4. Datalink Layer Protocols Flashcards
(36 cards)
1
Q
Datalink Layer Protocols define rules & procedures for:
A
- Framing
- Addressing
- Transmitting
- Receiving data on a network segment
2
Q
What does the Standard Ethernet do?
A
- Provides connectionless service
- connects devices within a LAN
- Uses CSMA/CD
- Uses 1-persistent method (send once channel is idle)
3
Q
Cable 10BaseX implies:
A
- data rate: 10Mbps
- digital signal
- X = type/length of cable:
- 5 = 500m coaxial
- 2 = 185m coaxial
- T = Twisted Pair (UTP)
- F = Fiber-optic
4
Q
Give details about Fast Ethernet
A
- 100 Mbps
- compatible with standard Ethernet:
- same frame format & size
- no CSMA as hosts don’t compete with each other.
- uses full-duplex links.
- uses switches with buffers to store frames.
5
Q
Describe 2 Ethernet Encoding methods
A
MLT-3:
- 3 levels (-1, 0, 1)
- signal level changes if next bit is 1
8B6T:
- Converts 8 bits to 6 base-3 symbols.
6
Q
Describe 4B/5B encoding
A
- Converts 4 bits into 5-bit codes
- Ensures frequent transitions for clock sync.
- 25% overhead (extra bits for reliability)
7
Q
Describe 8B/10B encoding
A
- Converts 8 bits into 10-bit codes
- high speed transmission
8
Q
Gigabit Ethernet
A
- Speed: 1000 Mbps (1 Gbps)
- same frame format & length
- same address size
9
Q
Ethernet addressing: define a MAC address
A
- 6 bytes in hex with colon every 2 bytes (xx.xx.xx.xx.xx.xx)
- assigned by NIC of station
10
Q
Define the Unicast address type
A
- Source address is always unicast (frame comes from only 1 station)
- destination address can be unicast, multicast or broadcast
11
Q
Define the Multicast address type
A
- defines group of devices that should receive the frame.
- First byte can’t be 0 (LSB = 1)
12
Q
Define the Broadcast address type
A
- defines all hosts
- Broadcast MAC address consists of all 1s
13
Q
Define the Preamble field of a frame
A
- to sync clocks
- receiver may miss some bits
14
Q
Define the SFD of a frame
A
- 1 byte (10101011)
15
Q
When does the Type/Length field of a frame define the type or the length?
A
- if more than 1500: defines length
- if less than 1500: defines protocol type of encapsulated payload
16
Q
Define the Data field of a frame
A
- carries data encapsulated from upper-layer protocols.
- if <46 pad with 0s
17
Q
Define the CRC (error detection) field of a frame
A
- calculated over address, type & data fields.
- if CRC = 0 discard frame
18
Q
How does CSMA/CD avoid collisions?
A
- Compare sent vs received bits
- if collision, wait & resend
- interframe gap (IFG): 12 bytes between frames
19
Q
Signal attenuation limits LAN range, what amplifying devices r used to balance this out?
A
- Repeaters
- Hubs
- Switches
- cut-through switching
20
Q
What does a Repeater do?
A
- join LANs together
- amplify/regenerate signals
- extend network range
21
Q
What does a hub do?
A
- join input lines
- broadcast signals to all ports
- is a collision domain
22
Q
What does a switch do?
A
Connect computers:
- extract destination addy from frame.
- looks up destination in table.
- forwards frame to appropriate host
- support simultaneous communication
23
Q
Pros of switches:
A
- reduces collisions
- each connection can send in both directions
- secure & private
- if busy, store frame in buffer
- frame sent only to necessary host
- can adjust sender & receiver transmission rates
24
Q
Disadvantages of switches
A
- high cost
- needs to build forwarding table
- forwarding delay (as needs to receive, parse, look-up store & forward)
25
What does the Cut-Through Mode do?
- check 1st 6 bytes (destination MAC addy)
- starts forwarding before full frame is received
- no CRC checking
26
Collisions deteriorate performance of broadcast networks.
Give a solution for this.
- Minimize broadcast messages
- Use unicast transmissions where possible
27
Switch learning:
when a frame arrives, the switch:
- inspects source MAC addy
- Associates it with incoming interface (port)
- Stores mapping in switch table
- Uses TTL to forget mapping later
28
What does a **Switch** / **Multi-switch network** do if the destination addy of the frame isn’t found
- S: forwards frame to all ports
- M-S: forwards frame to all connections (including to other switches)
29
Multiple switches may cause loops, which causes frame duplication.
Give a solution for this.
- disable redundant links
30
Define the Software defined LANs Virtual LANs (VLAN)
Segments physical network into multiple logical networks.
Provides:
- Flexibility
- security
- broadcast control
- optimization
31
VLANs with multi-Switch Networks:
Members of a VLAN can be connected to ports of different switches.
Continue defining this procedure.
- Each VLAN registers hosts connected to its ports as members of a VLAN.
- They tag broadcast frames with VLAN numbers.
- Frame structure holds VLAN tag (field used by switches not users)
32
Point-to-Point Protocol (PPP) are used in WANs.
What do they provide?
- framing
- Error detection
- Authentication
- Link negotiation (via LCP = Link Control Protocol)
33
Give the defined bytes used in Byte stuffing
- Start Flag: 01111110 (0x7E)
- Escape byte: 0x7D
- we XOR with 0x20
34
With Byte stuffing, start & end of frames can be searched by scanning for byte 0x7E. How?
If 0x7E found in payload, XOR with 0x20
35
How does Byte destuffing work?
Look for 0x7D, remove & XOR following byte with 0x20
36
Give the Wireless Datalink protocols differences from Wired
- medium is freely accessible
- collision detection may fail
- high attenuation
