Networks and Systems

Computer Network
A group of devices that are connected to one another in order to exchange information or share resources


Computer Network overview

  • Hosts - End systems
  • Communication links - Fiber, copper, radio, satellite
  • Bandwidth - Transmission rate
  • Packet switches - Forward packets (routers and switches)
  • Internet - A network of networks
  • Protocols - Control sending, receiving of messages
  • Internet standards:
    • RFC: Request for comments
    • IETF: Internet Engineering Task Force

What’s a protocol

Protocols define the format and order of messages sent and received among network entities, and actions taken on message transmission and receipt

Access Network


Use existing telephone line to central office DSLAM

  • data over DSL phone line goes to internet

  • voice over DSL phone line goes to telephone net

Asymmetric so much faster download than upload

Cable network

Cable network

HFC - hybrid fiber coax

Network of cable, fiber attaches hones to ISP router

  • Homes share access network to cable headend

  • Unlike DSL, which has dedicated access to central office

Shared line between a group of users

CMTS translates signal between the coaxial cable and the ISP

Ethernet (Enterprise access networks)


Home network

Home network

Wireless Access Networks

Shared wireless access networks connects end system to router via base station, aka “access point”

Wireless LANs(802.11): - Within Building (54-1300 Mbps)

Wide-area wireless access - 10s of km (1-10 Mbps)

Physical media

Bit: Propagates between transmitter/receiver pairs

Physical link: What lies between the transmitter and receiver

Guided media: Signals propagate in solid media (usually cables or fibers)

Unguided media: Signals propagate freely (radio etc)

Coax, Fiber

Twisted Pair:

  • Two insulated copper wires

  • Cat5: 10Mbps, 1Gbps

  • Cat6: 10Gbps

Coaxial cable:

  • Two concentric copper conductors

  • Can achieve high data transmission rates

Fiber optic cable

  • Glass fibre carrying light pulses representing bits

  • High speed operation

  • Low error rate


  • Signal carried in electromagnetic spectrum

  • No physical wire

  • Carry a signal for long distances

  • Propagation environment effects

    • Reflection

    • Obstruction by objects

    • Interference

Classified into three groups

  • Very short distance

  • LAN

  • Wide area

Network Security

Network Security

  • How bad actors can attack computer networks

  • How to defend networks against attacks

  • How to design architectures resistant to attacks

Internet originally designed with little security

  • Original vision: “a group of mutually trusting users attached to a transparent network”

  • Internet protocol designers playing “catch up”

  • Security considerations in all layers

Bad actors can “sniff” packets

  • Broadcast media

  • “Promiscuous” network interface reads/records all packets