1. The Virtual Network #

🔒 Lab Sheet and Recordings

What is a Computer Network? #

A computer network is a collection of devices that are all interconnected.

Components of a network include:

• Hosts (computers, printers, etc)
• Links (coaxial cable, twisted pair, optical fibre, radio, satellite)
• Switches/routers (intermediate systems)

The goal of a computer network is to provide ubiquitous access to resources, and allow remote users to communicate.

Network Operating System #

A Network Operating System, or NOS, expands the role of a normal OS, in that it offers:

• A remote file system
• Running of shared applications
• Input and output to shared network devices
• CPU scheduling of networked processes

Types of Networks #

Networks can be classified in different ways:

• Based on management method: Peer-to-peer, client/server
• Based on network size: LAN, WAN, and MAN
• Based on topology: Bus, Star, Ring, etc
• Based on transmission media: Wired (UTP, coaxial, fibreoptic), and Wireless

Peer-to-peer Networks #

A peer-to-peer network contains no fixed clients or servers.

Resources are hosted on other clients on the network.

Client/Server Based Networks #

The client/server model involves requests and replies.

Requests are sent to the central server from a client and the server responds to the client with the requested resource.

Microsoft Networking Models #

• Workgroups:
  • Peer-to-peer
  • A logical grouping of several computers whose work or users are connected and share their resources
• Domains:
  • Client/server
  • A type of workgroup that includes a server
  • Management made easy with Server Management application
  • Discretionary access control
  • A domain contains a domain controller, member servers (optional), and workstations or clients

Communications Model #

Multiplexing #

Multiplexing allows multiple channels or users to share link capacity.

Multiplexing devices or mechanisms:

• Multiplexor:
  • Accepts data from multiple sources
  • Sends data across a shared channel
• Demultiplexor:
  • Extracts data from a shared channel
  • Sends to the correct destination

Types of multiplexing:

• Frequency Division multiplexing
• Time Divison multiplexing
• Statistical multiplexing

Frequency Division Multiplexing (FDM) #

• Circuit mode (constant bandwidth) multiplexing technique
• Multiple items are transmitted simultaneously
• Each channel is allocated a particular portion of the bandwidth (known as bands)
• All modulated signals are carried simultaneously (as a composite analog signal)
• Example: TV, radio

Synchronous Time Division Multiplexing #

• Circuit mode (constant bandwidth) multiplexing technique
• Divides time into equal-sized quanta
• Assigns each quanta to flows on the physical link in a round-robin fashion.

Statistical Multiplexing #

With statistical multiplexing, each flow is broken into packets and sent to a switch, which can deal with the arriving packets according to a policy, such as, FIFO, round-robin, etc.

Unlike circuit mode multiplexing, statistical multiplexing allows for variable bandwidth.

Circuit Switching #

• Source first establishes a connection (circuit) to the destination
  • Each router or switch along the way may reserve some bandwidth for the data flow
• Source sends the data over the circuit
  • No need to include the destination address with the data since the routers know the path
• The connection is torn down

An example of circuit switching is the telephone network (analog).

• Advantages:
  • Fast and simple data transfer, once the circuit has been established
  • Predictable performance since the circuit provides isolation from other users, e.g. guaranteed bandwidth
• Disadvantages:
  • In bursty traffic, circuit will be idle for significant periods of time
  • Users with different bandwidth needs may need to use multiple circuits

Packet Switching #

• A form of statistical time-division multiplexing

• Source sends information as self-contained packets that have an address

  • Source may have to break up single message into multiple packets

• Each packet travels independently to the destination host

  • Packets are passed from node to node between source and destination
  • Routers and switches use the address in the packet to determine how to forward the packets

• Advantages:

  • Packets from different sources are interleaved
  • Efficient use of resources (since they are used on demand)
  • Can accommodate bursty traffic
  • Allows for flexibility and robustness. Packets can travel through alternative paths (adaptive routing)

• Disadvantages:

  • Undesired situations such as congestion; long delays may occur

Packet Details #

• Depends on the underlying network:
  • Minimum/maximum size
  • Format
• A hardware packet is called a frame

Internetworking #

A gateway or router is needed to interconnect one or more networks.

Host-to-host connectivity is only possible if there’s a uniform addressing scheme and a routing mechanism.

Messages can be sent to a single destination (unicase), to multiple destinations (multicast), or to all possible destinations (broadcast).

Routers determine the next network point on a data unit’s path. They are essentially the sorting offices for the Internet Protocol.

LAN/MAN/WAN Networks #

Computer networks can also be classified according to their geographical coverage:

• Local Area Network (LAN): Covers a small geographic area, such as a building
• Metropolitan Area Network (MAN): Covers a larger geographic area, such as a town or city
• Wide Area Network (WAN): Covers a very large geographic area, such as the Earth.