Download presentation source

Ethernet LAN
1
LANs

Local Area Networks

Limited Geographical Area
– Single office
– Single building
– University campus or industrial park

Generally, high speeds
– Now, most operate at around 10 Mbps
– 100 Mbps is emerging as the new “base speed”

Most Data Traffic is Local
2
Standards Setting

LANs are Subnets (single networks)

Subnet technology is Dominated by OSI Standards (true
for LANs)

IEEE Creates most LAN Standards
– Institute for Electrical and Electronic Engineers
– Submits its standards to ISO and ITU-T for ratification

IEEE 802 Committee
–
–
–
–
LAN standards are set by the IEEE 802 Standards Committee.
802.3 for Ethernet Standards
802.5 for Token-Ring LAN Standards
802.11 for Radio and Infrared Wireless LANs
3
LANs and OSI Architecture
 OSI
is a 7-layer architecture

LAN transmission only uses Layers 1 and 2

Layer 1: Physical Layer
– Connectors, Media, Electrical signaling

Layer 2: Data Link Layer
– Packaging data into frames
– Managing transmission over link (error handling, etc.)
– Access control: when each station may transmit
4
OSI Physical and Data Link Layers
Physical Layer (OSI Layer 1)
Physical (plugs, media, etc.); Electrical (voltages, timing, etc.)
Electrical Signal
Station
A
Connector
Plug
F4-1
Station
B
Transmission Medium
(telephone wire, etc.)
Connector
Plug
5
OSI Physical and Data Link Layers
Data Link Layer (OSI Layer 2)
Frame 2
Station
A
Frame 1
Station
B
6
Data Link Layer
 For
point-to-point transmission
– A point-to-point connection is a data link
– So is a transmission system shared by multiple devices,
only one of which can transmit at a time because of
collisions
Transmission
7
Data Link Layer
 First
function: Packaging of Data (1s and 0s)
– PDU at Data Link Layer
is called a frame

Second Function: Access Control
– Only one station can transmit at any time
– If another transmitted, their signals would scramble one
another
– Must control access to (transmission into) the
transmission medium
8
Data Link Layer
OSI
Data Link
Layer
(Layer 2)
OSI
Physical
Layer
(Layer 1)
Logical Link Control Layer
Media Access Control (MAC) Layer
802.3
802.3
10Base-T 10Base-5
802.3
Other
Physical
Layer
802.5
Physical
Layer
4 Mbps
802.5
Physical
Layer
16 Mbps
Other
Physical
Layer
OSI Data Link Layer is subdivided into two layers
Media access control
Logical link control
9
MAC Layer
Media
Access
Control
OSI
Physical
(Layer 1)
Media Access Control (MAC) Layer
802.3
802.3
10Base-T 10Base-5
802.3
Other
Physical
Layer
802.5
Physical
Layer
4 Mbps
802.5
Physical
Layer
16 Mbps
Other
Physical
Layer
MAC layer implements media access control:
When a station may transmit
Controls the framing of data along the wire
10
Logical Link Control Layer
LLC
802.2 Logical Link Control Layer
802.3 Media Access Control
(MAC Layer)
OSI
Physical
(Layer 1)
802.3
802.3
10Base-T 10Base-5
802.3
Other
Physical
Layer
802.5
MAC
4 Mbps
802.5
MAC
16 Mbps
Other
MAC
802.5
Physical
Layer
4 Mbps
802.5
Physical
Layer
16 Mbps
Other
Physical
Layer
Provides Control Function
Begin/end connections between stations
Error correction (optional)
11
Simple LAN Using Ethernet 10Base-T
10Base-T Hub (Multiport Repeater)
Unshielded Twisted Pair (UTP) Wiring
(4-Pair Bundle)
PC
RJ-45 jacks
10Base-T
UTP Wiring
Bundles:
4 Pairs
EIA
Category
3, 4, or 5
Network
Interface
Card
NIC
RJ-45
Jack
12
Ethernet 10Base-T (802.3u)
 Physical
Layer Standard
– 10 Mbps (10 in 10Base-T)
– Baseband signaling: Injects voltage changes directly
into the wires (Base in 10Base-T)

Hubs (Multiport Repeaters)
– Connect the stations together
10Base-T Hub
13
NICs
 Network
Interface Cards
– Implement Physical Layer

Plug and Electrical Signaling
– Implements the Data Link Layer (data packaging,
access control, etc.)

LLC (802.2)

MAC (802.3 MAC)
14
Wiring

Unshielded Twisted Pair (UTP)
– Twisted several times per foot to reduce interference, T
in 10Base- and Unshielded



No protection except for plastic coating
Distance limitation: 100 meters (attenuation, distortion, noise
and interference, crosstalk) -- propagation
Categories of UTP Wiring
– Category 5, 6: The best. Good for 100 Mbps
– Category 3 and 4: lower. May be OK for 100 Mbps

Wiring Plugs: RJ-45 Standard
– Similar to home (RJ-11) jacks, but wider
15
Physical Layer Transmission in 10Base-T
10Base-T Hub
Step 1
Station A
Transmits on
Upstream
Pair
(Wires 1&2)
Station
A
F4-4
Station
B
Station
C
16
Physical Layer Transmission in 10Base-T
10Base-T Hub
Step 2
Hub Repeats
(Broadcasts)
The Message
To All
Stations
On Downstream
Pairs
(Wires 3&6)
Station
A
Station
B
Station
C
Bus transmission
means broadcasting
17
MAC Layer
CSMA/CD Media Access Control
 Controls
when stations may transmit
– If two transmit at once, signals will be scrambled
X
Collision
Collisions will grow rapidly above 30% of line utilization.
Keep traffic moderate, or throughput will be affected.
18
CSMA/CD Media Access Control
 CS:
Carrier Sense
– Each NIC always listens for traffic on the line
– This lets it recognize messages sent to its address
– This also lets it know if the line is free

CSMA: Carrier Sense Media Access
– A station may transmit if it hears no traffic on the
network
19
CSMA/CD Media Access Control in Ethernet
10Base-T Hub
Station A
is Transmitting
Station B
must wait
Must
Wait
Station
A
Station
B
Station
C
20
CSMA/CD Media Access Control
 CD:
Collision Detection
– If two stations transmit at once ….
– Their signals collide, scrambling one another
– Because each sender listens (senses the carrier), both
know that there has been a collision
– Both stop and wait a random amount of time.
101010
X
001110
Collision
21
Layering in 802 Networks
Internet Layer
Data
Link
Layer
TCP/IP Internet
Layer Standards
(IP, ARP, etc.)
Logical
Link
Control
Layer
Media
Access
Control
Layer
Physical Layer
Other Internet
Layer Standards
(IPX, etc.)
802.2
Ethernet 802.3 MAC Layer
Standard
100BASETX
1000
BASESX
…
Non-Ethernet
MAC Standards
(802.5,
802.11, etc.)
Non-Ethernet
Physical
Layer
Standards
(802.11, etc.)
22
802.3 MAC Layer Frame
Preamble
Start of Frame Delimiter
Destination Address
Source Address
Length
In asynchronous transmission,
each byte is sent separately, in a
10-bit frame.
In 802.3 MAC Layer frames,
transmission is synchronous.
Many data bytes are sent in each
frame of variable length.
Data
Frames must be between 64 octets
and 1518 octets long, divided into
groups of bits called fields.
Pad
Field lengths are measured in
“octets”, eight bits.
“Octet” is a synonym for “byte”
Frame Check Sequence
23
Error Detection
 MAC
Layer Process Provides error detection
– Determines that an error exists
 Does
not provide error correction
– Merely discards the frame
– No request for retransmission if there is an error
 Logical
Link Control Layer may do error
correction
– Receiving LLC process detects discarded frames
– Receiving LLC process asks for retransmissions
24
Logical Link Control (LLC) Layer
Next Higher Layer
(Usually Internet)
LLC Layer
Process
802.2 can ask
for retransmission of
lost MAC frames.
Optional.
LLC Frame
Next Higher Layer
(Usually Internet)
LLC Layer
Process
LLC frame placed
within MAC frame
MAC Layer
Process
Station A
MAC Frame
MAC Layer
Process
Station B
25
Ethernet 10Base-T Network with Two Hubs
Hub 1
Station A
Transmits
to
Hub 1
100 m
Segment
Maximum
Hub 2
100 m
Segment
Maximum
Hub 1 Transmits
Out All Ports,
Including the
Port to Hub 2
100 m
Segment
Maximum
Hub 2
Broadcasts
the Message
Out All of Its
Ports
Station C
Receives the
Message
Station A
Station B
Station C
26
Ethernet 10Base-T LAN with Multiple Hubs
Hub 2
Hub 3
Hub 1
UTP
Wire
UTP
Wire
Station
A
Station
B
UTP
Wire
UTP
Wire
Hub 4
Daisy chain,
no Loops
allowed!
Maximum distance
between farthest Stations is
4 Hubs/5 100 meter segments
Station
C
UTP Wire
Station
D
27
Speed and Distance

Transmission speed worsens problems
– Error rates increase because bit periods are smaller and are
more likely to be damaged by brief noise spikes and
interference
– High speeds create high-frequency components in the
signal that attenuate more rapidly than lower-frequency
components
– In general, as speed increases, maximum distance
decreases, although improving technology can lessen the
decrease
28
Dealing with Propagation Effects
 Use
High-Quality, High-Cost Media
– Use media designed for long-distance propagation
– Optical fiber, coaxial cable
– Too expensive for runs to many desktops, good for hubto-hub

Use Inexpensive Media to the Desktop
– Improve the technology (allows 100 Mbps+ on UTP)
– Accept distance limitations (100 meters for UTP)
– More popular alternative to desktop because of low cost
29
Ethernet Physical Layer Standards
UTP Physical
Layer
Standards
10BASE-T
Speed
Maximum Medium
Run Required
Length
10 Mbps 100 meters 4-pair Category 3 or higher
100BASE-TX
100 Mbps 100 meters 4-pair Category 5 or higher
1000BASE-T
(Gigabit
Ethernet)
1,000 Mbps 100 meters 4-pair Category 5 or higher
100BASE-TX dominates access links today,
Although 1000BASE-T is growing in access links today
30
Ethernet Physical Layer Standards
Fiber Physical
Layer
Standards
Speed Maximum Medium
Run 850 nm light (inexpensive)
Length Multimode fiber
1000BASE-SX
1 Gbps
220 m
62.5
microns
160
MHz-km
1000BASE-SX
1 Gbps
275 m
62.5
200
1000BASE-SX
1 Gbps
500 m
50
400
1000BASE-SX
1 Gbps
550 m
50
500
The 1000BASE-SX optical fiber standard dominates trunk links today
S means that the standard uses short wavelength light (850 nm)
31
Shared media LANs
 Limits
to Shared Media LANs
– FDDI, 100Base-X, 1000Base-SX are all shared media
LANs

Only one station can transmit at a time, causing
latency

Every station hears every message, so as the number
of stations grow, the LAN saturates
– 100, 1000 Mbps speed only delays saturation
32
Shared media LANs
 Shared
Media Networks with Hubs (such as
10Base-T)
– Incoming frame arrives through a single port
– Hub broadcasts frames out all ports
– Congestion on output ports
Hub
33
Switched LANs
 In
a switched network
– Incoming frame arrives on a single port
– Frame sent out again only on a single port--the one
leading to the receiver
– No congestion on other ports
Switch
34
Switch
With a switch, multiple
stations may transmit
simultaneously: no
congestion as traffic
grows.
Station
A
Station
B
Switch
Station
C
Connection 1
A-C
Connection 1
A-C
Connection 2
B-D
Station
D
Connection 2
B-D
35
Switching in Perspective

Switching is dominant in LANs today
– Congestion does not increase as the number of stations
grows

Today, switches are no more expensive than
10Base-T or 100Base-X hubs

Read CISCO white paper
36
Switch connections

paths called connections must be pre-defined
between stations

a fixed logical data link (logical connection) is
established between stations before transmission
even begins

during the transmission, all traffic between the
stations must pass over that data link

unless a data link has been pre-established, two
stations may not communicate at all

only OSI Layer 2 (Data Link Layer) protocols are
needed
37
Ethernet Switches

Ethernet Hubs are Half Duplex

Most Ethernet Switches are Full Duplex
– No collisions are possible
– So two stations can both transmit to each other at the
same time (full duplex operation)
– Requires full duplex switches
– Requires full duplex NICs

Lowest-cost LAN switches

Not standardized, so buyers tend to get locked into
a single vendor
38
Data Link Using Multiple Switches
Received
Original Received Regenerated Signal
Received
Regenerated
Signal
Signal
Signal
Signal
Signal
Switches regenerate signals before sending them out;
this removes propagation effects
It therefore allows signals to travel farther
39
Multiswitch Ethernet LAN
40
Hierarchical Ethernet LAN
Ethernet switches must be arranged in a hierarchical topology
In a hierarchical LAN, there is only one possible path between any hosts
41
Routed LAN with Ethernet Subnets
When a routed LAN links multiple
Ethernet switched networks,
individual switched networks are
called subnets
42