Week 2 - Datalink Layer Flashcards
Where was Ethernet developed?
Originall developed at Xerox Palo Alto Research Centre 1973 and patented in 1976.
In 1980, the first formal Ethernet standard was published when DEC, Intel and Xerox (DIX) joined together to publish an ethernet spec.
In 1983 the IEEE 802 LAN/MAN standards committee published a spec for Ethernet (IEEE 802.3 CSMA/CD) Access Method.
Thus there are two types of “Ethernet”: DIX and IEEE.
What is the difference between IEEE 802.3 and DIX Ethernet Frame formats?
What is a MAC address?
Identify the network entities in the Ethernet LANs.
Unique for each LAN interface.
48 bits in length (22 bits identify organisation and 24 bits are vendor assigned).
Burned into the ROM of a network interface card (NIC)
What is MAC?
Medium Access Control refers to the sublayer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. CSMA/CD and Aloha are examples (methods to transmit the data and handle collisions).
What factors limit Ethernet length?
Collision Detection - timing.
Attenutation - signal gets weaker as it propagates.
Noise - longer wires pick up more noise.
What is a bridge?
Bridges, unlike repeaters, store the frames in a buffer. That way, if they detect a collision they can retransmit. So bridges separate collision domains. The bridge receives a frame on one port and transmits it on another port. Bridges are called layer 2 devices because they examine layer 2 info (source/address).
Learning:
Bridges examine the source addresses of every frame and maintains a table, or cache, of which MAC addresses are attached to each of its ports.
Filtering:
The bridge examines the destination, if the destination is on the same port as the source, the frame is not forwarded. The frame is forwarded only to the port the destination is attached to - eliminates unnecessary traffic.
Forwarding:
Bridges can broadcast a frame to all destinations by sending to every port - called “flooding”.
What is a switch?
A switch can be implemented using a crossbar or a spanning tree. Spanning trees ensure broadcasts would never be repeated due to the nature of the topology.
What is a broadcast domain and what is a VLAN?
A network interconnected by bridges comprises a broadcast domain. Broadcasts from one host are seen by every other host on the bridged network.
Broadcast domains have to be isolated because 100mbps traffic could thus congest a 10mbps network. Can be done using a Virtual Local Area Network (VLAN).
What are the following standards: 802.3, 802.5, 802.11, 802.15, 802.16?
802.3 = Ethernet CSMA/CD
802.5 = Token Ring
802.11 = Wireless Lan (“Wifi”)
802.15 = Wireless personal area network (WPAN) - Bluetooth
802.16 = Fixed broadband wireless access system (WiMax)
What is the difference between an infrastructure vs ad-hoc wifi network?
Infra is connected by wired network between access points. WLANs make use of the “Industrial, Scientific and Medical” (ISM) bands - these are unlicensed frequencies available for free in most countries. (2.4ghz bands, 5ghz bands, 60ghz bands etc.). There are a range of 802.11 wifi frequencies used depending on the target range.
What is the format of the 802.11 wifi frame?
Why can’t 802.11 wireless networks use CSMA/CD?
It is not possible to detect a collision: the power of a radio transmission decreases rapidly with distance, listening while transmitting only results in hearing yourself.
On a wireless network its not always possible for a station to hear all other stations, so a sending station that is free to transmit has no way of knowing if the receiving stations is free as well.
What is the Hidden Terminal Problem?
What is the Exposed Terminal Problem?
What is CSMA/CA?
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is the mechanism used by 802.11 in replacement of CSMA/CD. The carrier sense part remains the same: listen until nobody is talking then start.
For collision avoidance, the transmitters communicate with a 4-way handshake:
1. “Listen before you talk”
2. Send a short message: Ready to Send (RTS) which lets the other nodes know that a message packet is coming.
3. RTS contains destination address and duration - so tells everyone else to back-off for the duration.
4. If RTS reaches the destination successfully, the destination sends a Clear to Send (CTS) message after waiting a prescribed amount of time, called Short Inter Frame Space (SIFS).
5. After receiving the CTS, the transmitter transmits the packet.
6. Other nodes in range detect the CTS signal and refrain from transmittion for a time known as the Network Allocation Vector (NAV).
7. The receiver uses the CRC to determineif the packet has been recevied and sends out an ACK packet.
8. if not ACKd start again.
