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CS 455 > Data-Link Layer > Flashcards

Data-Link Layer Flashcards

1
Q

Introduction

Differences between layers

A

Transport Layer: communication between applications
Network Layer: communication between source and destination
Link Layer: communication between nodes (routers and hosts)

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2
Q

Introduction

Nodes

A

Hosts and routers

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3
Q

Introduction

Data-Link Layer

A
  • Communication between nodes
  • Has responsibility of transferring datagram from one node to a physically adjacent node over a link
  • 2-Layer Packet: frame encapsulates datagram
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4
Q

Introduction

Link

A

Communication channels that connect adjacent nodes along communication path

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5
Q

Introduction

Link Types

A
  • Wired
  • Wireless
  • LANs
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6
Q

Introduction

Link-Layer: Context

A
  • Datagrams are transferred by different protocols over different links (wifi, ethernet, …)
  • Each link protocol provides different services (may not be reliable)
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7
Q

Introduction

Link Layer: Services

A
  • Framing, link access
  • Reliable delivery between adjacent nodes
  • Flow control
  • Error detection and correction
  • Half-duplex and full-duplex
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8
Q

Introduction: Services

Framing, link access

A
  • Encapsulates datagram into frame, adding header/trailer
  • Controls channel access if shared medium
  • “MAC address” in frame headers identifies source/dest
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9
Q

Introduction: Services

Flow Control

A

Pacing between adjacent sending and receiving nodes

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10
Q

Introduction: Services

Error detection and correction

A
  • Errors caused by singal attentuation, noice
  • Receiver detects errors and signals retransmission or drops frames
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11
Q

Introduction: Services

Half-duplex and full-duplex

A

With half-duplex, nodes at both ends of link can transmit, but not at same time

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12
Q

Introduction

Where is the link-layer implemented?

A
  • In each and every host
  • Uses network interface card (NIC) or on a chip
  • Attaches into host’s system buses
  • Combination of hardware, software, and firmware
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13
Q

Introduction

Interface Communication: Sender Side

A
  • Encapsulates datagram in frame
  • Adds error checking bits, reliable data transfer, flow control, etc.
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14
Q

Introduction

Interface Communication: Receiver Side

A
  • Looks for errors, reliable data transfer, flow control, etc.
  • Extracts datagram, passes to upper layer at receiving side
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15
Q

Introduction

Error Detection

A

Datagram: d data bits + EDC
EDC: error detection and correction bits, larger EDC yields better detection
D: data protected by error checking, may include header fields

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16
Q

Introduction

Parity Checking: Single Bit Parity

A

Detects single bit errors
Even Parity: Set parity bit so there is an even number of 1s

17
Q

Introduction

Parity Checking: Two-Dimensional Bit Parity

A

Detect and correct single bit errors
(if more then one error, cannot correct)

18
Q

Introduction

CRC

A
  • Cyclic Reduncancy Check
  • Goal: choose CRC bits, R, such that <D, R> exactly divisible by G (mod 2)
  • If non-zero remainder: error detected
  • D: data bits (given, binary)
  • G: bit pattern (generator), of R+1 bits (given)
19
Q

Multiple Access Protocols

Link Types

A

Point-to-Point: link between Ethernet switch and host
Broadcast: Uses old-fashioned Ethernet, cable-based access network (shared)

20
Q

Multiple Access Protocols

Multiple Access Protocol

A
  • Used on single shared broadcast channels
  • Determines how nodes share channel (determines when node can transmit)
  • Collision: if simultaneous transmission/two or more signals received by node
21
Q

Multiple Access Protocols

Three Broad Protocol Classes

A
  1. Channel Partitioning
  2. Random Access
  3. “Taking Turns”
22
Q

Multiple Access Protocols

Protocol Classes: Channel Partitioning

A
  • Divide channel into smaller partitions
  • Allocate partition to node for exclusive use
23
Q

Multiple Access Protocols

Protocol Classes: Random Access

A
  • Channel not divided, allow collisions
  • Specifies how to detect collisions and how to recover from them
  • Examples: ALOHA/slotted ALOHA, CSMA/CD/CA
24
Q

Multiple Access Protocols

Protocol Classes: “Taking Turns”

A
  • Nodes take turns using the shared channel
  • Nodes with more to send can take longer turns
25
Q

Multiple Access Protocols

Channel Partitioning MAC Protocols: FDMA

A
  • Frequency Division Multiple Access
  • Channel spectrum divided into frequency bands
  • Each station assigned fixed frequency band
  • Con: Unused transmission time in frequency bands go idle
26
Q

Multiple Access Protocols

Channel Partitioning MAC Protocols: TDMA

A
  • Time Division Multiple Access
  • Access to channel is given in “rounds”
  • Each station gets fixed length slot in each round (length = packet transmission time)
    * Con: unused slots go idel
27
Q

Multiple Access Protocols

Random Access Protocols: Pure ALOHA

A
  • Simpler then slotted bc no synchronization (when frame arrives, transmit immediately)
  • Higher collision probability bc no synchronization
  • Vulnerable Time: 2t (if a single bit overlaps, then both frames with value t have to retransmit)
28
Q

Multiple Access Protocols

Random Access Protocols: Slotted ALOHA (Assumptions)

A
  • All frames same size
  • Time divided into equal size slots
  • Nodes to start transmission only at beginning of slot
  • Nodes are synchronized
  • If 2 or more nodes transmit in slot, all nodes detect collision
29
Q

Multiple Access Protocols

Random Access Protocols: Slotted ALOHA (Operation)

A

When node obtains frame, transmits in next slot:
If no collision: node can send new frame in next slot
If collision: node retransmits frame in each subsequent slot with prob b until success

Vulnerable Time: t

30
Q

Multiple Access Protocols

Random Access Protocols: Slotted ALOHA (Pros)

A
  • Single active node can continuously transmit at full rate of channel
  • Highly decentralized: only slots in nodes need to be in sync
  • Simple
31
Q

Multiple Access Protocols

Random Access Protocols: Slotted ALOHA (Cons)

A
  • Collisions, wasting slots
  • Idle slots
  • Nodes may be able to detect collision in less time then time to transmit packet
  • Clock synchronization
32
Q

Multiple Access Protocols

Slotted ALOHA Channel Utilization

A

Channel Utilization = # of slots / # of successes

CS 455 (11 decks)

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