PSDNs

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)

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 LeasedLine
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
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
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
Circuit-Switched PSDNs

• End-to-End Capacity is Guaranteed
• If you need it, it is always there
• When you dont 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