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ECE407 Link Layer Study Guide
Introduction to Computer Networking
Terms in this set (20)
Explain the role of the link layer.
Communication channels that connect adjacent nodes along communication path: link (wired, wireless, LAN), has the responsibility of transferring datagram from one node to physically adjacent node over a link
Explain what services can be provided by the link layer.
framing, link access - encapsulate datagram into frame, adding header, trailer, channel access if shared medium, MAC addresses in frame headers identify source and destination
reliable delivery between adjacent nodes - seldom used on low bit-error links, wireless links have high error rates
flow control - pacing between adjacent sending and receiving nodes
error detection - errors caused by signal attenuation, noise, receiver detects errors, signals retransmission or drops frame
error correction - receiver identifies and corrects bit errors without retransmission
half-duplex/full-duplex - with half duplex, nodes at both ends of link can transmit, but not at same time
Explain the role of error detection and error correction codes.
The receiver detects errors and either signals for a retransmission, drops the frame, or it identifies and corrects the bit errors without retransmission
Explain how the parity bit works.
Extra 1 or 0 is added to make the bit even or odd
If the parity bit isn't correct (bit line isn't either odd or even) then the data is rejected and resent
Explain how the two dimensional parity bit can work as an error correction or an error detection code.
It has more aspects that it can look at by crossing across the 2 dimensional space and seeing the exact bit that is wrong and flip it
Link Layer slide 11
Explain how the CRC is computed.
Link Layer slide 14
D data bits and G bit patter generator of r+1 bits,
<D,R> = 2*D^r
r is the CRC bits at the end of the data bits
R = remainder[D*2^r / G]
Given a message and a generator polynomial, compute the corresponding CRC.
Link Layer slides 15-16
Explain when and why MAC protocol may be needed.
when one node wants to transmit it can send at rate R, when M nodes want to transmit it can send at R/M rate, fully decentralized, no special node to coordinate transmissions, no synchronization of clocks, simple
identifies each device specifically
Given a MAC protocol, classify it as a Channel Partitioning, Random Access, and Taking Turns.
Link Layer slide 21
channel partitioning - divide channel into smaller pieces(time slots, frequency, code) and allocate piece to node for exclusive use
random access - channel not divided, allow collisions, recover from collisions
taking turns - nodes take turns but nodes with more to send can take longer
Explain how TDMA, FDMA, ALOHA, Slotted ALOHA, CSMA, CSMA/CD, polling and token passing work.
TDMA - time division multiple access, access to channel in rounds, each station gets fixed length slot in each round, unused slots go idle
FDMA - frequency division multiple access, channel spectrum divided into frequency bands, each station assigned fixed frequency band, unused transmission time in frequency bands go idle
ALOHA - random access, when frame first arrives, transmit immediately, collision probability, best efficiency is 18%
Slotted ALOHA - random access, assume all frame sizes are the same, time divided into equal size slots(time to transmit 1 frame), nodes start to transmit only slot beginning, nodes are synchronized, if 2 or more nodes in slot, all nodes detect collision, when node obtain fresh frame, transmits to next slot, if no collision, can send new frame into next slot, if collision, node retransmits frame in each subsequent slot
Link Layer slide 22
Explain why ARP is needed and how it works.
ARP(Address Resolution Protocol) - there is a table where each IP node on LAN has a table, IP/MAC address mappings from some LAN notes <IP, MAC; TTL> TTL time to live after which address mapping will be forgotten, typically 20 minutes
Explain the fields in the Ethernet header.
Preamble: used to synchronize receiver, sender clock rates
Addresses: 6 bytes source and destination MAC, if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to network layer protocol, otherwise adapter discards the frame
Type: indicates higher layer protocol, mostly IP, used to demultiplex up at receiver
CRC: cyclic redundancy check at receiver, error detected->frame dropped
Given the relevant parameters, compute the efficiency of a CSMA/CD network.
Carrier Sense Multiple Access with collision detection (CSMA/CD)
efficiency = 1 / [ 1 + 5tprop/ttrans ]
tprop = max delay between 2 nodes
ttrans = time to transmit max frame size
Explain the similarities and differences between a hub, a switch, and a router.
ROUTERS - network layer devices, examine network layer headers, compute tables using routing algorithms, IP addresses
SWITCHES - link layer devices, examine link layer headers, learn forwarding table using flooding, learning MAC addresses
Explain how transparent switches fill in their forwarding tables.
Process called learning - Switches do not need to be configured, they are plug and play. it stores and forwards ethernet frames, examines incoming MAC, selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment
If floods the all outgoing links with the first frames until it learns which links contain which MACs
Given a topology with transparent switches and hosts, as well as a list of packets sent from one host to another, list the resulting switch tables as well as which network elements receive the packets.
Floods until it learns all links
Explain how VLAN can be useful.
It can be a single broadcast domain, good for scaling, all layer 2 broadcast traffic must cross entire LAN, efficient, secure, private, operates at multiple virtual switches
Explain how VLANs can define membership.
Ports can be dynamically assigned membership along VLAN,
Explain how VLANs trunking networks over multiple switches.
The trunk port carries frames between VLANs defined over multiple physical switches, frames forwarded within VLAN between switches cant be vanilla 802.1 frames, they must carry the VLAN ID, 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports
Given a network topology with hosts, hubs, switches and routers, and a network operation to be performed (e.g., ping, retrieving a webpage, sending an email, a remote login, etc.), list the packets transmitted and received at different points in the network.
Link Layer slide 85+
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