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)
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–
–
–
–
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
–
–
–
–
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
PSDN
efg
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
– 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