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Material Type: Notes; Professor: Sengupta; Class: Comptr Netwrks/Distrbutd Procs; Subject: Computer Science; University: SUNY Institute of Technology at Utica-Rome; Term: Unknown 1989;
Typology: Study notes
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Packet Switching. Migration from a single-link network to a larger Computer network: Via a switch. A switch provides multiple linkages. Switch receives packets (or frames) and decide how to deal with them. Store and Forward switch (as a Router). Receive and Transmit switch (as a Bridge). Also, Cut-through Network 1 Network 2 Network 2 Network 3 Switch Switch queue Entering packets
switches. Also, hybrids between CT and SF (Fragment-Free switching). A number of packet or data streams concurrently flow on a link (multiplexer) as different sessions and then separated (demultiplexed) by the switch. Basic switching techniques. Circuit switching. The entire session flows over a dedicated set of switches. Initial setup phase sends a probe to discover a path through switches which would be followed by all packets (or messages) till it ends. A dedicated channel is a must. Ideal for data transmission in real-time. Also known as connection oriented network. A typical circuit-switched event. Switch Switch Switch
In Datagram mode: All packets contain destination address, and all are sent asynchronously over the routers. A connection-less service. Switch (or Routers) uses a routing table to see on which port the packet to be forwarded. Routing table entries are obtained empirically from packets received at a router. In Source Routing: Each packet contains the entire path from source to destination that has to be followed. In Virtual Circuit mode: A hybrid between the above two. Has setup phase of Circuit switching. Packet switching Datagram mode Virtual Circuit mode Source Routing Virtual circuit Permanent Virtual Circuit Switched Virtual Circuit
An SVC = Temporary VC A VC is a path between two points which appears to be a discrete physical path, but comprises managed pool of circuit resources from which specific circuits emerge. A connection-oriented service. ■ Explicit connection setup and tear down phases. ■ Subsequent packets follow same route. ■ Analogy: A caravan train? ■ Each switch maintains a VC table. LAN and VLAN. A VLAN (Virtual LAN) provides logically a single- link LAN even though it is comprised of different LAN segments physically separated. VLAN is configured through software (instead of hardware) making it very flexible. Even when a computer is physically moved to another location, it still may appear on the same VLAN without any hardware reconfiguration. More later. LAN Switching
■ If destination is ambiguous or not known, it would broadcast received frames to all ports. ■ Network looping with bridges causes problems. Hence, with multiple bridges (included for redundancy) spanning tree LAN segment layout is desired. Either keep B1 or B2, but not both at the same time. But, a spanning tree has to be identified by the participating bridges themselves in a distributed framework. Challenge 1: Each bridge must find out which of its ports could be safely included in the LAN configuration so as to yield a spanning tree. B LAN 1 LAN 2 A B C K L M X Y Z W B
Challenge 2: In the event of any bridge failure, how does one replace the failed bridge by another functional one without bringing a network to a halt? The STP (Spanning Tree Protocol IEEE 802.1d)
Port: 1, 2 (Switch 1), 4,5,6,7 (Switch 2) form a logical group VLAN A. Port: 3,4,5,6,7,8 (Switch 1), 1,2,3,8 (Switch 2) form another logical group VLAN B. In a port-based VLAN, system admin decides which port goes to which group. In a MAC addressed based VLAN, a machine cannot be member of more than one group. IEEE 802.1q is the VLAN standard.
