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Networks

Networks. Networking has become ubiquitous (cf. WWW) Two major types of networks depending on geographical distance Local Area Networks (LAN) Connect 100’s of computers within a maximum distance of a few kilometers Often limited to a single building

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Networks

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  1. Networks • Networking has become ubiquitous (cf. WWW) • Two major types of networks depending on geographical distance • Local Area Networks (LAN) • Connect 100’s of computers within a maximum distance of a few kilometers • Often limited to a single building • LANs can be connected together via bridges (also called switched networks) • Wide Area Networks (WAN or long haul networks) • Connect 1000’s of computers all over the world CSE378 Networks

  2. Network architectural issues • How are “nodes” connected? • What is a message? • Length • How are messages routed from one node to another etc. • What is the “quality of service” of the network? CSE378 Networks

  3. Some sample network topologies Ring - LAN Bus (e.g. Ethernet) - LAN Point to point Star Switch-based CSE378 Networks

  4. Typical network bandwidths • Ethernet (LAN) • originally 10 Mbit/sec; now 100 Mbit/sec. Soon Gigabit Ethernet? • Internet (successor of Arpanet - WAN) • Point to point originally with special processors (IMP: Interface message Processor); Currently copper co-axial cables yield bandwidths of 100 Mbits/sec. Fiber optics yield 1 Gbit/sec • ATM (Asynchronous transfer modes for switched networks; has been used in WANs and LANs) • 150 Mbit/sec to 2.5 Gbit/sec but high latency CSE378 Networks

  5. Message Format • Header • Request type; where it has to go; sometimes where it comes from; sequence number etc. • Payload • Data (from a few bytes to Kbytes) • Checksum • For checking the data received is correct • Long messages can be split into packets • Sending/receiving messages require complex protocols with several layers (TCP/IP, ISO standard etc. cf CSE/EE 461) CSE378 Networks

  6. A little more on LAN’s • Ethernet (shared-bus) • broadcast of messages; each node recognizes his own address • uses a “collision-detection” with back-off method • If 2 messages sent at the same time, garbage on the bus. Both senders recognize there is a garbage and resend • Since resend could be in sync, senders increase time between resends if there are lots of collisions. • Rings • Allows more than one message on the ring at a given time • ATM and “special” switches (Myrinet) are becoming popular CSE378 Networks

  7. A little more on WAN’s • Point to point topology • Store-and-forward routing • virtual circuits (telephone model). Connection is set-up for the duration of the message • packet switching • long messages split into packets • packets do not have to follow the same route. Can arrive in different order they were sent. Protocols reassemble the message • More complex hardware for routing , fault recovery etc. but better performance because can avoid congestion CSE378 Networks

  8. Parallel processors • Two basic paradigms • Shared-memory multiprocessors (communicate through “load-store” instructions) • Message-passing (communicate through “send-receive” primitives • Current trend • SMP’s (small number of processors) are shared-memory • Clusters of SMP’s connected either mode. Depends often on the type of interconnection network CSE378 Networks

  9. Interconnection networks for parallel processors • Shared-bus • Least amount of concurrency • Good only until the bus gets saturated (used to be 32 processors; now more like 4 or 8) • Cache coherence well understood • Cross-bar • Greatest amount of concurrency • Expensive in number of switches (grows like O(n2)) • Multi-stage interconnection networks • In between in terms of concurrency and complexity (O(nlogn)) CSE378 Networks

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