Logical Link Control Flashcards
What are the 5 problems we address in the Linked Layer?
- Encoding (transmit so receiving node can understand)
- Framing (put bits into complete messages)
- Error Detection
- Reliable Delivery
- Multi-Access Control
What does NRZ (non-return to zero) encoding do?
Follows the desired bit message after every clock cycle… does not change it
What does NRZI (non-return to zero inverted) encoding do?
For 1s, there is a transition at the positive edge of the clock cycle. For 0s no transition, same level of signal transmitted
What does Manchester encoding do?
For 1s, high to low signal within clock cycle. For 0s, low to high signal within clock cycle.
What does 4B/5B encoding do?
It inserts extra bits into the bit stream to break up long sequences of 0s or 1s. Specifically, every 4 bits of actual data are encoded in a 5-bit code that is then transmitted to the receiver.
00000 = line dead
111111 = line idle
The rest of the 5 bits are only valid if no leading 0 (1 zero) and no trailing 00s (2 zeros).
What are the 2 problems associated with NRZ encoding?
- BASELINE WANDER
adapter determines baseline (i.e. between low and high) based on average of incoming signals.. so if we have repeated 1s or 0s, our baseline is skewed and may not be able to used for comparison to find high vs. low. - CLOCK DRIFT
we have an adapter to encode and an adapter to decode… both of which use clocks and these clocks must be synchronized!
They get synchronized when we transition: 0→1 or 1→0. … once again, if we have many 1s or 0s may not sync for some time.
What is the problem associated with NRZI encoding?
fixes problems of NRZ for repeated 1s but not for repeaded 0s
What is the problem associated with Machester encoding?
baud rate is double bit rate… so efficiency = 50%
What is the efficiency of 4B/5B encoding?
80%, baud rate = 0.8 * bit rate
(5 bits/ signal transitions to represent 4 bits of information)
What is baud rate?
The rate at which the signal changes
What is a last-mile link?
Cost-effective link that allows millions of customers to be connected, but cannot be used to create networks from scratch.
The final segment of a telecommunications network that delivers services to end-users
What is a back-bone link?
A link that can be used to make a network from scratch. After its installation, can keep adding to network.
High-capacity network segments that connect different parts of a network, often spanning long distances.
What is a mobile link?
A wireless connection that provides network access to mobile devices
What is a Local Area Network (LAN)?
Network connections within a limited area, such as a building or campus.
i.e. Ethernet and Wi-Fi
What is the task of framing?
To recognize what which set of bits constitute a frame
What are 3 approaches to framing?
- BYTE-ORIENTED PROTOCOLS
- BIT-ORIENTED PROTOCOLS
- CLOCK-BASED FRAMING
What are the 2 approaches of Byte-Oriented Protocols?
- Use sentinel characters
[SYN] then [STX] [PAYLOAD] [ETX]
-includes character stuffing… if
[STX] or [ETX] show up in payload, adapter puts [DLE] to indicate that this [STX] or [ETX] is not for end/ start of frame.
EXAMPLE: PPP (Point-to-Point Protocol) - Include number of bytes in beginning of frame (frame header)
-can have error in this number in header… resulting in wrong number of bits being read.
What does “bit stuffing” refer to in the Bit-Oriented Protocols approach to Framing?
It refers to adding a 0 after 5 1s in the payload to differentiate it from the [Beginning Sequence] and [Ending Sequence].
The receiving adaptor knows that a 0 after 5 1s is “bit stuffing” and ignores it when extracting payload.
Explain the Bit-Oriented Protocol approach to Framing.
[Beginning Sequence] [Header] [Payload] [CRC] [Ending Sequence]
[Beginning Sequence] = [Ending Sequence] = 01111110 for HDLC
These sequences allow us to know when a frame starts and ends.
What are the extra bits added to frames to help detect error called?
Error-detecting codes or checksum
What are the two basic approaches to Error Detection?
Identify when error occurred and…
1. Ask sender to resend
2. Reconstruct original, correct message
What are the extra bits added to frames to help CORRECT error called?
Why wouldn’t we opt for these bits instead of the error-detecting ones?
What are the uses for these bits?
Forward error correction codes
While having to resend packet can take some time, having to send larger codes (which is required here) ALL THE TIME can take even more time.
So, Forward Error Correction Codes are often used for when errors are very likely (i.e. in Wi-Fi, wireless connections) and when the transmission time is large (i.e. sending signals to a satellite)
