1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

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
Virtual Circuit

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 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