2) Uses of Networks

Example Uses of Networks:

• Work:
– Email, file sharing, printing, …
• Home:
– Movies / songs, news, calls / video / messaging, e-commerce, …
• Mobile:
– Calls / texts, games, videos, maps, information access …

For User Communication:

• From the telephone onward:
– VoIP (voice-over-IP)
– Video conferencing
– Instant messaging
– Social networking
→Enables remote communication

– Need low latency for interactivity

For Resource Sharing:

• Many users may access the same underlying resource
– E.g., 3D printer, search index, machines in the cloud
→More cost effective than dedicated resources per user
– Even network links are shared via statistical multiplexing »

Statistical Multiplexing:

• Sharing of network bandwidth between users according to the statistics of their demand
– (Multiplexing just means sharing)
– Useful because users are mostly idle and their traffic is bursty
• Key question:

– How much does it help?
• Example: Users in an ISP network
– Network has 100 Mbps (units of bandwidth)
– Each user subscribes to 5 Mbps, for videos
– But a user is active only 50% of the time …
• How many users can the ISP support?
– With dedicated bandwidth for each user: 
– Probability all bandwidth is used: 
(assuming independent users)





• With 30 independent users, still unlikely (2% chance) to need more than 100 Mbps!
– Binomial probabilities
→Can serve more users with the same size network
– Statistical multiplexing gain is 30/20 or 1.5X
– But may get unlucky; users will have degraded service

For Content Delivery:

• Same content is delivered to many users
– Videos (large), songs, apps and upgrades, web pages, …
→More efficient than sending a copy all the way to each user

– Uses replicas in the network »
• Sending content from the source to 4 users takes 4 x 3 = 12 “network hops” in the example

For Computer Communication:

• To let computers interact with other computers
– E.g., e-commerce, reservations
→Enables automated information processing across different parties

To Connect Computers to the Physical World:

• For gathering sensor data, and for
manipulating the world
– E.g., webcams, location on mobile
phones, door locks, …

• This is a rich, emerging usage

The Value of Connectivity:

• “Metcalfe’s Law” ~1980:
– The value of a network of N nodes
is proportional to N2
– Large networks are relatively more

valuable than small ones

• Example: both sides have 12 nodes, but the left network

has more connectivity

Previous: 1-1 Goals And Motivation

Next: 1-3 Network Components