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IT 377 - Practical Telecommunication Networking > Homework Study Guide > Flashcards

Homework Study Guide Flashcards

0
Q

What is the main role of the level 3 network layer? How does this relate to the level 2 network layer?

A

The main role of the level 3 network layer is to provide routing; that is, an end-to-end route for an application over the network? The level 2 network layer in turn takes care of setting up point-to-point connections to make the larger end-to-end route possible.

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1
Q

Define network routing?

A

Network routing is the act of finding a network route/path between two ends of an application.

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2
Q

What is a frame?

A

A frame is a structure for containing data, usually formatted based on standards. It contains not only the data being sent by the sender, known as the “payload”, but it also contains the sender and receiver’s address, and other protocol information, like error checking.

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3
Q

Explain the concept of minimum frame size. Why is it important? How does it relate to network segment length?

A

Minimum frame size is a concept that was needed to make sure collision detection was a valid option. For collision detection to work, the sender must remain available to receive the collision signal. The amount of time the sender needs to be listening is the amount of time it would take for a packet to make it to the end of the segment, and then for a collision to “echo” back, that is, twice the length of time it’d take for the signal to propagate to the end of the line and back. The sending node, while it is sending, is listening for collisions. The minimum frame size ensures that it’s listening for long enough that it won’t finish transmitting before a collision could reach it from the very end.

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4
Q

What is FCS? How does it work?

A

FCS, or Frame Check Sequence, is a four-byte field that is used to perform error checking. When a frame is sent, its source node performs what’s called a cyclic redundancy check (CRC), which takes all the data in the frame before the FCS, and applies an algorithm which generates a unique, 4-byte field. The receiving node then can “unscramble” the FCS by applying the same algorithm, and confirm that the data is unchanged.

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5
Q

What is the preamble? What is it used for, and how long is it?

A

The preamble is a seven-byte segment that precedes an ethernet frame to inform the sender that a data flow is incoming. It is followed by a one-byte start-of-frame delimiter (SFD), which identifies where the data fields actually begin. In total, it makes for 8 bytes that aren’t generally counted in a frame’s size.

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6
Q

What is a header? What does it do, and how long is it?

A

The header is a 14-byte field at the beginning of each ethernet frame, containing information like the destination and source address (MAC address, 6 bytes each), and an extra field that can be used for multiple things, depending on the protocol.

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7
Q

What makes up the “frame” around the data in the ethernet protocol, and how long is it?

A

The frame around the data in the ethernet protocol is 18 bytes in total, with 12 bytes making up the sender and receiver address, 2 bytes taking up with “utility” space, and four bytes needed for the FCS.

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8
Q

What is the minimum and maximum amount of data able to be contained within an ethernet frame? What happens if the minimum is not hit? What is the total size (min/max) of an ethernet frame.

A

The minimum amount of data that can be contained in an ethernet frame is 46 bytes, and the maximum is 1,500. If the minimum isn’t met, “padding” is added to the data field. The total size of an ethernet frame is 64/1518 bytes.

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10
Q

Why do larger frame sizes usually result in faster transmission speeds?

A

Larger frame sizes result in higher transmission speeds due to the fact that the smaller frame sizes mean the need for more frames to be sent, and each frame has to take up additional space with its packet structure (header, etc), and CRC must be run on additional packets, slowing things down further.

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11
Q

Why is the network span (maximum distance) of a 100BaseT network (205 meters) approximately one tenth (1/10) of the network span of a 10BaseT network (2500 meters)?

A

The network span of a 100BaseT network is approximately one tenth of the network span of a 10BaseT network because both networks have minimum frame sizes, and the network spans are tied directly to the minimum frame transmission time (that is, the time it takes to transmit that specific minimum frame size) in order to be able to detect collisions.

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12
Q

What is the organization that specifies the standards for Ethernet?

A

The IEEE is the organization that specifies the standards for Ethernet.

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13
Q

Which of the Ethernet cable standards has the longest segment length (without using any extension repeaters)? Why?

A

Of the Ethernet cable standards, 10Base5 has the longest segment length.

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14
Q

What is wireless ethernet?

A

Wireless ethernet is a data link layer LAN networking technology, put in place by the IEEE 802.11.

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15
Q

What is Token Ring?

A

Token Ring is a layer 2 LAN networking protocol, which uses a three-byte “token” to signify which node is allowed to transmit. It uses a star topology, but is artificially forced into acting as a ring (thus, token-ring). It was standardized in IEEE 802.5.

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16
Q

What is FDDI?

A

FDDI, or Fiber Distributed Data Interface, is a LAN data transmission standard.

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17
Q

What is frame relay?

A

Frame relay is a WAN-based physical and data-link layer technology that uses packet switching.

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18
Q

What information is used by an Ethernet switch to build its MAC forwarding table?

A

A Ethernet switch’s MAC forwarding table is built by reading an incoming packet’s source MAC address. This allows it to determine what MAC addresses are lying beyond each of its ports.

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19
Q

What IEEE standard is FDDI similar to?

A

FDDI is the most similar to IEEE 802.5 (Token Ring), as they both use a ring topology with token passing.

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20
Q

What are the sublayers of the data link layer (L2)?

A

The sublayers of the data link layer are the Media Access Control layer, and the Logical Link Control Layer

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21
Q

Which ethernet technology doesn’t support CSMA/CD?

A

The only Ethernet technology that doesn’t support CSMA/CD is Ethernet in 10Gbps.

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22
Q

What is the purpose of the jamming signal in CSMA/CD? What does the jamming signal look like? Why is it the length that it is?

A

The jamming signal is a 32-bit signal that is sent by a node when it detects a collision. This signal indicates to anyone receiving it that the previous transmission was corrupted. The jam signal is 32 bits because it is designed to be put in the CRC field, which is very unlikely to be the correct CRC value, and will be the last thing received.

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23
Q

What is a bridge?

A

A bridge is a layer 2 device that connects two network segments. An ethernet switch is an example of a multi-port bridge.

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24
Q

What is a hub?

A

A hub is a layer one device that acts as a repeater, rebroadcasting frames it receives.

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25
Q

What are the minimum and maximum frame sizes of an ethernet frame?

A

The minimum frame size of an ethernet frame is 64 bytes, and the maximum size is 1518 bytes.

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26
Q

With what media would you directly connect one PC to another PC, using ethernet?

A

If you were to connect two PCs directly without using a hub or a switch, you would use a twisted-pair cable.

27
Q

What is the 5-4-3 rule?

A

The 5-4-3 rule refers to the ideal layout in a 10Mbps ethernet network, with no more than five segments, four repeaters, or three populated segments between any two nodes.

28
Q

What is the size of a MAC address?

A

A MAC address is 6 bytes long.

29
Q

What are the advantages of an ethernet switch?

A

An ethernet switch supports full duplex, has higher throughput, and has no network span limits.

30
Q

What is the network connector for 10Base2?

A

10Base2 uses the BNC-T connector.

31
Q

What is the copper standard for 10Gbps ethernet?

A

The copper standard for 10Gbps is 10GBase-CX.

32
Q

What standard has the best support for prioritized traffic?

A

The best support for prioritized traffic is 100VG.

33
Q

What is the difference between collision domain and broadcast domain?

A

Collision domains allow layer 1 signals to be sent without interruption, broadcast allows layer 2 signals to be sent without interruption.

34
Q

If a collision happens on the 2nd try, what is the probability of it happening again on the 3rd try?

A

On the first try, the random is [0,1], so each sender has a 50% chance, 2 X 50% X 50% = 50%. The X2 is due to there being 2 digits.
On the 2nd try, the random window is [0, 1, 2, 3], giving each sender a 25% chance, so 4 X 25% X 25% = 25%.

35
Q

What was the first copper standard for Gigabit Ethernet, and why was it not used by the industry?

A

The first copper standard for Gigabit Ethernet was 1000BaseCX (IEEE 802.3z), and wasn’t adopted because of the extremely limited network span of 25 meters. This was corrected by 1000BaseT, which extended the minimum frame size from 64 bytes to 512 bytes, increasing the network span to 200 meters. It also doesn’t use CSMA/CD, using full duplex, which removes the need for collision detection

36
Q

What is store-and-forward?

A

Store-and-forward is a operation mode of switches that involves the full reception of a frame, and the running of CRC error checking, before it sends the frame on.

37
Q

What is cut-through?

A

Cut-through is a method of operation in which a message is transmitted as soon as it is able, before it is even fully received. It sends generally as soon as the destination segment is received.

38
Q

What is broadcasting? How does it work?

A

Broadcasting is a layer 2 mechanism in which the message is sent to all ports on a device. This is done when the destination doesn’t have a clear route in the MAC forwarding table.

39
Q

What is STP? What does it do?

A

STP stands for spanning tree protocol, and it is a method to allow for the mesh topology to be used without worrying about loops or broadcast storms. Defined in IEEE 802.1D, it functions by identifying the “root” bridge, that is, the port on the device with the lowest “bridge ID”, a number based off the MAC address and priority number. From that bridge and every other bridge on the network, STP determines the shortest path through the network. It then intelligently disables ports, removing loops and ensuring that only the fastest routes are available.

40
Q

What is a root port?

A

A root port is a port on a bridge in a STP network that makes up the shortest path back to the root bridge.

41
Q

What is a designated port?

A

A designated port is the port on a network segment that is used to reach the root bridge. If one port on a switch is a root port, then the segment that leads out of that port connects to a designated port.

42
Q

What are the states of STP? Of RSTP?

A 
The STP states are as follows:
Blocking
Listening
Learning
Forwarding

The RSTP states are as follows:
Disregarding
Learning
Forwarding

43
Q

What are the timers that are used by STP? What are they used for?

A 
The timers used by STP are:
Hello Timer
Aging Timer
Max Age
Forward Delay
44
Q

What are the advantages of RSTP over plain STP? Why?

A

Compared to standard STP, RSTP has a considerably shorter fail-over time. It requires and uses full-duplex lines, and is able to recognize ports that aren’t in use (edge ports), and bring them online if a BPDU is heard.

45
Q

What is a BPDU?

A

A BPDU, or bridge protocol data unit, is a packet sent as a part of the STP system. It is used to test the “integrity” of the spanning tree, and contains information like the supposed path to the root bridge, the path cost of the route to the root bridge, and so on.

46
Q

How does half-duplex ethernet implement flow control?

A

Half-duplex ethernet uses flow control by sending a frame back the line that is transmitting, causing a jam.

47
Q

What is the MAC control frame defined by the 802.1Q standard?

A

The MAC control frame defined by the 802.1Q standard is the PAUSE frame.

48
Q

How many VLANS can be defined by the 802.1Q standard?

A

The 802.1Q standard can have up to 4096 VLANs.

49
Q

What is the maximum size of a tagged 802.1Q frame?

A

Since the default size of an Ethernet frame is 1518 bytes, and the tag adds an additional 4 bytes, the maximum size of the frame is increased to 1522 bytes.

50
Q

How do switches and bridges exchange control information with one another in regards to the STP?

A

Switches and bridges exchange control with one another via the Bridge Protocol Data Unit, or BPDU.

51
Q

How many priority queues can be specified in an 802.1p-enabled frame?

A

Up to eight priority queues can be specified in an 802.1p-enabled frame, as there are three bits reserved for prioritization.

52
Q

When is a VLAN ID added to an 802.1Q tagged frame?

A

VLAN ID is added to an 802.1Q tagged frame at the ingress switch.

53
Q

What are the two essential functions of a bridge?

A

The two essential functions of a bridge are to learn of its neighbors by building its MAC forwarding table, and to forward traffic.

54
Q

What are the four steps undertaken by the STP protocol?

A

The four steps taken by the STP protocol are as follows:

  1. Determine the root bridge from the lowest MAC address and priority.
  2. Determine the root ports on all non-root bridges.
  3. Determine the designated ports on all network segments.
  4. Block all remaining ports.
55
Q

How do ports in the “blocking state” behave in STP?

A

As long as a “blocked” port continues to receive a BPDU from its neighbors before its Max Age Timer expires, it will remain in blocking state. If it doesn’t, it will transition into the “listening” state, where it will participate in the next STP process.

56
Q

In STP, what does it mean for a port to be disabled?

A

When a port is disabled, it means that it was manually told to be in a blocking state, but to disregard BPDUs, only responding to further admin action.

57
Q

In STP, what is the aging timer?

A

In STP, the aging timer is the amount of time a particular entry is kept in the switch’s forwarding table after receiving a packet from said entry before it is discarded. It is generally 300 seconds.

58
Q

In STP, what is the Forwarding Delay Timer?

A

The forwarding delay timer, in STP, is the amount of time a bridge/switch remains in the Listening and Learning state before it enters the Forwarding state. There is a 15 second delay before leaving the Listening state, and a second 15 second delay before leaving the Learning state.

59
Q

What is the Hello Timer in STP?

A

In STP, the Hello Timer (usually 2 seconds) is the interval at which the root bridge and designated bridges (currently active network nodes) send, and forward, in turn, BPDUs in order to keep the topology of the network known.

60
Q

What are some of the problems associated with STP?

A

Some of the main problems associated with STP are:
- There is an excessively long fail-over time, between 45-60 seconds.
- When there is a failure, the entire network must be re-calculated, putting all ports in the “blocked” state.
RSTP solves these issues.

61
Q

What are the five port types used by RSTP?

A

The five main port types used by RSTP are:

  • Root Port: Same as STP
  • Designated Port: Same as STP
  • Alternate Port: This port is an alternate path to the root port, and is in discarding state until there’s a failure in the root port path.
  • Redundant Port: This port is a backup for a network segment, in case that segment fails.
  • Disabled Port: This port has no role whatsoever.
62
Q

Why are VLANs so beneficial, in terms of cost?

A

VLANs allow the use of switches to be used instead of routers, and switches are exceptionally cheaper.

63
Q

What sort of domains are VLANs?

A

VLANs are broadcast domains.

IT 377 - Practical Telecommunication Networking (4 decks)

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