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 LeasedLine

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

PSDN

AccessDevice

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

LeasedLine PSDN

POP

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

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

MultiplexedTrunk 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

VirtualCircuit

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

– 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