Networks week1

What is a computer network physically?

It is an “infrastructure” interconnecting “end-devices”

What is the infrastructure of a computer network?

Communication links and packet switches

What is the internet physically?

The internet is a computer network that interconnects billions of computing devices, called hosts or end-systems.

By what are end systems connected?

End systems are connected together by a network of communication links and packet switches

What are packets?

When a system has data to send it segments the data and adds header bytes to each segments. The result is called packets.

What does a packet switch do?

A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links

Through what do end systems acces the internet?

End systems acces the internet through Internet Service Providers (ISPs).

What is an ISP?

An Internet Service Provider is in itself a network of packet switches and communication links

What are protocols?

Protocols control the sending and receiving of information on the Internet

What are 2 of the most important protocols?

The Transmission Control Protocol (TCP) and Internet Protocol (IP) are 2 of the most important protocols

Who develops internet standards?

The Internet Engineering Task Force (IETF)

What are the IETF standard documents called?

They are called request for comments (RFCs)

What is the internet as a service?

The internet, as a service, is an infrastructure that provides services to applications.

Why are the applications connected to the internet said to be distributed applications?

Because they involve multiple end ststems that exchange data with each other

End system attached to the internet provide a socket interface. What is this?

A socket interface specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.

What does a protocol define?

A protocol defines the format and the order of the messages exchanged betweeen 2 or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

What are end systems also reffered as and why?

End systems are also referred to as hosts because they host (run) application programs.

Into what groups can hosts be devided?

Clients andservers

What is the acces network?

The acces network is the network that phsically connects an end system to the first router.

What are the 2 most prevalent types of broadband residential acces?

Digital Subscriber Line (DSL) and cable.

What does DSL use?

When DSL is used it uses the existing telephone line with a digital subscriber line acces multiplexet (DSLAM) located in the telephone company’s local central office.

What does cable Internet access use?

Cable Internet acces makes use of the cable television infrastructure. The cable modem termination system turns analog signal sent from cable modems into digital format

What does a Local Area Network (LAN) do?

A LAN is used to connect an end system to the edge router

What is the Network Core?

The Network core is the mesh of packet switches and links that interconnects the internet’s end systems

What system do most package switches use?

Most packet switches use store-and-forward transmissions

How does store-and-forward work?

Store-and-forward means that the packet switch must receive the entire packet before it can begin to trnasmit the first bit of the packet onto the outbound link

How long does it take to transmit an entire packet and for the entire packet to be received in the general case?

N(L/R)

Where N is the amount of links (routers + final destination)

Where L is an L-bit packet 

Where R is the speed of transmission in bps

What is the function of an output buffer (output queue)?

It stores packets that the router is about to send into the next link

What is the function of a forwarding table?

A forwarding table maps destination address to that router’s outbound links.

What is forwarding

Forwarding is a local action: it is moving arriving packets from input link to appropiate output link

What is routing

Routing is a global action: it is determing source to destination paths for packets

What are the 2 fundametnatl approaches for moving data?

Circuit switching and packet switching

What is circuit switching?

In circuit switching the resources needed along a path for communication are reserved for the duration of the communication session between the end systems

What is packet switching?

In pcket switching the resources are not resered which might mean a queue forms.

What kind of connection do circuit-switched networks use?

The network establishesa dedicated end-to-end connection between the two hosts

What kind of multiplexing can a circuit have?

A circuit in a link is either implemented with frequency-division multiplexing (FDM) or time-division multiplexing (TDM).

How does FDM work?

With FDM the frequency spectrum of a link is divided up among the connections established across the link. So, the link dedicates a frequency band to each connection.

How does TDM work?

For a TDM link, time is divided into frames of fixed duration and each frame is divided into a fixed number of time slots.

What are arguments against circuit switching?

Circuit switching can lead to dedicated switches being idle during silent periods

Packet switching offers better sharing of transmission capacity

It is simpler

It is more efficient

Less costly to implement

What are argument against packet switching?

It is great for “bursty” data but it can lead to excessive congestion

Of what kind of ISPs does the network of networks consist?

There are hundreds of thousands access ISPs

There are multiple global transit ISPs

In any region there may be regional ISPs which connect to the global ISPs

Content provider networks such as Google and Microsogt may also run their own network

Along the transmission path what are the most important delays a packet can suffer?

nodal processing delay

queueing delay

transmission delay

propogation delay

What is the nodal delay?

The sum of nodal processing, queueing, transmission and propagation delays

What is the processing delay?

The time required to examine the packet’s header and determine where to direct it

What is the queuing delay?

The queuing delay depends on the number of earlier-arriving packets that are queued

What is the transmission delay?

It is L/R; the time to push all of the packet’s bits into the link

What is the propogation delay?

The timer required to propogate (travel) from beginning to end of the line

To analyse nodal delay you can use the traffic intensity, what is the formula for this and what does the outcome mean?

La/R

if La/R = 0 delay is small

if La/R → 1 delay is large

if La/R = 1 delay is infinite

What is throughput?

And what are instanteneous and average throughput?

The rate at which bits are being sent from sender to receiver.

Instanteneous is the rate any given moment in time, average is rate over a longer period of time

What is a bottleneck link?

A bottleneck link is a link on an end-to-end path that constrains end-to-end throughput

What are the 5 layers of network protocols

Application layer

Transport layer

Network layer

Link layer

Physical layer

What does the application layer do?

The application layer supports network application

What does the transport layer do?

The transport layer transport application-layer messages between application end-points

What does the network layer do?

The network later moves network-layer packets, known as datagrams, from one host to another

What does the link layer do?

The link layer moves a packer from one node to the next node on the route

What does the physical layer do?

The physical layer moces the individual bits within link-layer packets (frames) frome one node to the other

What does encapsulation mean?

Encapsulation means that the lower layer messages also contain the messages from above