Network communications

by Gilbert Keith

Answering some questions at the end of Lecture #2 on NPTEL’s Internet Technologies course – http://nptel.iitm.ac.in/video.php?subjectId=106105084 

With respect to speed of data transfer, which of LAN or WAN is faster?

  • LAN is faster.

What is the typical speed of a LAN with an Ethernet backbone?

  • 10 Gbit/s, upto 100 Gbit/S

Why is circuit switching not suitable for computer to computer traffic?

  • Computer to computer traffic tends to occur in bursts – there might be moments of relatively high activity in a long time range with relatively low activity. Circuit switching involves a dedicated route between source and destination hosts, which means that resources have to be dedicated to that route. This can be inefficient. Furthermore, when data is being transmitted across a circuit switched route, the hosts cannot use those resources to facilitate any other communication – the route is blocked off until the data transfer is complete. If a large file is being transferred, this may occupy the circuit for a long period of time, making it impossible to do anything else between those computers. 

What are the steps in circuit switching?

  • Establish connection.
  • Transfer data.
  • Terminate connection – (resources are dedicated, so need to deallocate!)

Which of circuit switching/packet switching is more suitable for sharing of links?

  • Packet switching is more suitable. You cannot share links on a circuit switched network because resources are dedicated between hosts and will be terminated when not needed. With packet switching, since data is broken down into small chunks, it can be sent across a variety of links to optimize its transmission to the destination.

Describe the “Virtual Circuit” connection model.

  • In the packet switching paradigm, resources are not dedicated – you transmit data to a node which is closer to the destination and which is the fastest to get to, and keep hoping that the same occurs down the road. However, when dealing with a packet switching framework, it is possible to dedicate some resources to finding out a circuit that can connect you to your destination, and establish that route as a preferred method of reaching said destination – you establish a “virtual circuit” like in the circuit switching model, and all packets follow the same path. However resources aren’t actually dedicated for your connection with the destination.

 Which required less info in packet headers? Virtual circuit or datagram?

  • In a datagram, each packet is sent to a locally optimal next node, which itself sends it to another node, etc… SInce the route is not established, each packet needs to have information regarding the source/destination in its header so that each node can determine where to send the resources next.

Which makes better use of links – virtual circuit or datagram?

  • Datagram 

Which guarantees ordered delivery of packets in absence of errors?

  • Virtual circuit.

Under what circumstances is the datagram method useful? 

  • It is useful when you don’t really need for data to be transmitted in an ordered fashion. And/or you are okay with potential differences in propagation delays for the various packets. eg – transmitting an image?

For transmitting 5 packets is datagram or virtual circuit faster?

  • For 5 packets, datagram would be a faster method. For 5000000, maybe circuit switching is appropriate.

5Kb over a 10 Mbps transmission link ==> 5/10000 = 0.0005 seconds, or 0.5 ms

600 byte packet over 20Kbps link, with 10 ms propagation delay ==> 600/20000 = 3/100 = 0.03 s = 30 ms. 30 ms + 10 s = 40 ms.

Which of the 4 layers of the OSI model are host-to-host layers?

  • Application (what the end user is interacting with)
  • Presentation (things pertaining to the OS)
  • Session (things pertaining to the session with the host)
  • Transport (things pertaining to device communication/establishing handshakes/checking data integrity/flow control)

Network layer ==> primary responsibility is routing of messages/data (think IP address)

Data Link layer ==> responsible for forwarding/routing of data within a LAN? (think switches which use MAC addresses to figure this out)

What is the difference between a bridge and a router?

  • A bridge is a device that connects physical devices within different networks or connects switches within different networks. They help find devices within a network.

    Routers find networks given a network address. They work in the network layer as opposed to the data link or physical layers

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