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 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 Leased Line 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 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 Leased Line PSDN

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

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 PSDN

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

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

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

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 Virtual Circuit

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

PSDN Cost Savings Packet Switching Virtual Circuits Unreliability 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