Access till nätet Flashcards
1.
Vad står IEEE för?
Vad menas med IEEE 802.3 ethernet?
Institute of electrical and electronics engineering.
Helt enkelt världen största institut som tog fram standarder för hur kommunikation ska ske på nätet. Standarden förklarar då hur meddelanden ska skickas och tolkas.
802.3 ethernet är då en speciell standard.
ethernet 10BASE5 betyder?
Mer specifikt vad betyder numrena?
ethernet har ett visst sätt att transmittera informationen.
10-bithastigheten är 10Mbps
BASE-informationen transmitteras på detta sättet (basbandssignalering)
5-Den har en räckvidd på 5 x 100 meter.
Första typen av kommersiella ethernet. Idag är vi uppe på 100 Gbps.
VLAN står för vad?
Finns det någon skillnad i hur deras MAC-frames ser ut?
(ethernet VLAN)
Virtuellt - LAN
Istället för att koppla upp säg till en specifikt nätverk geografisk är ett virtuellt lan istället någon som bestäms på annat sätt. Exempelvis kan det vara roller inom ett företag. För att skilja på nätverken räcker det inte med samma frames som för vanligt ethernet.
Ett extra block beskriver prioriteten och vilket nätverk ramen skickas.
Vad är en MAC-frame?
Media Access Control address identiferar varje enhet på nätet, de skickar ramar i ett visst format på nätet som kan identifieras som deras.
Det finns framförallt två topologier för LANs, vilka?
Vad skiljer de åt?
Buses - Alla stationer delar på ett medium. Skickas en frame, får alla det. Det finns kontrollinfo och destination i ramen som kontrollerar att transmittioner inte sker samtidigt.
Stars - Det finns en central nod alla skickar till. Antingen funkar det likt en buss med broadcast där huben skickar till alla andra stationer. (Den addresseade mottagaren tar emot). Eller så funkar noden som en stor switch som routar transmittionen till rätt mottagare.
Bus or star.
Ett lan ägs och används inom en organisation.
What is controlled access?
In controlled access the timeslots for the signals are decided beforehand. Used primarily on wireless networks.
What is the reservation access method?
synchronous
The reservation access method is centralized but can be distributed. The method is either FDMA (frequency division multiple access) or TDMA (time division multiple access)
What is access method round robin/token ring?
Asynchronous
You get a maximum time to transmit, then the turn is relayed to the next station. When a lot of stations have a lot of data, RR can be highly effective. It can be centralized or distributed.
What is the polling access method?
Polling is used on contentionless access. A primary station sends out polls and picks the station which gets to send. This means the station sending will always have something to send, but only if the controller is reliable.
What is contention access method? When is it normally used?
If there is contention, first to send is first to get access to the medium. This is a simple method to implement and are under light to moderate load efficient enough, however, the method crumbles under heavy loads. When the traffic is bursty, a lot of idle time with some stations having a lot to send occasionally, this method is most aptly used.
Are you familiar with these contention based access schemes:
* ALOHA-net
* Slotted ALOHA
* CSMA (carrier sense multiple access)
* CSMA with collision detection (w CD)
* CSMA with collision avoidance (w CA)
- ALOHA-net: Stations sends to main server, server respons on another channel. If a collision occurs, wait.
- Slotted ALOHA: Stations are only allowed to send data during specific time slots, otherwise like ALOHA.
- CSMA: Listen to the medium, if free, send signals. Since signals propagate through the medium, collisions can occur.
- CSMA w CD: (like Ethernet) after a signal has been sent, the station listens for collisions. If a collision occurs, a jamming signal is sent and exponential backoff is used.
- CSMA w CA Avoids collisions by using interframe space, waits until the channel has been empty for a while, contention window, send during specific window and ACK.
For wireless access, there are two problems that can occur called:
Hidden nodes, Exposed nodes. What are they?
Hidden nodes is when a node is not detected since it is too far away. It thinks it can send, but it really can’t without interfering.
An exposed node thinks it can’t send when it in reality can.
Hubs are used on the first layer (physical layer)
What is a HUB?
Central element in star topology.
Signal goes in and is repeated to all outgoing lines.
Switches are used on the second layer.
(Data link layer)
What are switches?
Store-&-Forward switch / Cut-through switch
A switch takes an inputs and switches the signal to the correct output. Unlike with a hub, several signals can be transmitted to different stations simultaneously.
What are bridges and on which layer do they operate?
Bridges operate on the data link layer (2nd) and relay frames between similiar networks.
