Download 2.1 Chapter 2 Network Models

Chapter 2
Network Models
2.1
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Network Models
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Network Architecture:
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A) Hardware: at the core of any network;
e.g.) Tx/Rx devices, modems, codecs,
physical links, switches
B) Software: to derive the core H/W to
communicate end users. The S/W is so
complex as its task. Hence it is divided into
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subtasks each of which is confined to a set of
related processes to perform a specific
function.
--Each subtask S/W is called a “ Layer”
--Layers are stacked over each other forming
the net model stack, e.g., ISO, TCP/IP
2-1 LAYERED TASKS
We use the concept of layers in our daily life. As an
example, let us consider two friends who communicate
through postal mail. The process of sending a letter to a
friend would be complex if there were no services
available from the post office.
Topics discussed in this section:
Sender, Receiver, and Carrier
Hierarchy
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Figure 2.1
2.5
Tasks involved in sending a letter
2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards. An ISO
standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model. It was first introduced in the late 1970s.
Topics discussed in this section:
Layered Architecture
Peer-to-Peer Processes
Encapsulation
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Layered Architecture
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OSI model is composed of seven ordered
layers.
Figure 2.3 shows the layers involved when
message is sent from device A to device B
Each layer defines a family of function
distinct from those of the other layers.
The OSI model allows complete
interoperability.
Note
ISO is the organization.
OSI is the model.
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Figure 2.2 Seven layers of the OSI model
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Figure 2.3 The interaction between layers in the OSI model
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Network Models (cont..)
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􀂄 Each Layer has a “Peer-to-Peer” protocol
that seems to represent (and carry out) the
rest of the network task, yet it does only a
specific part and delegate the rest to the layer
beneath it (via its interface). It also has an “
interface” that defines the services that is
provided to the layer above it.
Figure 2.4 An exchange using the OSI model
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Encapsulation
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“Encapsulation”: Each layer has its own PDU
that’s passes (as a parameter) to the layer
beneath, which in turn adds a “ header ”(at
layer 2 also adds trailer”) before assign to the
next layer (except the physical layer).
Why “header” and “trailer”?
Physical movement of information PDU is
“vertical” yet the user thinks (At each peer –topeer) layer that info moves” horizontal” (pipe)
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
layer in the OSI model.
Topics discussed in this section:
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
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Figure 2.5 Physical layer
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1) Physical Layer
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1) Physical Layer: PDU , bit stream.
Moves bit sequence over a physical link.
Defines the following:
a) Physical characteristics of EIA interfaces and medium.
b) Bit representation: encoding/decoding, electrical/optical.
c) Data rate: (b/s) bit TX duration.
d) Bits synch: sender and receiver clock synch and same data rate.
e) Line configuration: Point-to-point, Multipoint
f) Physical Topology: Mesh, ring, bus, and hybrid.
g) Transfer mode: Simplex, F/d, and H/d
Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
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Figure 2.6 Data link layer
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2) Data Link Layer
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2) Data Link Layer: PDU frame with
header/trailer
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Functions:
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a. Framing
b. Physical Addressing: Sender/receiver addresses in the
frame header.
c. Flow Control: To prevent fast sender from flooding a
slower receiver with frames.
d. Error Control: To Increase physical layer reliability by
adding mechanism to detect and ReTx damages and lost
frames. (Trailer)
e. Access Control: Control the access to the physical medium
among all connected devices.
Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
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Figure 2.7 Hop-to-hop delivery
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Figure 2.8 Network layer
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3) Network Layer
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Network Layer: PDU packets (headers only)
Responsible for the source to destination
delivery of packet, possibly across multiple
networks.
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Address
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logical IP address
a. Source host system to destination host system delivery,
utilizing the data link layer for peer-to-peer delivery.
b. Physical addresses at the D.L are not enough; we need to
add logical addressing in the packet header, of the sender
and receiver.
Network Layer (cont..)
c. Routing: To route packets over the
subnet cloud of routers and switches,
make the optimal routing decisions
(src- destination)
 d. Internetworking: resolve any Net
protocols conflicts while moving in the
subnet.
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Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
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Figure 2.9 Source-to-destination delivery
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Figure 2.10 Transport layer
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4) Transport Layer:
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The most important layer since it abstracts
the complete details of the subnet to the
user.
It communicates a meaningful data unit
called message (group of related packets)
between users over the subnet trying for
the most optimal utilization of the subnet.
4) Transport Layer: (cont..)
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Responsibilities:
a) Service Access Point Addressing: (SAP)
The network logical address is for src_sys
to destination not src_user_process to
destination_user_process, hence we need
another address mechanism => SAP
addresses within the same system for user
message delivery.
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Transport Layer: (cont..)
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b) Segmentation and reassembly: of
segments=> packets
A Process (user) message is divided into
segments (if needed) each with n seg.
Sequence number to aid in assembly
(incorrect order) related segments into the
original message at the
destination/transportation layer.
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Transport Layer: (cont..)
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c) Connection control:
1) Connection reliable service (-no ACK, -no
guarantee)
2) Connectionless unreliable service
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• In connection oriented TCP, guarantees delivery in order
with ACK of segments.
d) Flow Control: As in the data Link Layer but all
the message level “end-users”.
Transport Layer: (cont..)
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e) Error Control: Like the DLL, but
process_to_process delivery of messages.
Errors(damaged, loss or duplicate)cause
reTransmission of messages.
Note
The transport layer is responsible for the delivery
of a message from one process to another.
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Figure 2.11 Reliable process-to-process delivery of a message
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Figure 2.12 Session layer
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5) Session Layer:
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Session Layer: ( Nwk dialog controller)
It established, maintain and synchronizes
the interaction among communicating
system.
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a. Dialog Controls H/Duplex or F/Duplex
b. Synchronization: Checkpoints are added to
data streams for dividing into units of
independent ACK. Communication robustness
in case of crashes.
Note
The session layer is responsible for dialog
control and synchronization.
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Figure 2.13 Presentation layer
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Note
The presentation layer is responsible for translation,
compression, and encryption.
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6) Presentation Layer
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Presentation Layer:
a. Translation: ASCII,--.EBCDIC. Abstract
syntax notation (ASN).
b. Encryption: To secure information Tx
for privacy
c. Compression: For efficient utilization of
bandwidth.
Figure 2.14 Application layer
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Note
The application layer is responsible for
providing services to the user.
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7) Application Layer:
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Application Layer:
1) Virtual terminal”putty”to allow remote
logins (emulations)
2) File transfer , access, and management
3) Mail Service,
4) Directory service.
SMTP, FTP, HTTP, DNS, SNMP, TELNET.
Figure 2.15 Summary of layers
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OSI
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2-4 TCP/IP PROTOCOL SUITE
The layers in the TCP/IP protocol suite do not exactly
match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers: host-tonetwork, internet, transport, and application. However,
when TCP/IP is compared to OSI, we can say that the
TCP/IP protocol suite is made of five layers: physical,
data link, network, transport, and application.
Topics discussed in this section:
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
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Figure 2.16 TCP/IP and OSI model
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1) Physical Layer: Very Vague, it can be
LAN, MAN, WAN.
2) Network Layer:
Internet Protocol (IP) is used best-effortdelivery unreliable connectionless
datagram protocol (no end control flow).
At the network layer there are other
protocol to help the poor IP:
a) Address Resolution Protocol: physical
(MAC) logical (IP)
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Address translation.
b) Internet Control Msg Protocol (ICMP):
Help in reporting any failure/congestion of
a subnet part(s), aiding in the subnet
robustness.
c) Internet Group Msg Protocol (IGMP)
Single source broadcasts to multi group
destinations.
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3) Transport Layer: Proc-to-Proc
client/server.
A) User Datagram Protocol: UDP
--- Connectionless unreliable Transport
Protocol, with very limited error checking
(checksum)
--- No error/ flow control
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B) Transmission Control Protocol: TCP
---Reliable connection oriented (stream) transport
protocol.
---Establishes connection src_dest, before data Tx.
---Ordered/ ACK segment Tx with segment numbers.
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C) Stream Control Tx Protocol:
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SCTP Connection oriented reliable transport protocol to
supports voice IP (Internet telephony) combining the
best of UDP and TCP
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2-5 ADDRESSING
Four levels of addresses are used in an internet employing
the TCP/IP protocols: physical, logical, port, and specific.
Topics discussed in this section:
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
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Figure 2.17 Addresses in TCP/IP
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Figure 2.18 Relationship of layers and addresses in TCP/IP
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Physical Address
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Physical address (Link address) is the address of
a node as defined by its LAN or WAN.
Lowest level address
Have authority over the LAN or WAN nwks.
The size and format of these addresses vary
depending on the nwk.
Eg : Ethernet uses 6-byte physical address.
Example 2.1
In Figure 2.19 a node with physical address 10 sends a
frame to a node with physical address 87. The two nodes
are connected by a link (bus topology LAN). As the
figure shows, the computer with physical address 10 is
the sender, and the computer with physical address 87 is
the receiver.
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Figure 2.19 Physical addresses
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Example 2.2
As we will see in Chapter 13, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
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Logical Address
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Necessary for universal communications
that are independent of underlying
physical networks.
Different nwks can have different address
format.
An address defined in network layer.
Logical address in the internet is 32-bit
address
Example 2.3
Figure 2.20 shows a part of an internet with two routers
connecting three LANs. Each device (computer or
router) has a pair of addresses (logical and physical) for
each connection. In this case, each computer is
connected to only one link and therefore has only one
pair of addresses. Each router, however, is connected to
three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each
connection.
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Figure 2.20 IP addresses
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Port addresses
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A method is needed to label the different
processes .( addresses are needed)
Process communicating with another
process
eg : Comp. A can communicate with
comp.C by using TELNET. At the same
time comp .A communicate with comp. B
by using FTP
Example 2.4
Figure 2.21 shows two computers communicating via the
Internet. The sending computer is running three
processes at this time with port addresses a, b, and c. The
receiving computer is running two processes at this time
with port addresses j and k. Process a in the sending
computer needs to communicate with process j in the
receiving computer. Note that although physical
addresses change from hop to hop, logical and port
addresses remain the same from the source to
destination.
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Figure 2.21 Port addresses
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Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
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Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
address represented by one decimal number as shown.
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A 16-bit port address represented
as one single number.
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Note
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
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Exercise 1
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Figure 1, shows an internet path between two hosts
involves a hop across network A, a packet-switching
network, to a router and then another hop across
packet-switching network B. Suppose that packet
switching network A carries the packet between the
first host and the router over a two-hop path involving
one intermediate packet switch. Suppose also that
the second network is an Ethernet LAN.
Sketch the sequence of IP and non-IP packets and
frames that are generated as an IP packet goes from
host 1 to host 2.
Figure 1
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