Wide Area Networks and OSI Model

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The term enterprise WAN is used to describe a WAN that is used and controlled by a single organization.

• use public networks owned by telecommunications companies for long-distance comms
• WAN Physical layer describes the media type and interface specifications
  • provider link is a copper cable
  • uses a modem, rather than a switch
    • performs modulation of outgoing signals and demodulation of incoming data
    • works only at Physical layer
    • Modulation means transforming an electromagnetic wave to represent information
      • e.g., using the amplitude (height) of the wave to represent bits
    • Legacy modems perform digital to analog modulation for transmission over voice lines
    • analog (or dial-up) modem only supports low bandwidths (up to 56 Kbps)
    • Digital modems perform a different type of modulation
      • transform digital signals received as Ethernet frames for transmission over the WAN media
      • types
        • data service units (DSUs) for leased lines
        • digital subscriber line (DSL) modems
        • cable modems
        • satellite modems
• At Data Link layer
  •  WANs use simpler protocols than Ethernet LANs
    • links are more likely to be point to point and do not need much complexity
    • though, Ethernet is increasingly being deployed for end-to-end connectivity over WANs
• Network layer
  • customer and provider sites are addressed using Internet Protocol
  • customer edge (CE) router connects to a provider edge (PE) router
    • via Link layer interface