Net

42 flashcards · Other · by Emanuel Gostner

1. MAC - media access control. Which layer/header-field and function

   Layer 2- Ethernet of IEEE divides layer into LLC (logicl link control) and MAC (media access control). LLC is defined in the lengh field among the upper-layer protocol, there are also control bits in the length field LLC provides mechanisms for Multiplexing and ARQ. If there is not used a protocol for ARQ in L2 (LLC), then Error correction and Flow control is done in L4 transport.

2. What form of media access control does Ethernet and WLAN use. What does it stand for?

   CSMA (carrier sense multiple access) Ethernet: CSMA/CD - collision detection WLAN: CSMA/CA - collision avoidance

3. Methods of media access control

   Contention based (behauptung, streit): only one host at a time (CSMA/CD) Controlled access: each host has its turn. (token, CSMA/CA)

4. Structure MAC-address

   OUI: organisationally unique identifier Highest order byte: left Lowest order bit: right

5. CSMA/CD. When used?

   detects if there was a collision and throws the frame. Happens after collision. Mostly wired jam signal to all devices so they know. stop sending and define timer until retransmission

6. CSMA/CA. When used?

   Listens to the network and regulates who should send or not to prevent collisions. Used in Wireless where it's hard to detect collisions

7. Flow control and main techniques

   Defines the amount a data a receiver can handle Dedicated stop and go: like stop-and-wait: Sender has to wait for an acknowledgment (ACK) Adaptive Window: window with dynamic (n) size. Sender can send n packets

8. OSI - TCP/IP Model comparison

9. difference delay/latency

   delay is the time it takes a data stream to reach its destination. Latency includes round trip time (RTT)

10. latency jitter

    Jitter: Packets that are sent have different delays. This difference/inconsistency in packet delay is called jitter. -Packet loss -congestion (verstopfung) ->To compensate there is a jitter buffer which stores the packets until they can be resend

11. Problems Layer 2 must solve and its solutions

    Problem Solution Where does each frame begin and end(Framing)? Preamble (new frame starts) - Character Count/Bit Byte stuffing to mark start/end of frame How should the data flow be regulated (Flow Control)? ARQ point-to-point what are source and dest. (Physical addressing)? Source/dest. MAC-address Does a bit stream have errors (Error Control)? FCS, CRC Who can use the medium and when (Medium access control)? CSMA/CD

12. Framing Method Character Count

    Character Count: At the start of each frame there is a character (8bits) that shows the amount of chars of this frame (CC included)

13. Framing Method Byte Stuffing

    A byte sequnce ex. 10101010 is added at start/end of frame. If occurence of this sequence in Payload it is escaped with the same sequence.

14. Framing Method Bit Stuffing

    After each 5 1er there is added a 0, this way the added sequence (01111110) can't happen by accident as in byte stuffing

15. What is FCS used for?

    A sequence of bits at the end of the framewith which it can be determined wether the payload is correct or not.

16. What does ARQ stand for and its main functions

    Automated Repeat request (L2/L4) stop and wait Go back'N (Sender has window) Selective repeat/reject request (Sender and Receiver has window)

17. What are sequence numbers and ACKs. Why/how are they used?

    Data is split into segments/frames and are transmitted frame by frame. Frames don't always arrive at the same time at its destination. The Sequence number shows the order of the frame so the receiver can reassemble them correctly. ACK contains the number of the frame that the receiver expects next.

18. Mechanisms of ARQ

    window sequence number ACKs Timeouts

19. What is flow control and its main teqniques

    Defines how much data can be send/the receiver can handle Dedicated stop and go (stop-and-wait) Adaptive windows (Go back'n, selective repeat)

20. Media access control main methods?

    Contention based: Only one host can send, listen to network who wants to send (CSMA) Controlled access: every host gets his time to send (token)

21. What form of media access control does Ethernet use?

    CSMA /CD Carrier Sense multiple access / collision detection

22. Multiplexing

    bundle multiple signals to send them at the same time trough a cable

23. Dublex/Duplexing and its variations

    Simplex: Data can only be sent in one direction Half-Duplex: Data can be send and receiver both ways but only one at a time Full-Duplex: Data can be sent and received at the same time

24. Structure MAC address

25. Organizations which define network standards/protocols

    L1/L2: IEEE, ITU-T L3/L4: IETF(internet engineering taskforce) ISO: developed OSI-model ANSI ISOC (promotes open standards) IANA (provides IP adresses)

26. IEEE protocols

    802.2 LLC 802.3 Ethernet 802.11 WLAN

27. OSI layers and its functions

    Physical: signaling, encode bits into electrical signals and vice versa (decoding) Data-Link: Framing, physical addressing, Flow control, error control, define upper-layer protocols Network: logical addressing, IPs. Packet forwarding to/trough networks, QoS (priority of packets), time to live Transport: Segmentation of Data, reassembling of data, establish temporary communication and deliver data, Flow control (window), Error control (ARQ), deliver data to port, manage reliability (TCP/UDP->bes…

28. PDU shortcut and name for each layer

    Protocol Data Unit L7/6/5: Data L4: Segment L3: Packet L2: Frame L1: Bits

29. Network devices. At which layer do they operate?

    L1: Hub, repeater, cables L2: Switch, Bridge L3: Router, L3 Switch L5/6/7: PCs

30. Protocols and function for each layer

    L1: ADSL, DSL, SDSL, USB L2: DIX Ethernet II, IEEE 802.3 Ethernet, 802.2 LLC, 802.11 WLAN L3: IPv4, IPv6 L4: TCP/UDP L5: L6: ASCII, jpg, png... L7: HTTP, POP/IMAP (send mail), SMTP (receive mail), FTP

31. transmission impairments

    Attenuation (dämpfung) Noise Delay

32. delay/latency factors

33. media access control Ethernet's algorithm and how does it work

    Carrier Sense multiple access collision detection (contention based) as soon as there happens a collision CSMA detects it and: sends a jam signal to all devices so everybody knows discards the frame stop sending and set timer for retransmission

34. Two main Ethernet tchnologies

    DIX Ethernet II type field >= 1536 max frame length used by IP more widely used Sequnce of 8 bytes before header (preamble) IEEE 802.3 Ethernet length field <= 1500 : LLC encapsulated L3 Data MAC encapsulates LLC Data Sequence of 8 bytes before header (preamble and Start delimiter (SD))

35. purpose of network models and layers

36. What are network protocols

    protocols define rules for standarized tasks

37. Why do we need networking protocols and standards

    to provide compatibility between devices of different vendors. optimized security optimized safety

38. 3 sublayers in IEEE 802.3

39. How does a switch learn mac<->port addresses

40. What does a switch do with a broadcast frame?

    It forwards it to every host within the network, except to the one the frame originated from

41. Ethernet legacy and today

42. Reason for Ethernet's success