Layer One and Two LAN

warmersafternoonNetworking and Communications

Oct 23, 2013 (3 years and 9 months ago)

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Layer One and Two LAN
Networking

Wires and connections

Station to station packet
transmission

OSI:

Open Systems

Interconnection

Reference Model

LAN

Application

Presentation

Session

Transport

Network

Data Link

Physical

©”A Guide to Networking Essentials”, 1998, Course Technology.

OSI Reference Model

Open Systems
Interconnection (OSI)


International Standard


Written to be a manufacturing standard,
but never built


Used as a vocabulary short hand


Applies to all extensions of networks

OSI Reference Model

OSI Layer 1

Local Area Network protocols

Physical layer:


standards for physical connections (e.g. plugs and
connectors)


Responsible for getting bits from one location to another.


Medium


Connectors


Representation of signals

Media


Twisted Pair Copper Wire

shielded vs unshielded

plenum vs PVC


Coaxial Copper Cable

thinnet and thicknet


Fiber

single mode and multi
-
mode


Microwave


Infrared, Radio

Unshielded Twisted Pairs


Cheapest alternative for LAN cabling


Already exists in most offices


Adapts to star wiring and hub systems


Trend in modern LAN installations


New technologies adapt to it


Shielded cable used where electronic
interference a problem

Cat 5 Twisted Pair Wiring

Twisted Pair Signals

Reverse phases to cancel noise

LINE 1 +

LINE 2
-


Cable categories

Twisted Pair Connectors


Wiring Pairs


blue, white
-
blue


orange, white
-
orange


green, white
-
green


brown, white
-
brown


RJ
-
11 and RJ
-
45


D connectors

RJ
-
45
(Registered Jack 45):

8 wires for Ethernet

RJ
-
11
(4 or 6 wire
versions) for telephones

Unshielded Twisted Pairs


Phones use 1 pair


10BaseT uses 2 pairs of Category 5
copper


100BaseT uses 2 pairs of Category 5
copper


100BaseFX uses multimode fiber

10BaseT Ethernet Wiring

Fiber Optic Cables


Longer distances


High data rate requirements


High interference situations


High security situations


Connections more difficult than with UTP

Fiber Optic

8/125 micron

62.5/125 micron

Single

Mode

Multimode

LED

Laser

Up to 50 miles

Up to 2000 meters

Fiber
-
optic Connectors

Wireless


Infrared


Radio


Microwave


Radio Frequency


Spread Spectrum & Wi
-
Fi (IEEE 802.11)


Wireless security

Wireless LAN Access

Local Area Networks

Layer 2

"Gentlemen! Start


your


laptops!"


Copyright 1998 Doug Adams

Local Area Networks


Networks that move data from station to
station using a common set of layer 1 and
layer 2 protocols.


Common broadcast domain


Local ownership


Common operating system


Machine ID addressing

Application

Presentation

Session

Transport

Network

Data Link

Physical

©”A Guide to Networking Essentials”, 1998, Course Technology.

OSI Reference Model

Data Link
-

Layer 2

2. Data link layer:


protocols for error free transmission from station to
station




The data link is responsible for node to node
validity and integrity of the transmission. The
transmitted bits are divided into frames; for
example, an Ethernet, Token Ring or FDDI
frame in local area networks. Layers 1 and 2 are
required for every type of communications.

Data Link
-

Layer 2
functions

Data Link Sub
-
layers


Media Access Control (MAC)


Standards for addressing and locating
nodes


Logical Link Control (LLC)


Standards for communication with
higher layers

LAN Operating Protocols


Ethernet


open standard, cheap, most common


Token Ring


IBM proprietary, high quality,
expensive


Others

Network Interface Cards


Build, send out and accept frames


Usually a daughter board on PC


Must match LAN and CPU


Require drivers to operate

Network Interface Cards
(NIC)

Ethernet

Carrier Sense Multiple Access/Collision
Detection

Header

Body

Trailer

Listen before transmit

Contention access

Retransmit on collision

Ethernet:

CSMA/CD
(IEEE 802.3)


Carrier Sense Multiple Access with
Collision Detection


Compare channel voltage to reference level


Any node can transmit if channel free


Collision detection during transmission


Jamming


Random back off


Slot time and minimum packet size

Manchester Encoding

(self
-
clocking bit stream)

0

1

0

High
-
to
-
Low = 0

Low
-
to
-
High = 1

Switch voltage at

each time point

Ethernet Packets


Synchronization


Data transparency


MAC addresses


Minimum length


Error check


Alternate Ethernet packet formats

Ethernet Packet Structure

Administration


Acknowledgements


Address announcements (identification)


Slot time


Card streaming


Hub access


10BaseT, 100BaseT, Gigabit Ethernet

10BaseT Specifications

(Twisted Pair Ethernet)


100 m to hub


1024 stations per network


10 mbps


UTP


Star/bus

LAN Logical/Physical
Topologies

Bus

Ring

Star

Network Logical
Topologies


Topologies are determined by the technologies
that run the network


Bus:

Ethernet


Ring:

Token ring

SONET, FDDI


Star:

Mainframe

Switched Ethernet

Ethernet: Physical Star,
Logical Bus

LAN with hubs

Hubs or Switches

Switches (Layer 2)


Route packets to destination nodes based
on MAC addresses


Limit traffic on unused branches


Provide additional security


Connect 10Mb and 100Mb branches


Operate in firmware

Switch (CISCO Catalyst
1928)

Ethernet
Switching

Token Ring
(IEEE 802.5)


Allocated access via electronic token


Priority access reservation


Confirmed packet delivery


Multiple monitor functions

Token
Ring

Token Ring

Message

CRC

Header

Body

Header

Body

Header

Body

Token access

Equal access

Collision avoidance

CRC

CRC

Empty Token Structure

Dataframe Token Structure

Topology: Logical or Physical?

Bus

Ring

Star

Network Topologies


Topologies are determined by the technologies
that run the network


Bus:

Ethernet


Ring:

Token ring

SONET, FDDI


Star:

Mainframe

Switched Ethernet