Layer One and Two LAN
Networking
Wires and connections
Station to station packet
transmission
OSI:
Open Systems
Interconnection
Reference Model
LAN
OSI Reference Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
©”A Guide to Networking Essentials”, 1998, Course Technology.
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
7.
6.
5.
4.
3.
2.
1.
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
User
Programs
WAN
LAN
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
Cable Type
Level 1
Level 2
Category 3
Category 4
Category 5
Use
Voice & low speed data
Data to 4 Mbps
LAN to 10 Mbps
LAN to 20 Mbps
LAN to 100 Mbps
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
Single
Mode
Laser
8/125 micron
Up to 50 miles
LED
Multimode
62.5/125 micron
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
OSI Reference Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
©”A Guide to Networking Essentials”, 1998, Course Technology.
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
Media access
Data delineation
Error control
Addressing
Transparency
Code independence
Gain access to the network
Start &stop characters,
delimiters, frame formats
Trailers: e.g. cycle
redundancy checks (CRC)
MAC addresses (48 bit
permanent device ID’s)
Special codes in data not
interpreted as delineation
ASCII, EBCDIC or any
other code accepted
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
Trailer
Body
Listen before transmit
Contention access
Retransmit on collision
Header
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
Section
Length
Contents
Preamble
7 bytes
10101010 (7X)
Start Frame 1 byte
Delimiter
Destination 6 bytes
Source
6 bytes
10101011
Length
2 bytes
Number of bytes in data field
Data
46 – 1500 Message
bytes
4 bytes
Cyclical redundancy check
field
Check
Sequence
MAC destination
MAC source
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
Token access
Equal access
Collision avoidance
Body
CRC
CRC
Header
Body
Body
Header
Header
Empty Token Structure
Block
Use
Bytes
Starting Delimiter Begin Token
1
Access Control
1
Ending Delimiter
Priority
Token
Monitor
Reservation
End Token
1
Dataframe Token Structure
Start delimiter
Access control
Frame control
Destination address
Source address
Data
Frame check sequence
Ending delimiter
Frame Status
Bytes
1
1
Code violations
Priority, Token,
Monitor, Reservation
Logical Link or MAC
1
6
6
To 4,500
4
Error detection
1
Code violations
1
Address recognized,
Copied?
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