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Data Link Layer Fundamentals: Ethernet LANs

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Data Link Layer Fundamentals: Ethernet LANs

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Data Link Layer Fundamentals: Ethernet LANs

OSI Layer 1:Think of this as "sending bits". Includes the details of cabling, purpose of each pin on connectors, etc.

  • For example, CAT5 cable, pin 1+2 transmit data, pins 3+6 receive data.
  • Straight-through cable: Pin 1 connects to pin 1, 2 to 2, and so on.

OSI Layer 2:Think of "knowing when to send the data, detecting errors, and identifying the host that needs to get the data."

  • Arbitration:When is it okay for data to be sent?
    • Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
  • Addressing: Ensures the correct recipient(s) receives and processes data
    • (Ethernet) MAC Addressing: 6-bytes long, 12-digit hex
    • (Frame Relay) Data Link Connection Identifier (DLCI): 10-bits
  • Error detection: Did the data make it across the medium intact?
    • Uses Frame Check Sequence (FCS) or Cyclic Redundancy Code (CRC)
    • Fields are in the link trailer
    • Use mathematical formula against sent/received data, if no match, discard
  • Identification of the encapsulated data: Type of header that follows data link header.
    • Logical Link Control (LLC) uses a field in header to identify type of data (Type field)

802.2 = LLC Sublayer

802.3 = Ethernet

802.5 = Token Ring


Early Ethernet Standards

10BASE2 and 10BASE5

  • Coaxial cables
  • Single electrical bus
  • Listen to wire, send when quiet, collisions occur, random "back off" time

Repeaters: Reduceattenuation (decreased signal strength over distance)

-> Repeaters operate at layer one, interpret 0's and 1's and generates new, clean signal


10BASE-T Ethernet

  • Allowed use of telephone cabling system
  • Make use of hubs, aka multiport repeaters
  • Shared Ethernet:'''All devices still sharing same 10-Mbps hub
  • Physical star, logical bus

Crossover cable:Pin 1 -> Pin 3, Pin 2 -> Pin 6 -For connecting 2 PCs or Switches/Hubs

Half-duplex NICs: Use loop back circuitry to send broadcast data to NIC card internally, so if packet is sent to that same NIC, detects collision.

Full-duplex NIC:No loop back circuitry, can send/receive at full data rate

Switches:Doesn't create single shared hub, but rather creates separate collision domains. Use memory buffers to hold incoming frames, analyze content, and forward to only those addresses as receiver of data.


Ethernet Addressing

Unicast = Identify single LAN card, 6 bytes long (48 bits), for Cisco devices they are usually written as 0000.0C12.1234

> First 3 bytes are the manufacturer's ID, or the Organizationally Unique Identifier (OUI). -> Also known as Burned-in Address (BIA) and Universally Administered Addresses (UAA)

Broadcast =Addresses all devices on the LAN, written as FFFF.FFFF.FFFF

Multicast= Used to address multiple addresses, but not all addresses (multicasting)

? Written as 0100.5exx.xxxx


Ethernet Framing

Defines how string of 1's and 0's is interpreted, defines the fields assumed to be in encapsulated data packets. Maximum transmission unit = 1500 bytes

Ethernet (DIX):

8 bytes 6 bytes 6 bytes 2 bytes Variable 4 bytes
Preamble Destination Source Type Data FCS

Ethernet (802.3):

7 1 6 6 2 1 1 1-2 Variable 4
Preamble SD Destination Source Length DSAP SSAP Control Data FCS

Ethernet (802.3 w/ SNAP):

7 1 6 6 2 1 1 1-2 5 Variable 4
Preamble SD Destination Source Length DSAP SSAP Control SNAP Data FCS

DSAP = Destination Service Access Point

SNAP = Subnetwork Access Protocol


Summary of Type Fields:

Field Name Length LAN Type
Ethernet Type 2 bytes DIX Ethernet
802.2 DSAP and SSAP 1 byte each IEEE Ethernet, IEEE Token Ring, ANSI FDDI
SNAP 2 bytes IEEE Ethernet, IEEE Token Ring, ANSI FDDI

Note: SNAP Header must be used to support TCP/IP over IEEE Ethernet

Fast Ethernet:IEEE 802.3u, uses CSMA/CD but can be disable to support full duplex

  • Higher bandwidth: 100 Mbps
  • Auto negotiation: Dynamically assign 10 or 100 Mbps speed, depending on NIC
  • If this fails,use half duplex at 10 Mbps

Gigabit Ethernet:IEEE 802.3z for optical, 802.3ab for electrical

  • Most likely to be used between routers/switches, or switch and server
  • Operates at 1,000 Mbps, or 1 Gbps


Summary of Ethernet

Function Implementation
Arbitration CSMA/CD algorithm
Addressing 6-byte MAC Address
Error detection FCS in trailer
Identifying type of packet Protocol Type (2 bytes) - DIX Ethernet
DSAP (1 byte) - IEEE 802.2
SNAP Type (2 bytes) - IEEE 802.2


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