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PSDNs

PSDN. PSDNs. Public Switched Data Networks Designed for data rather than voice Site-to-site switching is handled for you You merely connect each site to the PSDN “ cloud ” (No need to know internal details). PSDN. PSDNs. Connect each site to the PSDN using one leased line

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PSDNs

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  1. PSDN PSDNs • Public Switched Data Networks • Designed for data rather than voice • Site-to-site switching is handled for you • You merely connect each site to the PSDN “cloud” (No need to know internal details)

  2. PSDN PSDNs • Connect each site to the PSDN using one leased line • Only one leased line per site • With N sites, you only need N leased lines, not N* (N-1)/2 as with a full mesh 1 LeasedLine

  3. PSDN PSDNs • Access Device Needed at Each Site • Connects each site to access line • Often a router • Sometimes a device specific to a particular PSDN Technology Access Device

  4. PSDN PSDNs • Point of Presence (POP) • Place where you connect to the cloud • May be several in a city • May not have any POP close • Need leased line to POP • Separate from PSDN charges POP LeasedLine

  5. PSDNs in Perspective • Simpler than Networks of Leased Lines • Less staffing • Fewer leased lines to support • Less Expensive than Networks of Leased Lines • Less staffing • PSDN prices are very low • PSDN is less expensive overall • PSDNs are replacing many leased line mesh networks

  6. PSDN Circuit-Switched PSDNs • End-to-End Capacity is Guaranteed • If you need it, it is always there • When you don’t need it, you still pay for it • Expensive for data traffic, which usually has short bursts and long silences A bcd efg

  7. Packet-Switched PSDNs • Messages are divided into small units called packets • Short packets load switches more effectively than fewer long messages

  8. Packet-Switched PSDNs • Packet-Switched PSDNs Usually Operate at Layer 2 (Data Link Layer for Single Subnets) • Should be called frame-switched networks • Still called packet-switched networks

  9. Packet Switched PSDNs • Packets are multiplexed on trunk lines • Cost of trunk lines is shared • Packet switching lowers transmission costs • Dominates PSDN service today Multiplexed Trunk Line

  10. Packet Switched PSDNs: Virtual Circuits • All commercial packet switched PSDNs use virtual circuits • Eliminates forwarding decisions for individual packets • Reduces switching load, so reduces switching costs VirtualCircuit

  11. Unreliable PSDNs • Most commercial PSDNs are Unreliable • (Only obsolete X.25 PSDN technology was reliable) • No error correction at each hop between switches • Reduces costs of switching • Note that both virtual circuits and unreliable service reduce switching costs

  12. PSDN Cost Savings • Packet Switching • Reduces costs of transmission lines through multiplexing • Virtual Circuits • Reduces costs of switches because they do not have to make decisions for each frame • Unreliability • Reduces costs of switches because they do not have to do error correction

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