Transcript OSI layers

Ch.2 – Networking Fundamentals
Getting past some basics…
CIS 81 and CST 311
Cabrillo College and CSUMB
Rick Graziani
Fall 2005
Overview
Remember, we are just beginning to herd the cats.
Much of this will become clearer LATER!
The more we learn, the more all of this will come into focus!
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What is Networking?
Networking - the interconnection of workstations, peripherals, terminals
and other devices.
Whatis.com: “In information technology, networking is the construction,
design, and use of network, including the physical (cabling, hub,
bridge, switch, router, and so forth), the selection and use of
telecommunication protocol and computer software for using and
managing the network, and the establishment of operation policies and
procedures related to the network.”
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The Evolution of Data networks
•
Peer-to-peer: Enabling communications between two
computers, without relying on a computer server
(client/server).
• LANs (Local Area Networks): Enabling communications
between groups of computers and other devices within an
office, company, etc., including servers and printers.
• WANs (Wide Area Networks): Enabling communications
between individual computers, computers on different
etc.
Rick LANs,
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Network Media Symbols
Becoming
obsolete
Usually a LAN
connection
The Serial connection symbol usually represents some
sort of WAN connection such as leased line (T1),
ISDN, Frame Relay, ATM, asynchronous dial-up
(modem), etc.
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Symbols for Networking Devices
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Network topologies
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Network topologies
• Network topology defines the structure of the network.
• Physical topology: Actual layout of the wire or media.
•
– Bus
– Star, Extended Star
– Ring
Logical topology: Defines how the media is accessed by
the hosts for sending data.
– Broadcast or multi-access
– Token passing
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Bus Topology
“A bus topology uses a single backbone segment (length of
cable) that all the hosts connect to directly.”
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Other Bus Topologies
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In the 70’s I was usually working on my bus
topology…
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Ring Topology
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•
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“A ring topology connects one host to the next and the last
host to the first.
This creates a physical ring of cable.”
Becoming less common.
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Token Ring
• Ring topologies use a token passing (logical topology).
• Token Ring NIC and Hub (or MAU)
• Token Ring networks are becoming obsolete and not part
of this curriculum.
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Star Topology
• “A star topology connects all cables to a central point of
•
concentration.
This point is usually a hub or switch, which will be
described later in the chapter.”
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Extended Star Topology
•
“An extended star topology uses the star topology to be
created.
• It links individual stars together by linking the
hubs/switches.
• This, as you will learn later, will extend the length and size
of the network.”
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Mesh Topology
Circuit and Packet
Switched networks could
be full or partial mesh.
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A mesh topology is used when there can be absolutely no
break in communications, for example the control systems
of a nuclear power plant.
Each device has its own connections to all other hosts.
This also reflects the design of the Internet, which has
multiple paths to any one location.”
There are also full mesh and partial mesh topologies,
both physical and logical, which will be discussed in later
courses.
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Full Mesh and Switched Telephone Lines
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Network protocols
010010100111000111010011100101001000111101 ...
• Protocol - Set of rules and conventions that govern a particular aspect
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•
of how devices on a network communicate.
– Including: format, timing, sequencing, and error control in data
communication.
Protocol suites are collections of protocols that enable network
communication from one host through the network to another host.
Without protocols, the computer cannot make or rebuild the stream of
incoming bits from another computer into the original format.
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Network protocols
Examples:
• Ethernet
• TCP/IP protocol suite
– IP
– TCP
– UDP
– ARP
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Network Technologies
LANs
MANs
WANs
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Local-area networks (LANs)
Some common LAN technologies are:
• Ethernet
• Token Ring
• FDDI
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Wide-area networks (WANs)
Some common WAN technologies are:
• Modems (Asynchronous)
• Integrated Services Digital Network (ISDN)
• Digital Subscriber Line (DSL)
• Frame Relay
• US (T) and Europe (E) – T1, E1, T3, E3
•Rick Synchronous
Optical Network (SONET)
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Metropolitan-area networks (MANs)
• A MAN is a network that spans a metropolitan area such as a city or
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•
suburban area.
A MAN usually consists of two or more LANs in a common geographic
area.
For example, a bank with multiple branches may utilize a MAN.
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Importance of bandwidth
• Bandwidth - The amount of information that can flow through a
network connection in a given period of time.
•
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Available at http://www.thinkgeek.com
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Analogies
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Measurement
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In digital systems, the basic unit of bandwidth is bits per
second (bps).
Bandwidth is the measure of how much information, or bits,
can flow from one place to another in a given amount of
time, or seconds.
Later – The size of a bit!
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Limitations
•
•
Bandwidth varies depending upon:
– Type of media
– Type of technology and protocol (LAN, WAN, wireless,
etc.)
The physics of the media account for some of the
difference.
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Throughput
• Throughput - The amount of data transferred from one place to
•
another or processed in a specified amount of time. (wikopedia.com)
Often far less than the maximum possible digital bandwidth of the
medium that is being used. Internetworking devices
The following are some of the factors that determine throughput:
• Type of data being transferred
• Network topology
• Number of users on the network
• User computer
• Server computer
• Power conditions
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Digital versus Analog
DTE
DCE
digital
analog
PSTN
Dial-up network
Modulation
DTE
DCE
digital
analog
PSTN
Dial-up network
Digital SignalsDemodulation
• A signal whose state consists of discrete elements such as high or low,
GOLDMAN:
onDATACOMM
or off
FIG.02-14
Analog Signals
• A signal which is “analogous” to sound waves
• telephone lines are designed to carry analog signals
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Digital and Analog Bandwidth
Bandwidth = The width or carrying capacity of a communications circuit.
Digital bandwidth = the number of bits per second (bps) the circuit can
carry
• used in digital communications
• measure in bps
Analog bandwidth = the range of frequencies the circuit can carry
• used in analog communications such as voice (telephones)
• measured in Hertz (Hz), cycles per second
• voice-grade telephone lines have a 3,100 Hz bandwidth
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Sound Waves
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ISO and the OSI Model
• The International Organization for Standardization (ISO) released
•
the OSI reference model in 1984, was the descriptive scheme they
created.
“ISO. A network of national standards institutes from
140 countries working in partnership with international
organizations, governments, industry, business and
consumer representatives. A bridge between public and
private sectors.” www.iso.ch
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ISO and the OSI Model
• “According to ISO, "ISO" is not an abbreviation. It is a word, derived
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•
from the Greek isos, meaning "equal", which is the root for the prefix
"iso-" that occurs in a host of terms, such as "isometric" (of equal
measure or dimensions) and "isonomy" (equality of laws, or of people
before the law).
The name ISO is used around the world to denote the organization,
thus avoiding the assortment of abbreviations that would result from
the translation of "International Organization for Standardization" into
the different national languages of members.
Whatever the country, the short form of the organization's name is
always ISO.” www.whatis.com
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OSI Model
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•
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It breaks network communication
into smaller, more manageable
parts.
It standardizes network
components to allow multiple
vendor development and support.
It allows different types of network
hardware and software to
communicate with each other.
It prevents changes in one layer
from affecting other layers.
It divides network communication
into smaller parts to make learning
it easier to understand.
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OSI Model
• OSI (Open Systems Interface) was released as a suite of protocols to
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be used as the Internet standard.
However, TCP/IP became the de facto standard.
The OSI reference model is the primary model for network
communications.
Although there are other models in existence, most network vendors,
today, relate their products to the OSI reference model, especially
when they want to educate users on the use of their products.
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OSI Model
The use of this model can be
confusing and will become clearer
later!
• The OSI reference model allows you to
•
– view the network functions that occur at each layer
– a framework that you can use to understand how information
travels throughout a network.
– understand, visualize, and troubleshoot the sending and receiving
data on a network
– visualize how information, or data packets, travels from application
programs, through a network medium (e.g. wires, etc.), to another
application program that is located in another computer on a
network, even if the sender and receiver have different types of
network media
Note: The Application Layer of the OSI model refers to networking
applications, and not user applications.
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OSI layers
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OSI layers
Usually not referred to.
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Usually not referred to.
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OSI Layer 1 – Physical Layer
• The physical layer defines the
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•
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electrical, mechanical, procedural,
and functional specifications for
activating, maintaining, and
deactivating the physical link between
end systems.
Signals, network media (cables,
wireless, …), layer 1 devices
Layer 1 devices include:
– Repeaters
– Hubs
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OSI Layer 1 – Physical Layer
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Determines how are the bits to be transferred over the
physical medium.
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Repeaters
Medium
Max Distance
Twisted Pair
100 meters
Coaxial Cable
185/500 meters
Fiber Optic
2+ kilometers
• Signals can only travel so far through media before they weaken, and
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•
become garbled.
This weakening of signals is called attenuation.
Attenuation increases when:
• Media distances are lengthened
• Nodes are added to the media
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The Repeater
•
•
Repeaters are Layer 1 internetwork devices used to
combat attenuation.
Repeaters take in weakened signals, clean them up,
regenerate them, and send them on their way along the
network.
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Repeaters Extend Distances
NODE A
NODE A
100 M
100 M
REPEATER
NODE B
100 M
NODE B
By using repeaters, the distance over which a network can
operate is extended.
Example: 10Base-T (a wiring standard) is allowed to run
100 meters. One repeater can double this distance to 200
meters!
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Repeater: Layer 1 Device
Signal come in
… signal go out.
(after I amplify
it)


Repeaters are Layer 1 devices.
They do NOT look at Layer 2, Data Link (MAC, Ethernet)
addresses or Layer 3, IP Addresses.
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Hub
Hub is nothing but a multiport repeater.

Hubs are Layer 1 devices.

Data that comes in one port is sent out all other ports, except for the port
it came in on.
Hubs are sometimes called
 Ethernet concentrators
 Multiport repeaters
 In Token Ring nets, Multi-station Access Units (MAU or MSAU)

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Hub: Layer 1 Device
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
Hubs are Layer 1 devices.
They do NOT look at Layer 2, Data Link (MAC, Ethernet)
addresses or Layer 3, IP Addresses.
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Repeaters
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In the “old days”, repeaters were typically used to extend
the size or length of a bus-topology network.
Repeaters take a signal in on one end and regenerate that
signal out the other end.
In most networks (LANs), repeaters have been replaced
by hubs, which have been mostly replaced by switches.
MORE LATER!
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Hubs
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Hubs allow computers and other network devices to
communicate with each other, and use a star topology.
Like a repeater, a hub regenerates the signal.
Hubs have the same disadvantage as a repeater, anything
it receives on one port, it FLOODS out all other ports.
Wherever possible, hubs should be replace by switches.
More LATER!
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OSI Layer 2 – Data Link Layer
• The data link layer provides reliable
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•
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transit of data across a physical link.
In so doing, the data link layer is
concerned with physical (as opposed
to logical) addressing, network
topology, network access, error
notification, ordered delivery of
frames, and flow control.
Frames and Layer 2 protocols
Layer 2 devices include:
– Switches
– Bridges
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Switches
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Switches are the core devices in today’s modern LANs.
Switches are Layer 2 devices, like bridges.
Switches keep tables of MAC addresses.
Switches keep track of and examine Layer 2, Data Link addresses
(MAC addresses) – more later.
Switches learn about devices on each port and “decide” whether or not
it needs to forward the traffic: Flood or Filter.
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Switches
Etherfast 5-port 10/100
switch with one free nic
$47.99 Click here for lease
options! (Data Warehouse)
• Switches filter or flood. – more later
• Switches can have a number of different “layer 2 features”
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and may cost anywhere from $50 to over $100,000
What is a “layer 3 switch”? – Later, but it is nothing more
than a switch with a router.
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Switches
Switches will be discussed:

More this semester

More in CIS 83.
 More in course: Multilayered Switched Networks
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LAN design, media types, VLANs, VLAN Trunking Protocol, ISL,
802.1Q, Spanning Tree (802.1d), Inter-VLAN routing, Multilayer
Switching, Flow Masks, HSRP, VACLs, Multicasting, and IGMP.
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Switches
Switch
• Switches look a lot like hubs, but internally are much different.
• Switches can learn where devices are on the network, so they do not
•
have flood information (frames), but can FILTER them so the
information only goes out the port towards the destination device.
Switches also uses a star topology.
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Bridges
• A bridge is a two port switch.
• In the early days there were other types of bridges such as translation
bridges that joined two different types of networks such as Ethernet
and Token Ring.
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Network Interface Card (NIC)
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Network Interface Card (NIC)
Network Interface Card (NIC)
• www.whatis.com “A network interface card (NIC) is a computer circuit
board or card that is installed in a computer so that it can be connected
to a network. Personal computers and workstations on a local area
network (LAN) typically contain a network interface card specifically
designed for the LAN transmission technology, such as Ethernet or
token ring. Network interface cards provide a dedicated, full-time
connection to a network. Most home and portable computers connect
to the Internet through as-needed dial-up connection. The modem
provides the connection interface to the Internet service provider.’
• Data Link, Layer 2 Device
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Network Interface Card (NIC)
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Network Interface Card (NIC)
Network Interface Card (NIC)
• Layer 2, Data Link Layer, device
• Connects the device (computer) to the LAN
• Responsible for the local Layer 2 address (later)
• Common Layer 2 NICs:
– Ethernet
– Token Ring
• Common Bandwidth
– 10 Mbps, 10/100 Mbps, 10/100/1000 Mbps
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Network Interface Card (NIC)

Routers with Ethernet and Token Ring Interfaces also have
NICs.
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OSI Layer 3 – Network Layer
• The network layer provides
•
•
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connectivity and path selection
between two host systems that may
be located on geographically
separated networks.
IP Addressing, routing and Layer 3
protocols
Layer 3 devices include:
– Routers
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The Router
• The purpose of a router is to examine incoming packets, choose the
•
•
best path for them through the network, and then switch them to the
proper outgoing port.
Routers work at Layer 3, the Network Layer.
Routers examine Layer 3 addresses IP (legacy technologies: IPX,
Appletalk, etc.)
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Router
• Routers can also connect different Layer 2 technologies, such as
•
Ethernet, Token-ring, and different serial technologies such as ISDN,
PPP, etc.
However, because of their ability to route packets based on Layer 3
information, routers have become the backbone of the Internet, running
the IP protocol.
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Router
Juniper
Router
IMP
(Interface
Message
Processor)
• A router connects different networks or subnets.
• A router connects:
– LAN to LANs
– LANs to WANs
– WANs to WANs
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Router
• Much more on routing:
• CIS 82: Routing theory, routing protocols, router configuration
• CIS 83: EIGRP, Single Area OSPF, classful and classless routing
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protocols
CIS 185: Advanced Routing, multi-area OSPF, EIGRP, IS-IS, BGP,
VLSM, CIDR, route redistribution, summarization and optimization.
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Router
•
•
A router is used to separate or segment one network from
another network.
This will make more sense LATER!
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Devices and their layers
Transceiver
• Hosts and servers operate at Layers 2-7; they perform the
•
•
•
encapsulation process.
Routers: Layers 1 through 3, make decisions at layer 3
Switches and NICs: Layers 1 and 2, make decisions at layer 2
Hubs and transceivers: Layer 1, no decisions to make
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Ethernet and TCP/IP
Ethernet & TCP/IP are the most pervasive LAN
protocols, and are often used together.
Application Layer
Presentation Layer
Session Layer
TCP/IP
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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Ethernet
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Peer-to-peer communications
• In order for data to travel from the source to the destination, each layer
of the OSI model at the source must communicate with its peer layer at
the destination.
• This form of communication is referred to as peer-to-peer.
• During this process, the protocols of each layer exchange information,
called protocol data units (PDUs).
• Each layer of communication on the source computer communicates
with a layer-specific PDU, and with its peer layer on the destination
computer as illustrated in Figure
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TCP/IP model
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TCP/IP was developed as an open standard.
This meant that anyone was free to use TCP/IP.
This helped speed up the development of TCP/IP as a standard.
Although some of the layers in the TCP/IP model have the same name
as layers in the OSI model, the layers of the two models do not
correspond exactly.
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TCP/IP model
Some of the common protocols specified by the TCP/IP reference model layers.
Some of the most commonly used application layer protocols include the
following:
• File Transfer Protocol (FTP)
• Hypertext Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (SMTP)
• Domain Name System (DNS)
• Trivial File Transfer Protocol (TFTP)
The common transport layer
protocols include:
• Transport Control Protocol (TCP)
• User Datagram Protocol (UDP)
The primary protocol of the
Internet layer is:
• Internet Protocol (IP)
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OSI and TCP/IP
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Detailed encapsulation process
• All communications on a network originate at a source, and are sent to
•
•
a destination.
The information sent on a network is referred to as data or data
packets.
If one computer (host A) wants to send data to another computer (host
B), the data must first be packaged through a process called
encapsulation.
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Detailed encapsulation process
Networks must perform the following five conversion steps in order to
encapsulate data:
1. Build the data.
2. Package the data for end-to-end transport.
3. Add the network IP address to the header.
4. Add the data link layer header and trailer.
5. Convert to bits for transmission.
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Data Encapsulation Example
Application
Header + data
Application Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2:
Network
Layer
010010100100100100111010010001101000…
Layer 1: Physical
Layer
Let us focus on the Layer 2, Data Link, Ethernet Frame for
now.
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This will make much more sense later!
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Ch.2 – Networking Fundamentals
Getting past some basics…
CIS 81 and CST 311
Cabrillo College and CSUMB
Rick Graziani
Fall 2005