2. Layers + Broadcast Flashcards
What are the layers of a communication network charactarized by the OSI model?
(There are 7 layers, of which 5 are relevant to this course).
Open-Systems Interconnect (OSI) Model
An abstract way of representing the functions needed in a communication system.
Divides networking tasks into layers.
Has 7 layers in total.
Each layer uses the one beneath it and serves the one above it.
Define: Bandwidth.
Bandwidth – The amount of traffic we can send on the communication channel, given in bit/sec
Define: Throughput.
Throughput – The percentage of the bandwidth used to send traffic.
Define: Goodput.
Goodput - The percentage of the bandwidth used to successfully send traffic
What is the Slotted ALOHA protocol for sending information in a network in which all nodes broadcast and listen to the same channel?
What is the goodput of this protocol?
The goodput is 1/e = 0.37.
That is not very efficient.
What is the difference between Pure ALOHA and Slotted ALOHA, and what is the cost of this difference?
In Slotted ALOHA, information is only sent at the beggining of a slot. Synchronizing clocks is an expensive task, which Pure ALOHA does not do.
The effect of losing the synchronization is that now, no node (besides myself) should send in the slot before we want to transmit and the slot we transmit.
This results with Pure ALOHA having a goodput of 1/2e, which is half the goodput of Slotted ALOHA.
What is a NIC?
Wikipedia:
A Network interface card is an electronic device that connects a computer to a computer network, usually a LAN. It is considered a piece of computer hardware. Most modern computers support an internal network interface controller embedded in the motherboard directly rather than provided as an external component.
Network cards let a computer exchange data with a network. To achieve the connection, network cards use a suitable protocol, for example CSMA/CD. Network cards usually implement the first two layers of the OSI model, that is the physical layer, and the data link layer.
What is the CSMA/CD protocol?
In the CSMA protocol, in case of collisions we would want to wait before trying to send data again.
But how much time should we wait?
A standard approach is called “Exponential Backoff”
The time waited depends on a parameter c (usually c=2)
After the the k’th collision we will wait for j*T time units, where j is uniformly selected randomly from {0, … , ck-1}.
The intuition is that if there were many collisions - the network is very loaded, and it makes sense to wait more time before trying to transmit again.
What is a MAC Address?
Wikipedia:
A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment.
Each computer is given a MAC address by its computer maker. A MAC address has 48-bits. MAC addresses identify many things, such as the type of device being used.
What is Ethernet?
Wikipedia:
Ethernet is a way of connecting computers together in a local area network or LAN.
Systems communicating over Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses, and error-checking data so that damaged frames can be detected and discarded; most often, higher-layer protocols trigger retransmission of lost frames.
As per the OSI model, Ethernet provides services up to and including the data link layer.
Define “PROP” and “TRANSP”.
In order for CSMA/CD protocol to work, what must the relationship between PROP and TRANSP be?
מה הגודפוט של CSMA\CD, מה המשמעות של הרכיבים שמהם הגודפוט מורכב?
