Data communication part2

DATCOMM
Local Area Network
Technologies
Engr. Melvin K. Cabatuan, MsE
De La Salle University
February 2013

Objectives
- To brieﬂy discuss the technology of dominant
wired LANs- Ethernet, and other LAN media.

Objectives
- Describe Media Access Control (MAC) and
Carrier Sense Multiple Access/Collision
Detection (CSMA/CD)

Objectives
Detection (CSMA/CD)
- Explain Address Resolution Protocol (ARP)
and Bridges.

Objectives
Detection (CSMA/CD)
- Explain Address Resolution Protocol (ARP)
and Bridges.
- Discuss Switched Ethernet and
Virtual LAN (VLAN).

Local Area Network
LAN
- A computer network that is designed for a
limited geographic area such as a building or a
campus.

Local Area Network
LAN
- A computer network that is designed for a
limited geographic area such as a building or a
campus.
- LAN technologies: Ethernet, token ring, token
bus, FDDI, and ATM LAN.

IEEE Project 802
IEEE standard for LANs
- specify functions of the physical and data link
layer of major LAN protocols.

IEEE Project 802
IEEE standard for LANs
- specify functions of the physical and data link
layer of major LAN protocols.
- subdivided the data link layer into two
sublayers: logical link control (LLC) and media
access control (MAC).

Ethernet
- was created in 1976 at Xerox’s Palo Alto
Research Center (PARC).

Ethernet
- de facto standard technology that is used for
connecting LANs.

Ethernet
- de facto standard technology that is used for
connecting LANs.
- ﬁrst implemented by a group called DIX
(Digital, Intel, and Xerox).

Ethernet (802.3) Frame
- Preamble
contains 7 bytes (56 bits) of alternating 0s and
1s that alerts the receiving system to the
coming frame and enables it to synchronize its
input timing.

- Start frame delimiter (SFD)
(1 byte: 10101011) signals the beginning of the
frame; the last 2 bits are 11 and alert the
receiver that the next ﬁeld is the destination
address.

- Destination address (DA)
is 6 bytes and contains the physical address of
the destination station/s to receive the packet.

- Destination address (DA)
the destination station/s to receive the packet.
- Source address (SA)
the sender of the packet.

- Length or Type
◦ 802.3: length field to define the number of
bytes in the data field or
◦ Ethernet: type field to define the upper-layer
protocol using the MAC frame.

- Data
◦ carries data encapsulated from the
upper-layer protocols;
◦ a minimum of 46 and a maximum of 1500
bytes.

Understand
What if the upper-layer packet is less than the
minimum 46 bytes?

Understand
What if the upper-layer packet is less than the
minimum 46 bytes?
- Padding is added to make up the diﬀerence.

- Cyclic Redundancy Check (CRC)
veriﬁes that the data that left the source
computer did not change at all during the
transmission.

Understand
The 802.3 standard deﬁnes the maximum length of
a frame (without preamble and SFD ﬁeld) as 1518
bytes. Give the historical reasons for this
restriction.

Understand
restriction.
- Memory was very expensive when Ethernet
was designed: a maximum length restriction
helped to reduce the size of the buﬀer.

Understand
restriction.
- Memory was very expensive when Ethernet
was designed: a maximum length restriction
helped to reduce the size of the buﬀer.
- It prevents one station from monopolizing the
shared medium, blocking other stations that
have
data to send.

MAC Address
also referred to as the data link address or physical
address
- a 6 bytes (48 bits) physical address applied to
the network interface card (NIC) by the
manufacturer during production.

MAC Address
- normally written in hexadecimal notation, with
a colon between the bytes.

MAC Address
- normally written in hexadecimal notation, with
a colon between the bytes.
- Ex. Ethernet MAC address
4A: 30 : 10 : 21 : 10 : 1A

Source and Destination Addressing
Modes
- Source address is always a unicast address - the
frame comes from only one station.

Source and Destination Addressing
Modes
- Source address is always a unicast address - the
frame comes from only one station.
- Destination address can be unicast, multicast,
or broadcast.

Exercise
Deﬁne the type of the following destination
addresses:

Exercise
addresses:
- 4A : 30 : 10 : 21 : 10 : 1A

Exercise
addresses:
- 4A : 30 : 10 : 21 : 10 : 1A
- 47 : 20 : 1B : 2E : 08 : EE

Exercise
addresses:
- 4A : 30 : 10 : 21 : 10 : 1A
- 47 : 20 : 1B : 2E : 08 : EE
- FF : FF : FF : FF : FF : FF

Exercise
addresses:
- 4A : 30 : 10 : 21 : 10 : 1A
- 47 : 20 : 1B : 2E : 08 : EE
- FF : FF : FF : FF : FF : FF
Solution: Refer to the second hexadecimal digit
from the left:
even == unicast;
odd == multicast;
all F’s == Broadcast

Transmission of Addresses
- transmission is left-to-right, byte by byte;
however, for each byte, the least significant bit
is sent first and the most significant bit is sent
last.

Transmission of Addresses
- transmission is left-to-right, byte by byte;
however, for each byte, the least significant bit
is sent first and the most significant bit is sent
last.
- Ex. Show how the address
47 : 20 : 1B : 2E : 08 : EE is sent out on line.

CSMA/CD
Carrier Sense Multiple Access with Collision
Detection
- access method for traditional Ethernet
(10-Mbps) that senses the medium before
trying to use it.

CSMA/CD
Detection
- access method for traditional Ethernet
(10-Mbps) that senses the medium before
trying to use it.
- Ethernet stations can be connected together
using a physical bus or star topology but its
logical topology is always a bus.

CSMA/CD
Detection Algorithm

CSMA/CD
Minimum Frame Size
- a restriction on the frame size is required.

CSMA/CD
Minimum Frame Size
- before sending the last bit of the frame, the
sending station must detect a collision and
abort.

CSMA/CD
Minimum Frame Size
- before sending the last bit of the frame, the
sending station must detect a collision and
abort.
- thus, transmission time Tfr must be at least
two times the maximum propagation time Tp.

Exercise
In the standard Ethernet, if the maximum
propagation time is 25.6µs, what is the minimum
size of the frame?

Exercise
size of the frame?
Tfr = 2 × Tp = 51.2µs

Exercise
size of the frame?
Tfr = 2 × Tp = 51.2µs
10 Mbps ×51.2µs = 512 bits or 64 bytes

Exercise
size of the frame?
Tfr = 2 × Tp = 51.2µs
10 Mbps ×51.2µs = 512 bits or 64 bytes
- This is the minimum size of the frame for
Standard Ethernet.

Standard Ethernet
Implementation

Reading Assignment
- Report about the IEEE 802.3
Standard, in your own words.
- Submit through
www.turnitin.com.

Readings
It is the standards body that creates the Physical
layer speciﬁcations for Ethernet.

Readings
- EIA/TIA (Electronic Industries Association
and the newer Telecommunications Industry
Association)

Readings
- EIA/TIA (Electronic Industries Association
and the newer Telecommunications Industry
Association)
- EIA/TIA speciﬁes that Ethernet use a
registered jack (RJ) connector with a 4 5
wiring sequence on unshielded twisted-pair
(UTP) cabling (RJ-45).

UTP Connections (RJ-45)
- RJ-45 connector is clear so you can see the
eight colored wires that connect to the
connector’s pins. These wires are twisted into
four pairs.

- RJ-45 connector is clear so you can see the
eight colored wires that connect to the
connector’s pins. These wires are twisted into
four pairs.
- Four wires (two pairs) carry the voltage and
are considered tip. The other four wires are
grounded and are called ring.

8-pin modular connector

Ethernet Cabling
Straight-through cable: used to connect
- Host to switch or hub.

Ethernet Cabling
Straight-through cable: used to connect
- Host to switch or hub.
- Router to switch or hub

Ethernet Cabling
Straight-through cable: wires on both cable ends
are in the same order.

Ethernet Cabling
Crossover Cable: used to connect
- Switch to switch

Ethernet Cabling
- Switch to switch
- Hub to hub

Ethernet Cabling
- Switch to switch
- Hub to hub
- Host to host

Ethernet Cabling
- Switch to switch
- Hub to hub
- Host to host
- Hub to switch

Ethernet Cabling
- Switch to switch
- Hub to hub
- Host to host
- Hub to switch
- Router direct to host

Ethernet Cabling
Crossover Cable: wires on each end of the cable
are crossed - Transmit to Receive and Receive to
Transmit on each side, for both tip and ring.

Fast Ethernet (802.3u)
- designed to compete with LAN protocols such
as FDDI or Fiber Channel.

- upgrade the data rate to 100 Mbps.

- backward-compatible with Standard Ethernet.

- same frame format and 48-bit address.

- same frame format and 48-bit address.
- same minimum and maximum frame lengths.

Implementation

- MAC sublayer was kept untouched

- star topology: half duplex and full duplex

- access method is the same (CSMA/CD) for the
half-duplex

- access method is the same (CSMA/CD) for the
half-duplex
- autonegotiation allows two devices to negotiate
the mode or data rate of operation.

Address Resolution Protocol (ARP)
- accepts a logical address from the IP protocol,
then, identify and place the source and
destination MAC address in the frame

- operates at the Internet layer, but the the
MAC address is attached at the Network
Access layer.

- operates at the Internet layer, but the the
MAC address is attached at the Network
Access layer.
- maps a logical address to its corresponding
physical address

Position of ARP in TCP/IP protocol suite

Position of ARP in TCP/IP protocol suite
- Why do we need ARP?

ARP Operation
ARP request is broadcast
c If this is your IP address, send me your
MAC address. d

ARP Operation
ARP reply is unicast
c This is my MAC address. d

ARP Packet Format
- Hardware type
16-bit ﬁeld deﬁning the type of the network on
which ARP is running. Each LAN has been
assigned.
Ex. Ethernet: type 1

ARP Packet Format
- Hardware type
assigned.
- Protocol type
16-bit ﬁeld deﬁning the protocol.
Ex. IPv4 protocol: 080016

ARP Packet Format
- Hardware type
assigned.
- Protocol type
16-bit field defining the protocol.
- Hardware length
8-bit field defining the length of the physical
address in bytes.
Ex. Ethernet: 6 bytes

ARP Packet Format
- Protocol Length
8-bit ﬁeld deﬁning the length of the logical
address in bytes.

ARP Packet Format
- Protocol Length
address in bytes.
- Operation
16-bit ﬁeld deﬁning the type of packet
Ex. ARP request (1), ARP reply (2).

ARP Packet Format
- Protocol Length
address in bytes.
- Operation
16-bit field defining the type of packet
Ex. ARP request (1), ARP reply (2).
- Sender hardware address
variable-length field defining the physical
address of the sender.

ARP Packet Format
- Sender protocol address
variable-length ﬁeld deﬁning the logical address
of the sender.
Ex. IPv4 protocol: 4 bytes

ARP Packet Format
of the sender.
- Target hardware address
address of the target.

ARP Packet Format
of the sender.
- Target hardware address
address of the target.
- Target protocol address
of the target.
Ex. IPv4 protocol: 4bytes

Encapsulation of ARP packet
An ARP packet is encapsulated directly into a
data link frame.

Exercise
A host with IP address 130.23.43.20 and physical
address B2 : 34 : 55 : 10 : 22 : 10 has a packet to
send to another host with IP address 130.23.43.25
and physical address A4 : 6E : F4 : 59 : 83 : AB
(which is unknown to the ﬁrst host). The two
hosts are on the same Ethernet network. Show the
ARP request and reply packets encapsulated in
Ethernet frames.

Proxy ARP Router
- acts on behalf of a set of hosts.

Proxy ARP Router
- acts on behalf of a set of hosts.
- whenever a router running a proxy ARP
receives an ARP request looking for the IP
address of one of its hosts, the router sends an
ARP reply announcing its own hardware
(physical) address.

Proxy ARP Router
After the router receives the actual IP packet, it
sends the packet to the appropriate host or router.

Connecting Devices
- To connect LANs and WANs together we use
connecting devices.

Connecting Devices
- To connect LANs and WANs together we use
connecting devices.
- Ex. Repeaters (or hubs), Bridges (or two-layer
switches), and Routers (or three-layer
switches).

Repeater
A repeater forwards every bit; it has no ﬁltering
capability.

Bridge
Two-layer switch
- operates in both the physical and the data link
layers.

Bridge
Two-layer switch
layers.
- PHY: regenerates the signal it receives.

Bridge
Two-layer switch
layers.
- DLL: check the MAC addresses (source and
destination) contained in the frame.

Bridge
Two-layer switch
layers.
- DLL: check the MAC addresses (source and
destination) contained in the frame.
- has a table used in ﬁltering decisions.

Bridge
Example
The bridge consults its table to ﬁnd the departing
port.

Transparent Bridge
- a bridge in which the stations are completely
unaware of the bridge’s existence.

Transparent Bridge
- reconﬁguration of the stations is unnecessary
when added or deleted.

Transparent Bridge
- reconﬁguration of the stations is unnecessary
when added or deleted.
- forwarding function with dynamic forwarding
table

Bridging vs. LAN Switching
- Bridges are software based, while switches are
hardware based (ASIC for ﬁltering)

- A switch can be viewed as a multiport bridge.

- Switches have a higher number of ports than
most bridges.

- Switches have a higher number of ports than
most bridges.
- Both bridges and switches forward layer 2
broadcasts.

Layer 2 Switch Functions
- Address learning
remember the source hardware address of each
frame received, and save in forward/ﬁlter table.

- Address learning
- Forward/ﬁlter decisions
When a frame is received, the switch looks at
the destination hardware address and ﬁnds the
exit interface.

- Address learning
- Forward/ﬁlter decisions
When a frame is received, the switch looks at
the destination hardware address and ﬁnds the
exit interface.
- Loop avoidance
stop network loops while still permitting
redundancy.

Forward/Filter Decisions
Host A sends a data frame to Host D. What will the switch do
when it receives the frame from Host A?

Network Redundancy
Importance and Problem

Loop Avoidance
Spanning Tree Protocol (STP)
All root ports forward,
All nonroot ports block .

Virtual Private Network
VPN
- It enables a host computer to send and receive
data across shared or public networks as if it
were a private network with all the
functionality

Network Security Issue
- Ensure conﬁdentiality through use of
◦ User authentication
◦ Data encryption

Virtual Private Networks
- network connection that uses the Internet to
give users or branch oﬃces secure access to a
company’s network resources.

- use encryption technology to ensure that
communication is private and secure

- use encryption technology to ensure that
communication is private and secure
- Privacy is achieved by creating a tunnel
between the VPN client and VPN server.

Virtual Private Network
A tunnel is created by encapsulation, in which the inner packet
containing the data is encrypted and the outer headers contain
the unencapsulated addresses.

VPN Types/ Beneﬁts
- Remote access VPNs
Enable mobile users to connect with corporate
networks securely wherever an Internet
connection is available.

- Remote access VPNs
Enable mobile users to connect with corporate
networks securely wherever an Internet
connection is available.
- Site-to-site VPNs or intranet
Allow multiple sites to maintain permanent
secure connections via the Internet instead of
using expensive WAN links.

- Reduce costs by using the ISP’s support
services instead of paying for more expensive
WAN support.

- Reduce costs by using the ISP’s support
services instead of paying for more expensive
WAN support.
- Eliminate the need to support dial-up remote
access, which is a higher-cost solution requiring
more personnel.

IP Security
IPSec
- a collection of protocols designed by the
Internet Engineering Task Force (IETF) to
provide security for a packet at the network
level..

IP Security
IPSec
level..
- helps create authenticated and conﬁdential
packets for the IP layer.

IP Security
IPSec
level..
- helps create authenticated and conﬁdential
packets for the IP layer.
- operates in one of two diﬀerent modes:
transport or tunnel mode.

IPSec Transport Mode
IPSec in transport mode does not protect the IP
header;
it only protects the information coming from the
transport layer.
It is used when we need host-to-host (end-to-end)
protection of data.

IPSec Tunnel Mode
IPSec in tunnel mode protects the original IP
header.
It is used between two routers, between a host and
a router, or between a router and a host.

Tunnel vs. Transport Mode
◦ In transport mode, the IPSec layer comes
between the transport layer and the network layer.
◦ In tunnel mode, the ﬂow is from the network
layer to the IPSec layer and then back to the
network layer again.

Other Terms
- 1000BASE-CX, 1000BASE-LX, 1000BASE-SX,
1000BASE-T
The IEEE 802.3 standards for Ethernet implementation
with 1-Gbps data rate.
- 100BASE-FX, 100BASE-T4, 100BASE-TX, 100BASE-X
The IEEE 802.3 standards for Fast Ethernet
implementation with 100-Mbps data rate.
- 10BASE2, 10BASE5, 10BASE-F, 10BASE-E, 10BASE-L
The IEEE 802.3 standard for Thin Ethernet with 10-Mbps
data rate.

Other Terms
- Address Resolution Protocol (ARP)
In TCP/IP, a protocol for obtaining the physical address of
a node when the Internet address is known.
- Address space
The total number of addresses used by a protocol.
- Bandwidth
The diﬀerence between the highest and lowest frequencies
available for network signals. The term is also used to
describe the rated throughput capacity of a given network
medium or protocol.

Other Terms
- Bridge
A network device operating at the first two layers of the
OSI model with filtering and forwarding capabilities.
- Broadcast address
An address that allows transmission of a message to all
nodes of a network.
- Congestion
Excessive network or internetwork traffic causing a general
degradation of service. This can be seen in slower response
times, longer file transfers and network users becoming less
productive due to network delays.

Other Terms
- Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA)
An access method in wireless LANs that avoids collision by
forcing the stations to send reservation messages when they
ﬁnd the channel is idle.
- Carrier Sense Multiple Access with Collision Detection
(CSMA/CD)
An access method in which stations transmit whenever the
transmission medium is available and retransmit when
collision occurs.
- Collision
The event that occurs when two transmitters send at the
same time on a channel designed for only one transmission
at a time; data will be destroyed.

Other Terms
- Consultative Committee for International Telegraphy and
Telephony (CCITT)
An international standards group now known as the ITU-T.
- Defense Advanced Research Projects Agency (DARPA)
A government organization, which, under the name of
ARPA, funded ARPANET and the Internet.
- Ethernet
A local area network using the CSMA/CD access method.

Other Terms
- Extranet
A private network that uses the TCP/IP protocol suite that
allows authorized access from outside users.
- Flooding
Saturation of a network with a message. intranet A private
network that uses the TCP/IP protocol suite.
- Intranet
A private network that uses the TCP/IP protocol suite.

Other Terms
- Institute of Electrical and Electronics Engineers (IEEE)
A group consisting of professional engineers that has
specialized societies whose committees prepare standards in
members’ areas of specialty.
- Logical tunnel
The encapsulation of a multicast packet inside a unicast
packet to enable multicast routing by non-multicast routers.
- Physical address
The address of a device used at the data link layer (MAC
address).

Other Terms
- Request for Comment (RFC)
A formal Internet document concerning an Internet issue.
- Reverse Address Resolution Protocol (RARP)
A TCP/IP protocol that allows a host to ﬁnd its Internet
address, given its physical address.
- Switch
A device connecting multiple communication lines together.
- Switched Ethernet
An Ethernet in which a switch, replacing the hub, can
direct a transmission to its destination.

References
TEXTBOOK:
- Data Communications and Networking,
Behrouz Forouzan, 4th Edition, McGraw-Hill,
2007

References
SECONDARY SOURCE:
- TCP/IP Protocol Suite, Behrouz Forouzan, 4th
edition, 2010

References
SECONDARY SOURCE:
- Data and Computer Communications, William
Stallings, 2007

References
SECONDARY SOURCE:
- CISCO Networking Essentials, Troy McMillan,
2012

References
SECONDARY SOURCE:
- Network Fundamentals, Cisco Networking
Academy, 2007

Data communication part2

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Data communication part2