Chapter 4 - UAH Personal Web Pages

CHAPTER
4


Ethernet Basic

WHAT IS THE ETHERNET?


Ethernet:



A
family

of network technologies (standards) to build a LAN



Logical

bus

topology



Bus

physical

topology: The “older” Ethernet


Using coaxial cable as the bus cable



Star

physical

topology: The “newer” Ethernet (“
Star Bus

topology”)


Using UTP or Fiber Optic cable

OLDER VERSUS NEWER ETHERNET

Older: BUS Physical Topology

Newer: STAR Physical Topology

Older: Coaxial Cable as the Bus Cable

Newer: UTP Cable as the Patch
Cable and the Horizontal Run

WHAT IS THE ETHERNET?


Ethernet:


Ethernet frame



CSMA/CD

as the
network access method/media
access control
, i.e.,
a method to determine which
computer in the network can access the wire (in the
network) at a given moment


ETHERNET IN THE OSI 7 LAYER MODEL


ETHERNET FRAME


Ethernet frame



Created, sent, received, read, and destroyed by
Ethernet NIC



The advantages of using frame:


Frame defines the maximum data size, and therefore, large
data must be broken into smaller pieces. This provides two
benefits:



Benefit 1:

Preventing one computer from monopolizing

the shared wire in
the network (distributing the network use among all computers in the
network more evenly)


Benefit 2:

Whenever data is corrupted during transmission, the sending
computer only has to retransmit one (or few) frame(s) that contains the
corrupted data

ETHERNET FRAME


Seven major parts in Ethernet frame:


Part 1:
Preamble



64 bits of alternating 1s and 0s and ends with 11



Functions:


Enabling the receiving NIC to recognize the beginning of
a frame


Giving time to the receiving NIC to realize a coming of a
frame and therefore, can make all necessary
preparations to receive the frame properly

ETHERNET FRAME


Seven major parts in Ethernet frame:


Part 2 and 3:
Receiver/destination MAC address and
Sender MAC Address



A program/software called
SNIFFER

can enable NIC to run in
PROMISCUOUS MODE

to
process

ALL frames it receives,
regardless of their destination MAC addresses


http://www.tamos.com/htmlhelp/monitoring/monitoringusinghubs.htm

Hub

Computer A

Computer B

With “Packet Sniffer” this
computer will accept and
process the message

Accept the message

Drop/destroy the message

Drop/destroy the message

Drop/destroy the message

For Computer B

ETHERNET FRAME


Seven major parts in Ethernet frame:


Part 4:
Length


Information about the number of BYTES of DATA
inside the frame



Part 5:
Data


Data that is received from the upper layer of the OSI
seven
-
layer model (Network Layer): PACKET


Minimum number of bytes of data: 64 bytes


Maximum number of bytes of data: 1500 bytes

ETHERNET FRAME


Seven major parts in Ethernet frame:


Part 6:
Pad


If the data size is less than 64 bytes, the sender will
add more bytes in this part (PAD) to bring the data
up to the minimum 64 bytes



Part 7:
Frame Check Sequence


Error Detection Information (CRC for the Ethernet)


To detect data transmission error that causes data
modification during the transmission


CSMA/CD


Carrier Sense Multiple Access (CSMA)/Collision Detection
(CD)


Network access method (media access control)



Carrier Sense:


Every computer in the network must first examine (
sense
) the
BUS/SEGMENT before sending a frame


If the computer detects traffic in the BUS/SEGMENT, it will wait
for a moment, and then reexamine (re
sense
) the
BUS/SEGMENT


If the computer detects no traffic, it will send the frame to the
BUS/SEGMENT

CSMA/CD


Multiple Access:


All computers

(NICs) in a network perform CSMA/CD, hence,
have
equal access

to the wire (
no computer

has
greater access
(priority)
to the wire)



Collision Detection:


When
two or more computers
are sending frames to a network
simultaneously, a
collision

occurs, and all frames will be
corrupted/damaged/lost (the transmission is wasted)


During a frame transmission
(after sending a frame to a network),
the sender’s NIC will always monitor
(listen to) the network


If the sender’s NIC detects something else being transmitted in the
network, it recognizes that its frame has been corrupted by collision with
another frame


EARLY
ETHERNET

NETWORKS


10BaseT =
UTP cable (the “T”) based Ethernet



10BaseFL =

Fiber Optic cable (the “FL”) based
Ethernet



Using hub

10BASET: CABLING


CAT3 or higher
,
two pairs
,
UTP cable



RJ
-
45 connectors


Also known as CRIMP


Connecting UTP cable to RJ
-
45 connector: CRIMPING




Crimping standard (see Figure 4.12 on Page 67)


TIA/EIA 568 A

standard


TIA/EIA 568 B

standard

10BASET: SPECIFICATIONS


10
Mbps



Baseband



Maximum distance between the hub and the node
(computer/NIC):
100
meters



Maximum number of nodes per hub:
1024



Network topology:
Star Bus
‏



Cable:
UTP, CAT
3
or higher, two pairs, RJ
-
45
connectors

10BASEFL/10BASEF: SPECIFICATIONS


10

Mbps



Baseband



Maximum distance between the hub and the node
(computer/NIC):
2000

meters

(
2
kilometers)
‏



Network topology:
Star Bus



Cable: Multimode
Fiber Optic

10BASEFL/10BASEF: SPECIFICATIONS


Advantages:


Longer cable without repeater


Immune to electromagnetic interference (EMI)


More difficult to tap


More scalable


Scalability = ability to be adapt to (to be used with)
a new, faster technology



Disadvantages:


More expensive


More difficult to install


More fragile


MULTIPLE ETHERNET HUBS


Why using multiple Ethernet Hubs (multiple
segments)?



If one hub is down, the other hub(s) still works
→
NO SINGLE POINT OF FAILURE IN THE HUB

MULTIPLE ETHERNET HUBS


How to connect multiple Ethernet Hubs (multiple Ethernet
switches)?



Crossover cable

(UTP, two pairs, CAT
3
or higher): reversing the
sending and receiving pairs on the end of the cable (see page
70
)
‏


One end of a cable is connected to the
regular port
, the other end is
connected to the
regular port
at the other hub



Crossover Port

(also called
Crossover, Uplink, In
-
port, Out
-
port
):


Use
regular cable
, NOT crossover cable


One end of the regular cable (
NOT crossover cable
) is connected to the
Crossover Port
, the other end is connected to the
regular port
at the
other hub



Autosensing Port

(a regular port that can turn to be Crossover
Port automatically whenever necessary)
‏


One end of a regular cable is connected to the
Autosensing Port
, the
other end is connected to the
regular port
at the other hub

CONNECTING ETHERNET SEGMENTS


MULTIPLE ETHERNET HUBS


Hubs must be connected in
a single line (daisy
chain)


Why? This type of connection will simply extend the
“logical bus cable inside the hubs,” hence, maintain
the “logical bus topology”



Hubs
cannot

be connected in “
hierarchical
configuration
” (see page 70 figure 4.17)


Why? This type of connection will connect the “logical
bus cables inside the hubs” in a configuration other
than a “logical bus topology”


COLLISION DOMAIN


Collision Domain
(all computers are connected to
a
hub
(s)):


A network in which
there will be collision
if

two or more
computers

transmit frames simultaneously


Collisions can significantly slow down the network. Why?
Because every time there is a collision, the sender will
automatically
retransmit the frame (the original transmission
is wasted)



A
SWITCH
or

BRIDGE (i.e., a two
-
port switch)

can divide
a network into multiple collision domains

SWITCH

Computer A

Computer B

Accept the message

For Computer B

Switch

SWITCH


Switch does the switching by using its
lookup table
(i.e.,
an electronic database storing a list of ports and their
corresponding MAC addresses)


When a switch is just installed, as the lookup table is still
empty, it acts like a hub. However, every time it receives a
frame in a port, it will store the port number and the
sender MAC address of the frame in the lookup table
(see this animation:
http://www.cisco.com/image/gif/paws/10607/lan
-
switch
-
transparent.swf)

Port Number

MAC Address

1

2

3

Abc

Def

Ghi

SWITCH


Connecting multiple switches:


Use the procedures to connect multiple hubs



Spanning Tree Protocol


Enabling a switch to detect
bridge loop
(i.e., three or more
switches that are connected to one another forming a loop) and
to automatically shut down one port to break the loop


The problem with bridge loop


Broadcast storm


COLLISION DOMAIN


FAQ:


Can I break a network into multiple “collision
domains” by using a Router?


Yes, you can



So, why do I use Switch instead of a Router?


Switch is cheaper (although the price of Router
continues going down)


Switch is easier to install (e.g., no (not much)
configuration is needed)


HUB (REPEATER) VERSUS SWITCH (BRIDGE) VERSUS ROUTER


When do we use hub (repeater), switch (bridge), and
router?


Hub


Switch


Router


Router can do what a switch can do, but it is more expensive and
more difficult to install (e.g., requires more configuration).


Only router can connect two different networks.


Switch: To connect computers within a LAN


Router: To connect one LAN to another LAN


If you want to install packet sniffer to monitor incoming traffics to
your network, you can get hub


More sophisticated,

more expensive,

more difficult to install