Chapter 4 - UAH Personal Web Pages

canoeornithologistΔίκτυα και Επικοινωνίες

26 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

80 εμφανίσεις

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