDS lines

T1 Channels

T3

• Equal to 28 T1 lines
• 672 channels
• Data rate of 44.736 Mbps
• Both T3 & fractional T3 can be leased
• It is common for several T1s to be replaced by a T3

Switched 56K

• Older point to point digital link
• On demand link
• Leased from local and long distance telcos
• Usage charged on per minute basis

Packet Switching Networks

Packet Switching Networks

• Data is segmented into packets
• Labeled with sequence order & address
• Individual packet is sent
• Header is read on receipt
• Data is reconstructed, any missing packets requested to be resent

Virtual Circuits

• Temporary pathway between two points
• No cable between endpoints
• Virtual circuit of logical connections
• Switched virtual circuits (SVC)
• Permanent virtual circuit (PVC)

Virtual Private Networks (VPN)

• Temporary or permanent connections
• Across a public network - the Internet
• Encrypted data for security
• Point to Point Tunneling protocol (PPTP)
• Access through an ISP
• Single modem on server
• Usually accessed by local phone number
• Extends the span of a network
• Saves money for user & network

Advanced WAN Technologies

• X.25 Public Data Networks (PDN)
• Popular outside North America
• Data Terminal Equipment (DTE)
• Data Communications Equipment (DCE)
• Being replaced by frame relay & ATM

ISDN

• Integrated Services Digital Network
• Single channel links of 64 Kbps
• Sufficient for 10Mbps networks
• Small office/home office (SOHO)
• 2 to 4 times faster than POTS(plain old telephone system)

Frame Relay

• Point to point PVC
• Fiber-optic links mostly used
• Error checking left to end devices
• Use charged by bandwidth allocation
  • Committed Information Rate (CIR)
• CSU/DSU router or bridge at end

Asynchronous Transfer Mode

• ATM
• Packet switching of 53 byte fixed length
• Rates of 155-622Mbps
• ATM protocol data unit (PDU) = cell
• Interoperates with other network types

FDDI

• Fiber Distributed Data Interface
• Operates at 100Mbps
• Fiber-optic media
• Dual rings allow multiple users to transmit simultaneously

FDDI Network

Synchronous Optical Network

• SONET
• Optical carrier levels (OC)
• OC-1 = 51.84 Mbps
• OC-2 etc are multiples of OC-1
• Specified to OC-24 2.48Gbps

SMDS