Digital Networks

Digital Networks

Black Boxes Interaction

• contingency of the network > (entangle cables, flashing lights)complexity BUT hidden structure

client --> server
client <-- server

        _________
       |         |
client |black box| server
       |_________|


client --> |bb| --> IP address

only interactions with the black box. No direct link between computers

Routers

info pass from many intermediate to reach the server the info is redirected and find its way through the address the intermediate is called router'

traceroute >> track the passages to arrive at the server

Black boxes are recursive:

information web > 
reliablility (track data loss) > 
router (transport data) > 
wifi/ethernet/fiber (transform digital signal into modulated to be transmitted on the physical link)

it is important to define in each level the expected functionality

Networks components

the software relies on an OS which manages the resources of the computer the os manage also the network, makes the communication reliable

HTTP > TCP > IP > WI-FI

1. wi-fi allows several equipments to connect, but a protocol make sure that only one computer can talk at a time (IP), otherwise, info is lost if overlap- Also adaptation of the modulation to network quality.
2. the IP packet is sent over the ADSL link and received by a router of the provider that makes sure that the traffic converges in links with higher bit rates in order to minimize interconnection costs. But usually, the server is not part of the same provider so it is sent to another provider or neutral zones (internet exchange points). They work on gigabit instead of megabit.

Problem of data loss when long communications (as in videos)

• process errors data losses
• build tables to guide the info through the network
• manage overloads that can occur

Analogy with postal service

one of the very first networks was the postal service

client > read and write

rules address > a place where mail can be received. Strict format + specify own address to allow the recipient to respond back or postal company to send back the mail if failure.

2 interactions

• client drop the letter in a public interface
• the postal service delivers the letter to the recipient public interface

advantages > allow correspondence with a distant correspondent limitations > postal service limit the weight of the letters, so we can send a definite amount of pages X letter. For a heavy document, we have to split it in different letters that are sent separately BUT variable duration to cross the network.

neither the recipient nor the sender knows how the letter is transported + the postal service doesn't care about the content, only looks the envelope

Timing diagram

    Td|.   |
      | .  |
      |  . |
      |   .|Ta

delivery time = Ta - Td

Impossibility to guarantee a perfect transfer duration. The receiver could not t receive the letters in the same sequence in which they are sent develop strategies to correct this problem:

• chef and the kitchen hand > numbering the letters only the right order make the recipe, but it correct only part of the problem.

Those strategies are similar to a protocol:

• set of rules known from both sides
• enhancing a communications service

Global view VS local view > The sender and the receiver are aware of only part of the service

Digital Network notions

Telephone Network

another kind of interaction with the user

take the telephone|
   dial the number|\  |
                  | \ |
                  |  \|ring
                  |   |take the telephone
              talk|   |talk
              talk|   |talk
              talk|   |talk
                  |   |close the telephone
                  |  /|
                  | / |
               end|/  |
                  |   |

3 kinds of interaction: open / transfer / end

Before telephone networks were digital, made by the connections of copper cables. With the voice digitalizing they become like small blocks similar to packets, but the same circuit principle is kept in a virtual way.

Datagrams

In a network where you have to send data from agency to agency, an unrealistic approach consists in creating links between each agency but:

• very expensive
• small traffic
• lack flexibility > if you have a new node you have to create all the links

It is better to have a backbone to limit the number of links and consequently lower communications costs BUT other problems: an agency is connected only through a single cable > you don't know who transmitted them.

• you could schedule the communications but if there is nothing to communicate you waste communication capacities
• store and forward - you could ask for a route but no resources reservation because the message is stored in each intermediate node. Optimization of the link usage but delays.

If we don't limit the size of the message transmitted the intermediate nodes would need almost infinite resources to store all the messages advantages:

• if an error on a block we don't need to resend the whole message shorter duration because we can transmit a block and receive the next one at the same time.

These data with the reduced size is called packets and the transmission duration is important because it could disadvantage other communications >> limit the duration as much as possible: have links with a higher bit rate to increase communication quality.