---

Creating links in NS

---

---

• Creating Links in NS
  • Links in NS are used for:

    to provide connectivity between Nodes (routers) Output buffering Because a link is in fact a buffer, you must pick (select) a buffer management algorithm to manage the buffer (e.g., what to do when the buffer overflows)

  • Properties of links:

    Duplex (bi-directional) or Simplex (uni-directional) BTW, a duplex link is actually 2 simplex links.. Bandwidth (data transmission rate) Units: b (#bits/sec), or Mb (#Megabits/sec) Propagation delay Units: s (seconds), or ms (milliseconds) Queue management: specifies how packets in the queue are managed Some commonly used buffer management: DropTail: FIFO queue where the node will drop the last arriving packet when queue is full RED: Random Early Drop method (this method tries to break the TCP Synchronization Syndrome) DDR: Deficit Round Robin method (research material) FQ: Fair Queuing method (very advanced research material) SFQ: Stochastic Fair Queuing method (another advanced research material) We will only use DropTail

    ---

  • Example: duplex (bi-directional) link, 10 Mbps, 10 msec delay

    set n0 [$ns node] set n1 [$ns node] $ns duplex-link $n0 $n1 10Mb 10ms DropTail

  ---

• NS Programming trick: More flexible way to create links
  • Often, you want to experiment with the same network configuration but using different bottleneck link capacities.

    For example, the bottleneck link is f times the normal bandwidth in other network links

  • Trick:

    set bw 10 set f 0.5 $ns duplex-link $n0 $n1 [expr $bw]Mb 10ms DropTail $ns duplex-link $n0 $n1 [expr $f*$bw]Mb 10ms DropTail

  ---

• Link: Changing its buffer size
  • You can change the buffer size of one particular link using the $ns queue-limit method

  • Example: set the queue length to 10 packets

    $ns duplex-link $n0 $n1 10Mb 10ms DropTail $ns queue-limit $n0 $n1 10

  ---

• Link: Changing the default buffer size of all links
  • Links in NS are created with a certain default queue size

  • Each class (DropTail, RED, FQ, etc) of queue has its own default queue size

  • You can change the buffer size of a class of queue using the set queue-limit command in the class Queue/QUEUE-CLASS

  • Example: set the default queue length of DropTail queues to 10 packets

    Queue/DropTail set queue-limit $n0 $n1 10

  ---

  ---

• Routing in NS
  • After you created the nodes and connected them through links, NS will provide Internet routing automatically

  • So, after created the nodes and connected them through links, you have an Internet.

  • All you need to do now is:
    • Add transport protocol to the end point nodes (no need to do so to intermediate nodes)

    • Add traffic (load)