Lecture 1

OutlineStatistical MultiplexingInter-Process Communication

Building Blocks

• Nodes: PC, special-purpose hardware…– hosts– switches

• Links: coax cable, optical fiber, wireless…– point-to-point

– multiple access…

Switched Networks

– two or more nodes connected by a link, or

– two or more networks connected by two or more nodes

• A network can be defined recursively as...

Strategies

• Circuit switching: carry bit streams– original telephone network

• Packet switching: store-and-forward messages or packets– Internet

Addressing and Routing

• Address: byte-string that identifies a node– usually unique

• Routing: process of forwarding messages to the destination node based on its address

• Types of addresses– unicast: single destination node– broadcast: all nodes on the network– multicast: some subset of nodes on the network

Summary

• Network constructed from a nesting of sub-networks implemented by a physical medium– >2 nodes connected by a physical link

– >2 networks connected by at least 1 node

• Providing host-to-host connectivity requires an address for each node and a means to route messages across the network to that address

• Broadcast and multicast also required functions

Resource Sharing or Multiplexing

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

STDM and WDM

• Synchronous time division multiplexing– Users assigned time quantum on a round robin basis

• A T1 line supports 24 users –packet size fixed

• Wavelength division multiplexing– Users assigned fixed frequency slot or band

• Deficiencies– Wasted time slots or bandwidth

– Cannot accommodate unknown number of users

Statistical Multiplexing• On-demand time-division• Schedule link on a per-packet basis• Packets from different sources interleaved on link• Buffer packets that are contending for the link• Buffer (queue) overflow is called congestion

…

Fairness

• Fixed packet size

• First-in-first-out

• A network that allows priority to be assigned to a packet-or max delay etc. is said to support Quality of Service (QOS)

Summary

• Statistical Multiplexing defines an efficient way for host-to-host data flows to share network resources (links and nodes) in a fine-grained manner.

• Switches schedule links on a per packet basis

• Fair allocation of capacity and congestion control are key issues.

Inter-Process Communication• Turn host-to-host connectivity into process-to-

process communication.• Fill gap between what applications expect and

what the underlying technology provides. Host Host

Application

Host

Application

Host Host

Channel

What functionality should a channel provide?-It depends on

the application.• FTP requirements

– Reading• Client-small message requesting transfer

• Server-large messege containing requested file

– Writing• Opposite

Video application requirements

• Video Conferencing• Frame=352x240x24/8=247.5 Kbytes

• Frame rate=30 /sec

– Sensitive to jitter

– Two way traffic for all users

• Video on demand– Files transferred and used

– Timing is not critical

– One way traffic

What Goes Wrong in the Network?

• Bit-level errors (electrical interference)• Packet-level errors (congestion)• Link and node failures• Messages are delayed or dropped• Messages are delivered out-of-order• Third parties eavesdrop

Interprocess or protocol must overcome these deficiencies!

Summary

• To define a useful channel one must understand the application and the limitation of underlying technology

• Challenge is to fill the gap between the application expects and what the technology can provide-The Semantics Gap