Computer Network notes1

Computer Networks and the Internet

defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

Access Network

  • DSL, Cable, FTTH, and 5G Fixed Wireless
    DSL: digital subscriber line
    FTTH: fiber to the home
  • Ethernet and WiFi
    LAN: local area network
    Ethernet: by far the most prevalent access technology in corporate, university, and home networks.

Physical Media

  • guided media
  • unguided media

Network Core

  • Packet Switching (for which there are two predominant types, routers and link-layer switches)
    L: packet bits
    R: transmission rate bits/sec

    • Store-and-Forward Transmission
    • Queuing Delays and Packet Loss
      Delay: packets suffer output buffer queuing delays
      Loss: arriving packet may find that the buffer is completely full with other packets waiting for transmission.
    • Packet head contains info for router who has a forwarding table to find the destination
  • Circuit Switching

Delay in Packet-Switeched Networks

  • processing delay
  • queuing delay
  • transmission delay
  • propagation delay

The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packet’s length and the transmission rate of the link, but has nothing to do with the distance between the two routers. The propagation delay, on the other hand, is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers

d_nodal = d_proc+d_queue+d_trans+d_prop

Queuing Delay and Packet Loss

a denote the averagee rate at which packets arrive at the queue (a is in units of packets/sec)
L: all packets consist of L bits
R is the transmission rate in bits/sec
==> La/R called traffic intensity


  • protocol layer
    implement the protocol in layers
    the services that a layer offers to the layer above—the so-called service model of a layer
    the protocols of the various layers are called the protocol stack
  • application layer
    HTTP: provides for Web document request and transfer
    SMTP: provides for the transfer of e-mail messages
    FTP: provides for the transfer of files between two end systems
    packet of information at the application layer ==> message
  • transport layer
    TCP: provides a connection-oriented service to its applications
    UDP: provides a connectionless service to its applications
    transport-layer packet ==> segment
  • network layer
    moving network-layer packets known as datagrams from one host to another.

    IP Protocol: defines the fields in the datagram as well as how the end systems and routers act on these fields.

    routing protocols: determine the routes that datagrams take between sources anddestinations.

  • link layer
    refer to the linklayer packets as frames.
  • physical layer
  • the osi model
    Open Systems Interconnection (OSI) model