Lecture 1 Outline Statistical Multiplexing Inter-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 R3 Switch 1Switch 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 Application Host Application 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