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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
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