CIT 384: Network Administration

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CIT 384: Network Administration

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1

CIT 384: Network Administration

LANs and WANs

CIT 384: Network Administration

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2

Topics

1.
Ethernet

2.
Hubs

3.
UTP Cabling

4.
Switches

5.
Ethernet Addresses

6.
Ethernet Frames

7.
WAN Protocols

8.
Frame Relay

CIT 384: Network Administration

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3

Ethernet

Most LANs use Ethernet.


Historically token ring, ATM, FDDI, etc.


Consists of a broad range of protocols.

Cheap and ubiquitous


Most motherboards have gigabit Ethernet now.


Ethernet cabling is cheaply available.

CIT 384: Network Administration

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4

Ethernet Media

Common Name

Speed

Name

IEEE
Standard

Cable
Type

Ethernet

10Mbps

10BASE
-
T

802.3i

Copper

10Mbps

10BASE
-
F

802.3j

Fiber

Fast Ethernet

100Mbps

100BASE
-
TX

802.3u

Copper

100Mbps

100BASE
-
FX

802.3u

Fiber

Gigabit

1000Mbps

1000BASE
-
T

802.3ab

Copper

1000Mbps

1000BASE
-
LX,

1000BASE
-
SX

802.3z

Fiber

CIT 384: Network Administration

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5

History of Ethernet

1973
: Invented by Robert Metcalfe at Xerox. (2.94 Mbps)




1980
: DEC
-
Intel
-
Xerox publish 10Mbps Ethernet standard.

1985
: IEEE published 802.3 standard (thicknet: 10BASE5)

1985
: Thinnet 10BASE2 standard published as 802.3a.

1990
: 10BASE
-
T twisted pair std published.

1995
: 100BASE
-
T fast Ethernet.

1998
: 1000BASE
-
X gigabit over fiber.

1999
: 1000BASE
-
T gigabit over twisted pair.

2005
: 10 gigabit Ethernet over fiber and Infiniband.

2007
: Working group for 100 gigabit project auth request.

CIT 384: Network Administration

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6

How 10BASE2 Ethernet Works


Single electrical circuit (bus) shared by all
computers on LAN.


Transmitted signal received by all
computers on the bus.


If two transmit at once, a
collision

results.

CIT 384: Network Administration

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7

CSMA/CD

Carrier sense multiple access/Collision detection

1.
Device listens until Ethernet LAN is quiet.

2.
When quiet, device begins sending frame.

3.
Device listens for collisions while sending.

4.
If collision occurs, the sending devices each
send a jamming signal to ensure all devices
recognize collision.

5.
After jamming, each sending device waits a
random time then tries again.

CIT 384: Network Administration

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8

Hubs

Advantages of 10BASE
-
T


In 10BASE2, a single cable failure takes down
the entire LAN.


Twisted
-
pair cabling instead of coax 10BASE2.

Hubs


Repeat transmitted signal on each port.


Increases max distance.


Star topology.

CIT 384: Network Administration

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9

UTP Cabling

Unshielded Twisted Pair


2 or 4 twisted pairs


RJ
-
45 connectors


Cat 3: 10 Mbps


Cat 5: 100Mbps


Cat 5e: 100+1000

CIT 384: Network Administration

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10

UTP Cabling Pinouts

Colors


green


orange


blue


brown

Stripes


green/white


orange/white


blue/white


brown/white

CIT 384: Network Administration

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11

Straight
-
through Cable


PC transmits on 1,2


Hub receives on 1,2


Hub transmits on 3,6


PC receives on 3,6

CIT 384: Network Administration

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12

Crossover Cable


Swaps transmit/receive wire pairs.


Used for two PC network without a hub.


Used for switch/switch communications.


Some switches can work with either cable.

CIT 384: Network Administration

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Hubs

1.
NIC sends a frame.

2.
Hub receives signal
on one port.

3.
Hub interprets
signal as bits.

4.
Hub sends bits out
all other ports.

CIT 384: Network Administration

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14

Switches

1.
NIC sends a frame.

2.
Switch receives
frame on one port.

3.
Switch looks up
destination MAC
address.

4.
Switch forwards
frame on port where
that destination can
be reached.

CIT 384: Network Administration

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15

Switches


Switch interprets Ethernet header.


Maintains table of address/port mappings.


Buffers frames and sends one at a time.


Reduces collisions


Forwards frames to single port using table.


Buffered frames are sent one at a time.

CIT 384: Network Administration

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Switches vs. Hubs

Hubs



Layer 1



Frequent collisions.



Bandwidth per hub, i.e.
100Mbps 12
-
port hub
has 100 Mbps total.


Switches



Layer 2



Infrequent collisions.



Bandwidth per port, i.e.
a 100 Mbps 12
-
port
switch has 1200 Mbps.

CIT 384: Network Administration

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17

Half Duplex

Receiving (RX) line monitored.


If frames seen, no frames sent until clear.


If frame received while transmitted on TX, a collision
occurs.

Hubs can only work in half
-
duplex mode.

CIT 384: Network Administration

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18

Full Duplex

RX, TX lines always available.


Transfer in both directions simultaneously.


No collisions possible.

Misleadingly advertised as 200 Mbps Ethernet.

CIT 384: Network Administration

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Auto
-
Negotation

Protocol where both sides of link decide on


Speed


Duplex

If only one side set to auto
-
negotatiate, it fails.


Both switch and NIC must support to succeed.

If auto
-
negotation fails, parallel detection used


Parallel detection can only determine speed.


Not duplex.


Assumes half
-
duplex for 10/100 Mbps.


Assumes full duplex for 1000 Mbps and faster.

CIT 384: Network Administration

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20

Auto
-
Negotiation Failure

Common failure case


One side hard coded to 100/full.


Other side set to auto
-
negotiate.

Result: one side half
-
duplex, other full duplex


Full
-
duplex side sends frames w/o checking RX line.


Half
-
duplex side sees many collisions, won’t send until
RX clear.

CIT 384: Network Administration

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21

Ethernet MAC Addresses


OUI assigned by IEEE.


Burned in address (BIA) stored in ROM.


Can be replaced with a local address.

CIT 384: Network Administration

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22

Address Types

Unicast

addresses
represent a single
device.

Multicast

addresses
represent a subset of
devices on the LAN.
They begin with
0100.5E

The
broadcast

address
represents all devices
on the LAN. It is


FFFF.FFFF.FFFF

CIT 384: Network Administration

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23

Ethernet Frames

Three header formats.

In combined length/type field:


If value in range 0..1536, it is a length.


If value > 1536, it is a type (ex: IP is 2048).

CIT 384: Network Administration

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Ethernet Fields

Field

Length

Description

Preamble

7

Synchronization; 7 octets of 01010101.

Start Frame Delimeter

1

1 octet of 11010101

Destination Address

6

Identifies recipient.

Source MAC Address

6

Identifies sender (who to reply to.)

Length

2

Length of data field.

Type

2

Type of protocol encapsulated in frame.

Data and Pad

46
-
1500

Encapsulated data from higher protocol.

Frame Check Sequence

4

CRC checksum to detect errors.

CIT 384: Network Administration

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25

IP over Ethernet


To create a Type field for frames that use
Length field, 1 or 2 headers added after
Ethernet header and before data.


802.2 Logical Link Control (LLC) header


Subnetwork Access Protocol (SNAP) header.

CIT 384: Network Administration

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Error Detection

CRC used to detect transmission errors.


Frames with bad checksums are discarded.


There is no provision for retransmission.


Transport layer protocols can handle that.

CIT 384: Network Administration

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Why WANs?

The obvious answer: distance


Ethernet can’t travel over many miles.


You don’t have the rights
-
of
-
way to run a cable
over many miles between sites.

Solution: point
-
to
-
point leased line.

CIT 384: Network Administration

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WAN Components

CSU
: Channel service unit.

demarc
: division between customer/telco
responsibility.

CPE
: customer premises equipment.

CIT 384: Network Administration

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29

WAN Serial Cabling

CIT 384: Network Administration

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30

Synchronization


CSU, WAN switch, and router must
synchronize clock rates to communicate.


DCE
: data communications equipment,
device that provides clock signal.


DTE
: data termination equipment, device
that receives clock signal, typically router.

CIT 384: Network Administration

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31

Link Speeds

Name of Line

Link Speed

DS0

64 kbps (digital signal audio channel)

DS1 (T1)

1.544 Mbps (24 DS0s + 8kbps overhead)

DS3 (T3)

44.736 Mbps (28 DS1s + overhead)

OC
-
1

51.84 Mbps (optical carrier)

OC
-
3

155.52 Mbps (3 * OC
-
1)

OC
-
24

1244.16 Mbps

OC
-
48

2488.32 Mbps

OC
-
96

4976.64 Mbps

OC
-
192

9953.28 Mbps

CIT 384: Network Administration

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32

Layer 2 Protocols

HDLC: High
-
level Data Link Control


Address field no longer used.

PPP: Point to Point Protocol


Framing identical to Cisco HDLC.

CIT 384: Network Administration

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Frame Relay

Leased lines don’t scale well.


To add a 10
th

site to network, need to add 10
new leased lines, one to each site.


Need routers that support 10 lines each too.

Frame relay


Only need one line per site.


Packet switching provided within telco network.


Given a
virtual circuit
, not a physical one.

CIT 384: Network Administration

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Frame Relay

Frame Relay contains address field


Data Link Connection Identifier (DLCI)


Telco network switches packets based on DLCI.

CIT 384: Network Administration

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Virtual Circuits

Virtual circuit is a logical path through telco network.


Acts like a point
-
to
-
point circuit.


Provider will guarantee minimum bandwidth.

CIT 384: Network Administration

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Frame Relay Example

CIT 384: Network Administration

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References

1.
Cisco, Cisco Connection Documentation,
http://www.cisco.com/univercd/home/home.htm

2.
Cisco, Internetworking Basics,
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.ht
m

3.
Gary A. Donahue,
Network Warrior
, O’Reilly, 2007.

4.
IEEE 802.3 Ethernet Working Group, http://www.ieee802.org/3/

5.
Wendell Odom,
CCNA Official Exam Certification Library, 3
rd

edition
, Cisco Press, 2007.

6.
Priscilla Oppenheimer and Joseph Bardwell,
Troubleshooting
Campus Networks
, Addison
-
Wesley, 2002.

7.
Charles E Spurgeon,
Ethernet: The Definitive Guide
, O’Reilly, 2000.

8.
W. Richard Stevens,
TCP/IP Illustrated
, Addison
-
Wesley, 1994.