Internet Protocols and Software

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24 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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TCP/IP Protocol Suite

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COMP 416

Internet Protocols and Software

Instructor: Zhijun Wang


Note:


Projects are available online


The first quiz will be given in next lecture (Oct. 15)


It covers the first 4 lectures



Today’s contents


Underlying Technology


Virtual LAN

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Chapter 3

Objectives


Underlying Technology



Understand the Ethernet



Understand the types of point
-
to
-
point WANs



Understand the types of switched WANs



Differentiate between repeaters, bridges, routers, and hubs

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3.1 Local Area Networks

A
local area network (LAN)

is a data communication system
that allows a number of independent devices to communicate
directly with each other in a limited geographic area such as
a single department, a single building, or a campus. A large
organization may need several connected LANs. The most
popular LANs are Ethernet and wireless LANs.

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Figure 1
Carrier sense multiple access with collision detection (
CSMA/CD
)

LAN is a communication medium shared by multiple users.

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Figure
2

Ethernet layers

Response for Flow

and error control

Responsible for the

operation of

the CSMA/CD

access method

Ethernet layer is above physical layer including logical link control and

Media access control sub
-
layers

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Figure 3

Ethernet frame

Used for synchronization

Identify the frame start

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3.2 Point
-
to
-
Point WANs

A second type of network we encounter in the Internet is the
point
-
to
-
point wide area network. A
point
-
to
-
point WAN

connects two remote devices using a line available from a
public network such as a telephone network.

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Figure
4

56K modem

PCM: Pulse code modulation

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ADSL (Asynchronous digital subscriber line) is
an asymmetric communication technology
designed for residential users;

it is not suitable for businesses.

Note:

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Figure
5

Bandwidth division

in ADSL

Telephone call

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Figure
6

ADSL and DSLAM

Digital subscriber line access multiplexer

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Figure
7

Cable bandwidth

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Figure
8

Cable modem configurations

Cable modem transmission system

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Table 1 Synchronous Optical Network (
SONET) rates

5 Gbps

10 Gbps

STS: synchronous


transport signal

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3.3 Switched WANs

The backbone networks in the Internet are usually switched
WANs. A
switched WAN

is a wide area network that covers a
large area (a state or a country) and provides access at
several points to the users. Inside the network, there is a
mesh of point
-
to
-
point networks that connects switches. The
switches, multiple port connectors, allow the connection of
several inputs and outputs.


X.25

Frame relay

Asynchronous Transfer Mode (ATM)

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A cell network uses the cell as the basic unit
of data exchange. A cell is defined as a
small, fixed
-
size block of information.

Note:

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Figure
9

Architecture of an ATM network

UNI: User
-
to
-
network interface

NNI: Network
-
to
-
network interface

ATM: asynchronous transfer mode

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Figure
10

Virtual circuits

VCI: Virtual circuit identifier

VPI: Virtual path identifier

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Note that a virtual connection is defined by
a pair of numbers:

the
VPI

and the
VCI
.

Note:

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Figure
11

An ATM cell

ATM cell is a 53
-
byte packet

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3.4 Connecting Devices

LANs or WANs do not normally operate in isolation. They are connected
to one another or to the Internet. To connect LANs or WANs, we use
connecting devices
. Connecting devices can operate in different layers of
the Internet model. We discuss three kinds of connecting devices:
repeaters (or hubs),

bridges (or two
-
layer switches),

and
routers (or
three
-
layer switches
). Repeaters and hubs operate in the first layer of the
Internet model. Bridges and two
-
layer switches operate in the first two
layers. Routers and three
-
layer switches operate in the first three layers

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Figure
13

Connecting devices

Router is a three
-
layer device (physical, data link and network layers)

Bridge is a two
-
layer device (physical and data link layers)

Repeater is a physical layer device

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Figure
14

Repeater

A repeater connects segments of a LAN.

A repeater forwards every bit;

it has no filtering capability.

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Figure
15

Function of a repeater

A repeater is a regenerator, not an amplifier.

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Figure
16

Bridge

A bridge has a table used in filtering decisions.


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Figure
17

Learning bridge

A bridge does not change the physical (MAC) addresses in
a frame.

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Figure
18

Routing example

A router is a three
-
layer physical, data link, and network) device.

A router changes the physical addresses in a packet.

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A repeater or a bridge connects segments of
a LAN.

A router connects independent LANs or
WANs to create an internetwork (internet).

Note:

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Gateways


The term “Gateway” is used with different meanings in
different contexts


“Gateway” is a generic term for routers (Level 3)


“Gateway” is also used for a device that interconnects
different Layer 3 networks and which performs translation
of protocols (“Multi
-
protocol router”)


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Many times it is necessary to connect a local area network to
another local area network or to a wide area network.

Local area network to local area network connections are often
performed with a bridge
-
like device.

Local area network to wide area network connections are usually
performed with a router.

A third device, the switch, can be used to interconnect segments
of a local area network.

Interconnection

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To separate / connect one corporate division with another.

To connect two LANs with different protocols.

To connect a LAN to the Internet.

To break a LAN into segments to relieve traffic congestion.

To provide a security wall between two different types of users.

Why Interconnect?


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An Example of Internetworking


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A transparent bridge does not need programming but observes all
traffic and builds routing tables from this observation.

This observation is called backward learning.

Each bridge has two connections (ports) and there is a routing
table associated with each port.

A bridge observes each frame that arrives at a port, extracts the
source address from the frame, and places that address in the
port’s routing table.

A transparent bridge is found with CSMA/CD LANs.

Transparent Bridges
-
I

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A transparent bridge can also convert one frame format to
another, but this does not happen too often anymore since most
networks are CSMA/CD.

Note that some people / manufacturers call a bridge such as this
a gateway or sometimes a router.

The bridge removes the headers and trailers from one frame
format and inserts (encapsulates) the headers and trailers for the
second frame format.

Transparent Bridges
-
II

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A remote bridge is capable of passing a data frame from one
local area network to another when the two LANs are separated
by a long distance and there is a wide area network connecting
the two LANs.

A remote bridge takes the frame before it leaves the first LAN
and encapsulates the WAN headers and trailers.

When the packet arrives at the destination remote bridge, that
bridge removes the WAN headers and trailers leaving the
original frame.

Remote Bridges

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An Example of Remote Bridges

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What happens if you have many LANs interconnected
with multiple bridges?

Data that leaves one workstation could travel to a
bridge, across the next network, into the next bridge,
and back onto the first network.

A packet may continue to cycle like this forever!

Loop in Connected LANs

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Consider the two LANs that are
connected by two bridges.


Assume
host n

is transmitting a

frame F with unknown destination.

What is happening?


Bridges A and B flood the frame

to LAN 2.


Bridge B sees F on LAN 2 (with

unknown destination), and copies

the frame back to LAN 1


Bridge A does the same.


The copying continues

Where’s the problem? What’s the
solution ?

Danger of Loops

F

F

F

F

F

F

F

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How do we stop this from happening?

Disconnect one of the bridges? Maybe we want bridge
redundancy in case one bridge fails.

How about applying the
spanning tree algorithm.

How is the algorithm applied?

Spanning Tree Algorithm

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Spanning Tree Protocol (IEEE 802.1d)


The Spanning Tree Protocol
(SPT) is a solution to prevent
loops when forwarding
frames between LANs



The SPT is standardized as
the IEEE 802.1d protocol



The SPT organizes bridges
and LANs as spanning tree in
a dynamic environment


Frames are forwarded only
along the branches of the
spanning tree


Note: Trees don’t have loops


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Spanning Tree Protocol (IEEE 802.1d)


Bridges that run the SPT
are called transparent
bridges



Bridges exchange
messages to configure
the bridge ( Configuration
Bridge Data Unit or
BPDUs) to build the tree.


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Virtual LANs

A virtual LAN, or VLAN, is a logical subgroup within a local
area network that is created via switches and software rather than
by manually moving wiring from one network device to another

Even though the employees and their actual computer
workstations may be scattered throughout the building, LAN
switches and VLAN software can be used to create a “network
within a network.”

Virtual LAN (VLAN)

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A relatively new standard, IEEE 802.1Q, was designed
to allow multiple devices to intercommunicate and
work together to create a virtual LAN

Instead of sending a technician to a wiring closet to
move a workstation cable from one switch to another,
an 802.1Q
-
compliant switch can be

remotely configured
by a network administrator

VLAN

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Review of LAN

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What is a VLAN

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VLAN

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Why do we need VLAN

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VLAN Tagging

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IEEE 802.1Q:Features
-
I

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IEEE 802.1Q:Features
-
II

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IEEE 802.1Q:Features
-
III

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IEEE 802.1Q:Features
-
IV

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VLAN Tag

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Tagging rule

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Tagged Frame Format

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Communications in VLANs

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Metro Ethernet

All information of Metro Ethernet can be found on


The Metro Ethernet Forum (MEF):


http://en.wikipedia.org/wiki/Metro_Ethernet_Forum


A
Metro Ethernet

is a computer network based on the Ethernet

standard and which covers a metropolitan area. It is commonly

used as a metropolitan access network to connect subscribers

and businesses to a Wide Area Network, such as the Internet.

Large businesses can also use Metro Ethernet to connect

branch offices to their Intranets.


Advantages of Metro Ethernet:


Cost
-
effective equipment


Easy network management


Fast speed


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Excises:

1.
Match the following to one or more layers of OSI model




(a) interface to transmission media


(b) provides access for the end user


(c) route selection


(d) communicates directly with user’s application medium


(e) define frames


(f) ensure reliable transmission of data

2.
(a) What are the differences between a repeater and a bridge?


(b) What are the differences between a bridge and a router?



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Answers

1.
Match the following to one or more layers of OSI model




(a) interface to transmission media


( Physical layer)


(b) provides access for the end user


(Application layer)


(c) route selection


(Networking layer)


(d) communicates directly with user’s application medium


(Application layer)


(e) define frames


(Data link layer)


(f) ensure reliable transmission of data


(Data link and transport layers)

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Answers
-
Cont.

2.
(a) What are the differences between a repeater and a bridge?


(b) What are the differences between a bridge and a router?


Ans: (a) A repeater only operates in physical layer, but a bridge operates


in both physical and data link layers;


A repeater has no filter function, a bridge has filter function.


(b) A bridge connects segments of a LAN, and a router connects LANs


and WANs;


A bridge operates in physical and data link layer and a router


operates in physical, data link and networking layers;


A bridge cannot change the physical address in a packet


A router does