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NWC202-5-LINK LAYER (Introduction (Services (framing, link access (channel…
NWC202-5-LINK LAYER
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Services
framing, link access
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“MAC” addresses used in frame headers to identify source, dest
encapsulate datagram into frame, adding header, trailer
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Error Detection
errors caused by signal attenuation, noise.
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Implemented
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combination of hardware, software, firmware
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Adaptors communicating
Sending Side
adds error checking bits, rdt, flow control, etc.
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Receiver Side
extracts datagram, passes to upper layer at receiving side
looks for errors, rdt, flow control, etc
- Error detection and correction
Introduction
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D
Data protected by error checking, may include header fields
not 100% reliable!
protocol may miss some errors, but rarely
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Parity Checking
Single Bit Parity
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For odd parity schemes, the parity bit value is chosen such that there is an odd number of 1s
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Internet Checksum
Goals
detect “errors” (e.g., flipped bits) in transmitted packet
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Cyclic Redundancy Check
Includes
choose r+1 bit pattern (generator), G
choose r CRC bits, R, such that
<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
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view data bits, D, as a binary number
Multiple Access Links
Two types of Link
Point-to-Point
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point-to-point link between Ethernet switch, host
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Introduction
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What is it?
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
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Characteristics
- when M nodes want to transmit, each can send at average rate R/M
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no synchronization of clocks, slots
- when one node wants to transmit, it can send at rate R.
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Addressing
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MAC
LAN Address
48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable
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Function
used ‘locally” to get frame from one interface to another physically-connected interface (same network, in IP-addressing sense)
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ARP
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ARP table
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each IP node (host, router) on LAN has table
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Ethernet
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Ethernet Frame Structure
Preamble
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used to synchronize receiver, sender clock rates
Addresses
if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol
otherwise, adapter discards frame
6 byte source, destination MAC addresses
Type
indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
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Unreliable
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data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost
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Ethernet Switch
Characteristics
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link-layer device
store, forward Ethernet frames
examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment
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VLAN
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switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.
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Spanning Multiple Switch
Trunk Port
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frames forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info)
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Goals
Priciples of Link Layer
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error detection, correction
local area networks: Ethernet, VLANs
instantiation, implementation of various link layer technologies
- Link Virtualization: MPLS
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- A day in life of a web request