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Komtek > Modules 4-7 Ethernet Concepts > Flashcards

Modules 4-7 Ethernet Concepts Flashcards

1
Q

What is the purpose of the physical layer?

A

Purpose of physical layer:

  1. Make connection between two datalink entities.
  2. Transform and encode the data frame from datalink layer into a series of signals representing bits.
  3. Then the next devices on the path(NIC) will re-encapsulate the frame and decide what to do next.
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2
Q

What is PHY encoding, give examples of encoding algos and their speed.

A

Converting a stream of data bits into a pre-defined code

Manchester(10 Mbps Ethernet) Slow

4B/5B 100Base (Fast ethernet)
8B/10B 1000 base Gigabit encoding

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3
Q

What is 4B/5B encoding and Manchester encoding?

A

4 bits of data into 5 bits of transmission using a dictionary.

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4
Q

PHY signaling

A

How the bit values (0 1) are represented in PHY medium.

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5
Q

What is a self-clocking signal

A

A signal that can be decoded without the need for a seperate clock signal or other source of synchronization.

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6
Q

Explain bandwidth

A

A medium’s capacity to carry data.

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7
Q

What is bit-rate and what is baud rate?

A

BIt rate: Bits per second

Baud rate: Number of symbols(signals) transmitted per unit of time

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8
Q

What is latency(delay,lag)

A

Amount of time, including delyas for data to travel form one given point to another, often expressed in milli-or-micorseconds.

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9
Q

What is throughput

A

The measure of the transfer of bits across the media over a given perid;
Expressed in Xbps

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10
Q

What is goodput:

A

The measure of usable data transferred over a given period.

Goodput = throughput - traffic overhead

Overhead can be protocol overhead(e.g headers), lost/retransmitted data

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11
Q

What are the 3 types of copper cable we have

A

UTP, STP and Coaxial

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12
Q

What are the benefits and disadvantages of copper cable?

A

Cheap, easy to install, low resistance to electrical flow.

Limitation with signal attenuation(weakens over distance)

susceptible to EMI, RFI and crosstalk.(noice on the wire)

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13
Q

What is UTP cabling? And what does it rely on to limit crosstalk?

A

Four pairs of color-coded copper wires twisted together and encased plastic, no shielding.

Cancellation: Each wire in pair uses opposite polarity. One negative, other positive, twisted together and the magnetic fields cancel each other and outside EMI (Electromagnetic Interference) RFI (Radio Frequency Interference)

Variation in twists per foot in each wire: each wire is twisted a different amount which helps prevent crosstalk amongst the wires in the cable.

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14
Q

What do you use a straight-through cable for

A

It is for connecting a host to a network device.

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15
Q

What is a Crossover UTP cable used for?

A

Used to connect two similar devices (transmit feeds into receive)

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16
Q

How does a fiber optic cable work? And what are some advantages and disadvanteges

A

Uses a laser or LED to encode as as light pulses

High bandwidth, long distance, more expensive than UTP, less susceptible to attenuation(signal loss) and no EMI/RFI

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17
Q

Where do you usually use fiber optic cables?

A

Enterprise networks: Interonncing infrastructure devices
Fiber-to-the-home: Always on broadband services to homes and small businesses
Long-haul networks: Connecting countries and cities
Submarine cable networks: Under water cables, high speed, high capacity and can survive in harsh undersea environments.

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18
Q

How does wireless media work? What freguency is it on?

A

Wireless carries electromagnetic signals representing binary digits using radio (<300 Ghz) or microwave (300Mhz - 300Ghz) frequencies. This provides the greates mobility option.

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19
Q

What are some limitation of wireless media?

A

Coverage area: effective coverage impacted by the physical characteristic of the deployment location.

Interference: Susceptible to interference and can be disrupted by many common devices

Security: Anyone can access the transmission medium

Shared medium: half-duplex WLANs (only one sneding/receiving device at a time)
Many users lead to reduced bandwidth for each.

Basically, area, intereference, security and shared medium(reduced bandwitdh)

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20
Q

Which standards does the PHY specifications dictate:

A

Data to radio signal encoding methods.

Frequency and power of transmission

Signal reception and decoding requirements

Antenna design and construction

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21
Q

What is Wi-FI, Bluetooth, WiiMAX and Zigbee?

A

Wi-Fi: Wireless LAN (WLAN)
Bluetooth ( Wireless personal Area network (WPAN)

WiMAX Point to point multipoint topology to provide broadband wireless access.

Zigbee Low data-rate, low power-consumption communications, primarily for interner of things IoT application.

22
Q

What is the binary number system, and how do you convert from binary to decimal?

A

Consists of 1s and 0s, called bi

128,64,32,16,8,4,2,1

23
Q

What is the purpose of the datalink layer

A

Responsible for communication between end-point NICs; allows upper layers to access PHY media and encapsulates L3 packets.

Transfer of frames, data transfer between adjacent stations connected by one physical channel

Data transmission, error detection & correction, flow control, configuration management.

24
Q

What are the two sublayers in the Datalink layer

A

Logical link control (LLC)
The LLC sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers.
Places information in the frame to identify the used network layer protocol.

Media Access Control (MAC)
The MAC sublayer is responsible for data encapsulation and media access control.

25
Q

What are p2p, hub and spoke and mesh? How do they work?

A

They are WAN topologies

Point to point (P2P)
Persistent link between two endpoints; most common/simplest WAN topology as all frames can only travel between two nodes.

Hub and Spoke:
Central cite interconnects other via p2p; similar to star topology.

Mesh:
Offers high availability (many to many); every system connected to every other end-system.

26
Q

What is a star, bus and ring topology? How do they work?

A

Star topology is a network topology in which each network component is physically connected to a central node such as a router, hub or switch. This can be where u have many routers where each router has their own “star”

Bus: Systems chained together and terminated at ends
Rings: Each connected to neighbour forming a ring.

27
Q

What is half duplex and full duplex

A

Half duplex : bi-directional, but only one at a time - e.g WLANs and old bus topology
Basically data can only go one way at a time

Full-duplex: bi directional, in both directions simultaionsly e.g Ethernet
Data can go both ways at a time.

28
Q

What is a contention based access control method?

A

Contention based: All nodes in half-duplex will have to contend to access the medium

29
Q

What is ALOHA and CSMA

A

ALOHA is a media access control method where users transmit whenever they have data; retry after a random time if collision. Efficient under low load/delay

CSMA; Improvement of ALOHA by sensing the channel users don’t send if they sense someone else!
1- Persistant(greedy) sends as soon as idle
Non-persistent: waits a random time then tried again
p-persistent: Sends with probability when idle

30
Q

What is CSMA/CD and CSMA/CA

A

CMSA/CD : Improvement to detect/ abort collisions; reduced contention time improves performance.

Legacy Ethernet LANs operating in half-duplex.
Devices back off after a random period of time and retransmit data!

CSMA/CA
– Operates in half-duplex mode (only one transmission at a time)
– Devices include time duration needed for transmission
– Other devices on the shared medium receive the time duration information and
know how long the medium will be unavailable
– Inserts back-off slots to avoid collision
– MAC uses ACKs/retransmissions for wireless errors

31
Q

What is the link data frame?

A

Data is encapsulated by Datalink layer with a header and trailer to form a frame

Divided up in Header, Data, Trailer ( Vary in size depending on the DLL protocol)

MAC address, physical address in frame header.

MAC address used for local delivery of a frame on the link

Gets updated on each hop

32
Q

What is Ethernet?

A

Family of “wired” networking technologies specified in a bundle of physical and datalink layer protocols.

33
Q

What are the three elements you need for data encapsulation?

A

Ethernet frame; Internal structure of Ethernet frame

Ethernet addressing: Source and destination Mac addrress used on the same LAN

Ethernet error detection: frame check sequence (FCS) trailer used for error detection - Cyclic

34
Q

What is LLC sublayer? What does it do?

A

One of the two subplayers needed to operate the data link layer.

The LLC Subplayer communicates between the networking software at the upper layers and the device hardware at the lower layers.

Places information in frame that identifies which network layer protocol is being used for the frame.

35
Q

What is the Ethernet MAC sublayer responsible for? And what does it consist of?

A

Data encapsulation, media access control, and DL layer addressing

Consists of 48 binary bits

36
Q

What is the min | max an ethernet frame can have? And what happends if they are smaller or bigger?

A

length < 64B “collision fragment” or “runt frame”; automatically discarded
length >1500B “jumbo” or “baby giant frames”

If the size of a transmitted frame is less than the minimum, or greater than the maximum, the receiving device drops the frame. Dropped frames are likely to be the result of collisions or other unwanted signals. They are considered invalid. Jumbo frames are usually supported by most Fast Ethernet and Gigabit Ethernet switches and NICs.

37
Q

What is frame processing and what are the steps?

A

Frame processing is

  1. sending/forwarding device including source and destination MAC address.¨
  2. Receiving NIC checks if the destination MAC address matches its own MAC address stored in its RAM.
  3. If yes, it will send the frame up the OSI layers, else frame will be discarded. (if its multicast or broadcast they will also be accepted if host is in same group)
  4. Successful frame will be decapsulated by upper layer.
38
Q

What is a Controlled Access access method?

A

Controlled access:

Deterministic access where each node as its won share of stranmission time; used on legacy token ring - using either reservation, polling or token access.

39
Q

What is Unicast MAC address

A

Unique address used for one-to-one (src to dst)

Src MAC addr should always be unicast.

40
Q

What is Address Resolution Protocol (ARP) and Network Discovery (ND)?

A

The process of finding a destination MAC address associated with an IPv4(ARP) or IPv6(ND) address.

41
Q

What is a broadcast MAC address? And what is its properties?

A

A MAC address that is received and processed by every device on Ethernet LAN.

It has a destination MAC address of FF-FF-FF-FF-FF-FF in hexadecimal (48 ones in binary).
It is flooded out all Ethernet switch ports except the incoming port.
It is not forwarded by a router.

42
Q

What do we need to think about when encapsulating iPv4 broadcast?

A

The host portion of the Ipv4 dst address should be all 1s e.g 192.168.1.(255)

Basically, all hosts on local network will accept the packet.

43
Q

What is a multicast MAC address? And what are its properties?

A

A mutlicast mac address is a Ethernet multicast frame received by a group of devices that belong to the same multicast group

Flooded out all Ethernet switch ports except the incoming port, unless the switch is
configured for multicast snooping.

Not forwarded by a router, unless the router is
configured to route multicast packets.

Represent a group of addresses (host group) and can only be used as dst MAC addr.
Source MAC addr will always be a unicast.

As with the unicast and broadcast, the multicast IP address requires a corresponding
multicast MAC address.

44
Q

How does forwarding work

A

(if the dst MAC address is unicast.)

The L2 ethernet switch maintains a MAC address table to decide which port to forward the incoming frame to(except the incoming port)

If no match, forward the frame o all other ports except incoming. (unknown unicast)¨
If multicast or broadcast, also flood on all ports except incoming.

The table is empty on startup

45
Q

Which forwarding methods do we have?

A

Store and forward switching

Cut-through switching

46
Q

What is Store-and-forward switching? Any advantages?

A

Store and forward switching is a forwarding method where it receives the whole frame and computes the cyclic redundancy check(CRC). If the CRC is valid, look up destination address, and determine the outgoing interface. Then forwards the frame to the correct port.

Advantage: Can determine error before propgation saving bandwidth.
It can do this because it gets the whole frame at once.

47
Q

What is Cut-through switching? And which two variants does it have?

A

Forwarding method where it forwards the frame before it is entirely received. At minimum, the destination address must be read, does not check for errors!

-Fast-Forward switching :
Typical cut-through method; offers the lowest level of latency by immediately forwarding a packet after reading destination. Because it starts forwarding before the entire packet is received, there may be packets relayed with errors. The destination NIC discards the errenous packet upon receipt.

Fragment free switching:
Mix between fast forward and store and forward. The switch stores and performs an error check on the 64 bytes of the frame before forwarding. This is because most network errors and collisions occur uring the first 64 bytes, this ensure that a collision has not occured before forwarding the frame.

48
Q

What is the MAC subplayer?

A

One of the two Data Link sublayers

Implemented in hardware, and is responsible for data encapsulation and media access control.

Provides data link layer addressing

Integrated with various physical layer technologies

49
Q

What is Port-Based memory?

A

Frames stored in queues linked to specific incoming/outgoing ports.

A frame is sent to the outgoing port only when all frames in queue have been successfully transmitted

Possible for a single frame to delay the transmission of all frames in memoery because of busy destination port.

The delay occurs even if the other frames could be transmitted to open destination ports.

50
Q

What is shared memory?

A

Deposits all frames into a common memory buffer shared by all switch ports and the amount of memory required by a port is dynamically allocated.

The frames in the buffer are dynamically linked to the destination port enabling a packet to be received on one port and then transmitted to another port, without moving it to a different queue.

Results in larger frames that can be transmitted with fewer dropped frames. THis is important with assymetric switching which allows for different data rates on different ports. More bandwidth can be dedicated to certain ports.

51
Q

What is autonegotiation?

A

Enables two devices to automatically negotiate the best speed and duplex capabilities.

52
Q

Duplex mismatch

A

When one or both ports on a link are reset, the autonegotioation process does not result in both link partners having the same config.

Or configures one side and not the other.

Komtek (10 decks)

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