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CS 455 > Data-Link Layer 2 > Flashcards

Data-Link Layer 2 Flashcards

1
Q

Multiple Access Protocols

Types of Random Access Protocols

A
  1. ALOHA
  2. CSMA
  3. CSMA/CD
  4. CSMA/CA
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2
Q

Random Access Protocols: CSMA

CSMA

A
  • Carrier Sense Multiple Access
  • Listen before transmit
  • Reduce possibility of collision
  • Cannot completely eliminate possibility of collision
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3
Q

Random Access Protocols: CSMA

Simple CSMA

A

If channel sensed idle: transmit entire frame
If channel sensed busy: defer transmission

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4
Q

Random Access Protocols: CSMA

CSMA: Persistance Method Types

A
  1. 1-persistent
  2. Non-persistent
  3. p-persistent
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5
Q

Random Access Protocols: CSMA

CSMA: 1-Persistent

A
  • Continuously senses at the beginning of each time unit.
  • Transmits when channel finally idle.
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6
Q

Random Access Protocols: CSMA

CSMA: Non-Persistent

A
  • Wait to sense for a random amount of time.
  • Transmits when channel finally idle.
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7
Q

Random Access Protocols: CSMA

CSMA: p-Persistent

A
  • Continuously sense at the beginning of each time slot.
  • When channel finally idle, send based on probability p (doesn’t send every time).
  • If probability p prevents sending, wait for set amount of time and try to retransmit with probability p again. (repeats until successful)
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8
Q

Random Access Protocols: CSMA

CSMA: Persistence Method Downfall

A
  • If 2 stations send frame at same time, collision occurs but station continues transmitting until whole frame sent
  • Wastes time instead of immediately interrupting transmission
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9
Q

Random Access Protocols: CSMA

CSMA/CD

A
  • Carrier Sense Multiple Access/Collision Detection
  • Collisions detected within short time
  • Colliding transmissions aborted, reduces amount of time wasted in collisions
  • Collision detection easy in wired, difficult in wireless
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10
Q

Random Access Protocols: CSMA

CSMA: Collisions

A
  • Collision: entire packet and transmission time wasted
  • Collisions can still occur even if carrier is sensing (two nodes can start at exact same time)
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11
Q

Random Access Protocols: CSMA

CSMA/CD: Algorithm

A
  1. NIC receives datagram from network layer, creates frame
  2. NIC senses channel (if idle: transmit, if busy: wait until channel idle)
  3. If NIC transmits entire frame without collision, then complete
  4. If NIC detects collision, abort and send jam signal
  5. After aborting: NIC enters Binary (Exponential) Backoff
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12
Q

Random Access Protocols: CSMA

CSMA/CD: Binary (Exponential) Backoff

A
  • After mth collision, NIC chooses K at random from {0, 1, 2, …, 2^m - 1}
  • NIC waits K x 512 bit times, returns to step 2 in algoritm
  • More Collisions = longer backoff interval
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13
Q

Random Access Protocols: CSMA

CSMA/CA

A
  • Carrier Sense Multiple Access/Collision Avoidance
  • Specficially designed for wireless networks
  • In Wired: if collision occured then energy of received signal almost doubles, allowing station to sense possibility of collision
  • In Wireless: can’t increase energy bc most of it used for transmission and can only increase energy by small amount
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14
Q

Random Access Protocols: CSMA

CSMA/CA: Three Strategies

A
  1. InterFrame Space (IFS)
  2. Contention window
  3. Acknowledgments
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15
Q

Random Access Protocols: CSMA

CSMA/CA Strategies: InterFrame Space (IFS)

A
  • When station finds channel busy: it senses the channel again
  • When station finds channel idle: it wait for a period of time called IFS time before transmitting
  • Can also be used to define priority of station or frame (higher IFS means lower priority)
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16
Q

Random Access Protocols: CSMA

CSMA/CA Strategies: Contention Window

A
  • Amount of time is divided into slots
  • Station that is ready to transmit chooses random value for wait time
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17
Q

Random Access Protocols: CSMA

CSMA/CA Strategies: Acknowledgments

A

Positive ACKs and time-out timer are used to help guarantee successful transmission

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18
Q

Random Access Protocols: MAC Protocols

MAC Protocol Types

A
  1. Channel partitioning MAC protocols
  2. Random access MAC protocols
  3. “Taking turns” protocols
19
Q

Random Access Protocols: MAC Protocols

MAC Protocols: Channel Partitioning

A
  • Share channel efficiency and fairly at high load
  • Inefficient at low load; delay in channel access (1/N bandwidth allocated even if only 1 active node)
20
Q

Random Access Protocols: MAC Protocols

MAC Protocols: Random Access

A

Efficient at low load: single node can fully utilize channel
High load: collision overhead

21
Q

Random Access Protocols: MAC Protocols

MAC “Taking Turns” Protocol Types

A
  1. Polling
  2. Token Passing
  3. Reservation
22
Q

Random Access Protocols: MAC Protocols

MAC “Taking Turns”: Polling

A
  • Master node “invites” other nodes to transmit in turn
  • Typically used with “dumb” devices
  • Concerns: polling overhead, latency, single point of failure (master)
23
Q

Random Access Protocols: MAC Protocols

MAC “Taking Turns”: Token Passing

A
  • Control token passed from one node to next sequentially
  • Token message
  • Concerns: token overhead, latency, single point of failure
24
Q

Random Access Protocols: MAC Protocols

MAC “Taking Turns”: Reservation Method

A
  • Stations reserve specific time-slots
  • When ready to send, uses reserved time slot
  • When all time slots reserved, waits until one is avaliable
25
Q

LANs: MAC Addressing

IP Addresses

A
  • 32-bit IP address (uses decimal)
  • Network-layer address for interface
  • Used for network layer forwarding
  • Like postal address
  • Not portable, changes upon IP Subnet change
26
Q

LANs: MAC Addressing

MAC Addresses

A
  • 48-bit MAC address (uses hex)
  • Each interface on LAN as unique MAC address (permenant at manufacturing)
  • Function: used “locally” to get frame from one interface to another physically-connected interface (like IP addressing but for same subnet)
  • Like social security number
  • Portable, always the same
27
Q

LANs: MAC Addressing

ARP

A
  • Address Resolution Protocol
  • Used to determine interface’s MAC address knowing its IP address
  • Uses ARP Table
28
Q

LANs: MAC Addressing

ARP Table

A
  • Each IP node (host, router) on LAN has table
  • IP/MAC address mappings for some LAN nodes (as well as TTL for mapping)
  • TTL: time after which address mapping will be forgotten
29
Q

LANs: MAC Addressing

ARP Steps

A
  1. Sender broadcasts ARP query with target IP address
  2. Target responds with ARP response
  3. Sender receives targets reply and adds MAC address to ARP table
30
Q

LANs: Ethernet

Data-Link Layer Responsibility

A

Tells what kind of medium (wired or wireless)

31
Q

LANs: Ethernet

Physical Layer Responsibility

A

Tells what specific version of medium type (if wired, then what kind of wire?)

32
Q

LANs: Ethernet

Ethernet Frame

A

Sending an interface encapsulates IP datagram into an Ethernet Frame
~~~
<preamble, dest address, src address, type, data, crc>
~~~
Preamble: used to synchronize receiver/sender clock rates
Addresses: 6 bytes src/dest MAC addresses (if adapter gets not matching dest address then discards, otherwise passes to network layer)
Type: indicates what higher layer protocol (usually IP)
CRC: cyclic redundancy check at receiver

33
Q

LANs: Switches

Switch

A
  • Link-layer device that takes an active role (are Ethernet participants)
  • Transparent and plug-and-play/self-learning
  • Able to handle multiple messages at once without collisions (as long as not same path)
  • Stores and forwards Ethernet frames
  • Examines incoming frame’s MAC address and selectively forwards frame to one-or-more outgoing links
34
Q

LANs: Switches

Hubs

A
  • Simple repeaters
  • Plug and play
  • Not able to handle multiple message at once, collision will occur
  • Don’t interpret Ethernet, just repeats PHY signals
35
Q

LANs: Switches

Plug-and-play

A

Device does not need to be configured

36
Q

LANs: Switches

Transparent

A

Hosts unaware of presence of device

37
Q

LANs: Switches

Self-Learning

A
  • Switch learns which hosts can be reached through which interfaces and records in switch table
  • If dest location unknown: flood (broadcast to all hosts in LAN)
  • If dest location known: selectively send on just one link
38
Q

LANs: Switches

Switch Forwarding Table

A
  • holds pair of <MAC address of host, interface to reach host, TTL>
  • Looks like routing table
39
Q

LANs: Switches

Small Institutional Networks

A

Switch:Info meant for specific department is flooded to all departments if dest location not known
Router: Info meant for specific department can be directed to just one specific department

40
Q

LANs: Switches

Switches VS Routers

A

Both are store and forward:
Routers: network-layer devices (examine network-layer headers)
Switches: link-layer devices (examine link-layer headers)

Both have forwarding tables:
Routers: compute tables using routing algorithms, IP addresses
Switches: learn forwarding tables using flooding, learning, MAC addresses

41
Q

LANs: VLANs

VLAN

A
  • Virtual Local Area Network
  • Switches supporting VLAN capabilities can be configured to define multiple virtual LANs over single physical LAN infrastructure
42
Q

LANs: VLANs

Port-Based VLAN

A

Switch ports are grouped so that single physical switch operates as multiple virtual switches

43
Q

LANs: VLANs

VLAN Frame Format

A

( preamble, dest address, src address, type (2-byte tag protocol ID + tag control info), data, crc )

2-byte tag protocol ID: indicates that dest device is on a VLAN
Tag control info: indicates which VLAN the dest device is in

CS 455 (12 decks)

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