2. Ethernet Networking Flashcards
Formal definition of a broadcast domain
A broadcast domain refers to a group of devices on a specific network segment that hear all the broadcasts sent out on that specific network segment
What device can you use to split up a collision domain?
A switch
What device can you use to break up a broadcast domain?
A router
Does a wireless access point (AP) segment a network?
No, an AP only extends the LAN with an unknown number of hosts that are all still part of one broadcast domain
What is used to further break up broadcast domains?
A virtual LAN (VLAN) can be used to further break up a broadcast domain to give greater bandwidth to individual users
What does CSMA/CD stand for?
Carrier Sense Multiple Access with Collision Detection
What is CSMA/CD?
CSMA/CD is an Ethernet protocol.
It helps devices share the bandwidth evenly, by trying to prevent two devices from transmitting simultaneously on the same network medium
How does CSMA/CD work?
- A host checks for the presence of a digital signal on the wire. If all is clear, it will proceed with its transmission.
- The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting.
- If the host detects another signal on the wire it sends out an extended jam signal (this causes all nodes on the network to stop sending data).
- The nodes respond to that jam signal by waiting before attempting to re-transmit. (Back off algorithms determine when the collided stations can re-transmit.)
- If collisions keep occurring after 15 tries the nodes attempt to transmit will timeout.
- All hosts have equal priority to transmit after the backoff time as expire.
Is CSMA/CD half duplex or full duplex?
Half duplex
What’s the physical difference between half duplex and full duplex?
Half duplex uses a single pair of twisted wires to send digital signals in both directions. Thereby collisions are possible.
Full duplex uses two pairs of wires, so transmitting and receiving can occur simultaneously and no collisions occur. This allows for a point to point connection between the transmitter and receiver, giving faster data transfer rates
Between which devices can you use for full-duplex ethernet?
(Basically anything that doesn’t involve a hub)
With a connection from:
Switch to switch
Router to router
Host to host
Switch to host
Router to switch
Router to host
What is the Ethernet auto-detect mechanism?
This is the process where an ethernet port when first powered on connects to the remote end (port) and negotiates the connection speed and type i.e. 10, 100, 1,000 Mbps (or more) and whether it can run full-duplex, (if it can’t, it will run half-duplex)
What is the default behaviour if auto detect mechanism fails?
Default behaviour of 10Base-T and 100Base-T is 10 Mbps, half duplex
What is a MAC?
Media access control.
This is a unique (6 byte) address burnt into each and every ethernet network interface card (NIC)
MAC address fields
I/G = Individual/Group
0 = individual address
1 = broadcast or multicast address
bit 47
G/L = Global/Local (Universal/Local)
0 = globally administered address assigned by IEEE
1 = locally administered address
bit 46
OUI = Organisationally Unique Identifier
Assigned by IEEE to each manufacturer
22 bits long (24-45)
0-23 Vendor assigned, often used as part of serial number
Ethernet (II) frame fields
Preamble
SDF
DA
SA
Type
Data
FCS
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Preamble
7 bytes
Alternating 10 pattern provide a clock at the start of each packet which allows the receiving devices to lock the incoming bitstream
SFD Start Frame Delimiter/Synch
1 byte
10101011, where the last pair of ones allows the receiver to come into the alternating 10 pattern somewhere in the middle and still sync up to detect the beginning of data
DA Destination Adress
6 bytes
The DA can be an individual, broadcast or multicast MAC address. Transmitted least significant bit (LSB) first.
SA Source Address
6 bytes
Used to identify the transmitting device, it uses LSB first. Broadcast and multicast address formats are illegal within the SA field.
Type
2 bytes
Used to identify the upper network layer protocol.
Data
46-1500 bytes
This is a data packet sent down from the network layer. The ethernet frame is designed to encapsulate this data and transmit it safely.
FCS Frame Check Sequence
4 bytes
Used to store the cyclic redundancy check (CRC). If a receiving host runs the CRC and the answer is not the same the frame is discarded assuming errors have occurred.
What’s a difference between an Ethernet_II and a 802.3 ethernet frame?
The old 802.3 protocol used a length instead of a type field, hence it cannot identify the upper layer protocol and must be used with a proprietary LAN-IPX
Who first implemented ethernet?
A group called DIX
Digital, Intel and Xerox
EIA/TIA
Electronic industries alliance and telecommunications industry association
This is the body that creates the physical layer specifications for ethernet
What does the RJ stand for in RJ-45?
Registered Jack
What is cable attenuation?
How can you reduce it?
This is a loss of strength as the signal travels the length of a cable and is measured in a decibels (dB)
Attenuation can be reduced by having more wire twists per foot, this reduces crosstalk between wires
10Base-T
10 Mbps
802.3
Cat 3 UTP
100m
RJ-45
Physical star topology and a logical bus
100Base-TX
802.3u
100 Mbps
Also called ‘Fast Ethernet’
Cat 5, 5e, 6 UTP 2pair wiring
100m
RJ-45
Physical star topology and a logical bus
100Base-FX
802.3u
100 Mbps
62.5 core, 125 cladding multimode (F)ibre
412m
ST, SC media-interface connectors
Point to point topology
