Information Science Institute of Sripatum University
IS516
Computer Communication and Networks
ื่ สารคอมพิวเตอร์และเครือข่าย
การสอ
Asst.Dr.Surasak Mungsing
surasak.mu@spu.ac.th
mungsing@gmail.com
http://www.spu.ac.th/teacher/surasak.mu
Sripatum University
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Lecture 05:
Guided Networks
2
LAN
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Line Configuration


Computers are connected for data
communication through transmission Medium,
which may be either Guided Media or
Unguided Media
Two types of connection configuration, pointto-point and multipoint
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Point-to-Point Line Configuration



A pair of computers or network devices have specific connection
points for connecting only those pairs of computers or devices
Communication capacity dedicated to the two connected
computers or devices
Transmission media may be either guided or unguided medium
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Multipoint Line Configuration

Use common medium for two or more
computers and devices
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Topology


Connection structure of computer
network(computers or network devices)
Five types of topology: Mesh, Star, Tree,
Bus and Ring
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Mesh Topology



All nodes are directly connected (linked) to other
nodes in the network
Number of connection lines n(n-1) / 2 where n
is number of computers in the network
Each node must have (n-1) I/O ports
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Mesh Topology







Advantages:
- better transmission rate and system reliability
- easy to detect errors
- better security and privacy
Disadvantages:
- many connection point and I/O ports required
e.g. 100 nodes requires 4950 connection lines
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Star Topology
•
•
•
Each node directly connects to the center device, called “hup” ,
which a node must send data to other nodes through the hub
Saves a lot of connection lines in comparison with mesh topology
In star topology, as well as the mesh, if a line fails only the two
connected nodes fails to communicate. But if the hub fails the
whole network fails to work.
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Tree Topology




Based on star from star topology, in such a way that each
node connects to a hub, either active hub or passive hub
The center of the network structure is an active hub and
repeaters are used to extend signal distance
Passive Hub is directly connected to nodes
Longer distance network than star topology
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Bus Topology


Bus is Multipoint connection, but the previous (Mesh, Star,
and Tree) are point-to-point connection
All node are connected to a cable, which acts as a
backbone for the network, so that all nodes can
communicates to each other through this cable
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Bus Topology


Use less connection lines and connection ports than
Mesh, Star, and Tree
Cable Terminators required at the two ends and all nodes
in the network tap to the cable

Disadvantage:

- if the cable fails, then the network fails
- Adding new node is more difficult than Mesh, Star,
and Tree topologies
- low data transmission rate due to sharing of a
common cable



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Ring Topology
•
•
Is a point-to-point connection between two nodes (a node and
its predecessor node)
Communication between nodes takes place from one node to
another in one direction until the intended received is reached
while each node in the ring acts as a repeater to relay data to
the next node
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Ring Topology


Easy to install and change, e.g. adding a node only
requires two connections
Requires longer time for sending data from a node
to another due the moving is in one direction, in
particular when there are many nodes in the ring
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Hybrid Topology

Connection in a network may be a hybrid of
more than one topology
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IEEE 802 LAN



The Institute of Electrical and Electronics Engineers
or IEEE (read eye-triple-ee) is an international non-profit,
professional organization for the advancement of
technology related to electricity. It has the most members
of any technical professional organization in the world, with
more than 395,000 members in around 150 countries
The services and protocols specified in IEEE 802 map to the
lower two layers (Data Link and Physical) of the seven-layer
OSI networking reference model
IEEE 802 describes CSMA/CD (Carrier Sense Multiple
Access with Collision Detection), (Token Bus) and Token
Ring LAN, which are different in Physical and MACsublayer
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IEEE 802
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Project 802
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IEEE 802.3 Ethernet (1)


IEEE 802.3 began from 100-nodes set up at Xerox
that can send data as far as 1 km. at the rate of
2.94 Mbps, called Ethernet
Follows by cooperation of Xerox, DEC and Intel in
developing Ethernet standard that can operate at
the rate of 10 Mbps, the IEEE 802.3
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IEEE 802.3 Ethernet (2)


The 802.3 describes LANs that use the principle of
CSMA/CD (Carrier Sense Multiple Access with
Collision Detection), which operates at the data
rate of 1 ถึง 100 Mbps and on variety of
transmission media
IEEE 802.3 and Ethernet have some different
information in the Header (IEEE 802.3 field length
is used to identify type of Packet in the Ethernet
standard ) Therefore IEEE 802.3 describes
LAN that use CSMA/CD but Ethernet means a
product of IEEE 802.3 LAN
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IEEE 802.3 Ethernet (3)

วิธก
ี ารรับสง่ ข ้อมูลของแลน IEEE 802.3 ซงึ่ เป็ นแบบ
CSMA/CD ก็ทางานในลักษณะเดียวกัน คือโหนดใดที่
ื่ กลางการสง่ ข ้อมูล จะตรวจสอบ
ต ้องการสง่ ข ้อมูลลงในสอ
ั ญาณในสอ
ื่ กลาง ถ ้าหากสอ
ื่ กลางในการสง่ ข ้อมูลว่าง
ดูสญ
ก็จะทาการสง่ ข ้อมูลได ้ทันที แต่หากโนดตัง้ แต่ 2 โนดขึน
้
ื่ กลางพร ้อมๆกัน สญ
ั ญาณข ้อมูลจะ
ไปสง่ ข ้อมูลลงไปในสอ
เกิดการชนกันขึน
้ ทุกๆสถานีจะต ้องหยุดการสง่ ข ้อมูลแล ้ว
่
รอเวลา ซงึ่ ชว่ งเวลาของการรอแต่ละครัง้ จะทาการสุม
ขึน
้ มา (Random Time) หลังจากหมดเวลารอแล ้วก็จะทา
ั ญาณในสอ
ื่ กลางเพือ
การตรวจสอบสญ
่ สง่ ข ้อมูลลงไปใหม่
อีก
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Collision Detection (1)


ั ญาณข ้อมูลแล ้ว เวลาจะถูก
เมือ
่ เกิดการชนกันของสญ
่ งๆ (slots) แต่ละชอ
่ งมีชว่ งเวลา 51.2
แบ่งออกเป็ นชอ
ไมโครวินาที (นั่นคือเวลาสถานีทส
ี่ ง่ ข ้อมูลรู ้ว่าเกิดการ
ชนกันของข ้อมูลหรือไม่ สาหรับความยาวของแลน
2,500 เมตร อัตราการสง่ ข ้อมูล 10 Mbps) หลังจากการ
่ (Random)
ชนกันครัง้ แรก แต่ละสถานีจะสร ้างตัวเลขสุม
่ 2^1 ค่า)
ทีม
่ ค
ี า่ 0 หรือ 1 (เลขสุม
่ งเวลา 0 และ
สถานีทไี่ ด ้ค่า 0 จะสง่ ข ้อมูลออกไปในชอ
่ งเวลาที่ 1 หากสอง
สถานีทไี่ ด ้ค่า 1 จะสง่ ข ้อมูลในชอ
่ เดียวกันและสง่ ข ้อมูลภายในชอ
่ งเวลา
สถานีได ้ค่าเลขสุม
เดียวกัน จะเกิดการชนกันอีกครัง้
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Collision Detection (2)

หลังจากการชนก ันครงที
ั้ ่ 2 แต่ละสถานีจะสร้าง
่ ทีม
่
ต ัวเลขสุม
่ ค
ี า
่ 0,1,2, หรือ 3 (นน
่ ั คือเลขสุม
่ งเวลาของตนเอง
2^2 ค่า) แล ้วสง่ ข ้อมูลภายในชอ
่ จานวน 2^3 ค่า
หากชนกันอีกก็จะสร ้างเลขสุม
กล่าวคือหลังจากการชนกัน i ครัง้ แต่ละสถานีก็จะมี
่ ตัง้ แต่คา่ 0 ถึง 2^i-1 ค่า และสถานีก็
การสร ้างเลขสุม
่ งเวลาของตนเอง กระบวนการใน
จะสง่ ข ้อมูลภายในชอ
การแก ้ไขการชนกันของข ้อมูลแบบนีเ้ รียกว่า Binary
Exponential Back off ซงึ่ จะเห็นได ้ว่ากระบวนการ
นีท
้ าให ้โอกาสในการทีจ
่ ะเกิดการชนกันของข ้อมูลมี
น ้อยลง เมือ
่ จานวนครัง้ ของการชนกันของข ้อมูลมาก
ขึน
้
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Cable for IEEE802.3 LAN
Type
Cable
Length Number
of
of nodes
per
Segme
Segment
nt
10Base
5
Thick
Coaxial
500
meters
100
Use as
Backbone
10Base
2
Thick
Coaxial
200
meters
30
Least expensive
10Base
T
Twisted
Pair
100
meters
1,024
Easy to maintain
10Base
F
Fiber
Optic
2,000
meters
1,024
Connection
between
buildings
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Advantage
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Figure 12-9-continued
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10BASE5
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Ethernet Segments
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10BASE2
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10BASET
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Fast Ethernet



High speed demand due to multimedia applications
IEEE decided to improve 802.3 standard to 803.2u,
called Fast Ethernet
Fast Ethernet operates at the speed of 100 Mbps,
with no change in frame structure and collision
detection and control but reduce time of
transmission of each bit from 100 nanoseconds to
10 nanoseconds to obtain 10 time faster
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Gigabit Ethernet

Gigabit Ethernet operates at the speed of 1000
Mbps or 1 Gbps by improving data encoding and
data transmission technique and using Fiber Optic
media instead of Twisted Pairs
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IEEE 802.4 LAN



Token Bus is IEEE 802.4 standard, which
cannot guarantee whether a node can send data
at the time it wants
Physical topology is Bus but operation is logical
ring
Each node knows the addresses of node on its
left and node on its right
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IEEE 802.4 LAN


When ring is established, a token will be send
from one node to another in the same direction
The node that wants to send data will have to
wait for the token with available flag to have the
right to send data therefore there will be only
one sender at a time
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IEEE 802.5 LAN


Each node in IEEE 802.5 LAN or Token Ring LAN
connects to the ring (which is different from
Ethernet and Token Bus)
Signals move in one direction from sender
passing other nodes to the receiver
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Token Ring
A
C
่ ข้อมูล ถึง
สง
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WAN
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Wide Area Networks

Relation between hosts on LANs and the subnet.
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Switching Techniques


Circuit switching
Packet switching
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Circuit Switching Characteristics

Channel capacity dedicates to the sender and
receiver, even through there is no data sending

Once communication line connected, users feel like
they have direct connection with no more delays

Was developed for voice communication, but now
also used for data communication
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Circuit Switching Applications

Public Telephone Network (PSTN)

Private Branch Exchanges (PBX)

Private Wide Area Networks (often used to
interconnect PBXs in a single organization)

Data Switch
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Traditional Circuit Switching Illustration
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Packet-Switching Networks

Developed in 1970s for long distance data
communication due to the limitations of circuit
switching


During communication with circuit-switching, data
traffic on the transmission line is rather light, so
inefficiently use of data transmission line
sender and receiver must communicate at the same
data rate
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Packet Switching Operation

Data are arranged into small pieces called
packets, which can be sent to the destination in
different routes

Advantage: more efficiently use of transmission
line, signals can be transmitted on different line,
and priority can be arranged

Disadvantage: delays of data at nodes, each data
packets arrive at different time (jitter), and all
packets have extra information, e.g. addresses
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Packet
Switching
Illustration
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Integrated network access using dedicated
channels
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Integrated network access using
public switched WAN
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X.25

The first public data network developed in 1970s

Is connection-oriented network

Support switched virtual circuit and permanent virtual
circuitม similar to Leased Line

Replaced by newer network, the Frame Relay, in
1980s

How to use X.25 network
First establish connection between sender and receiver by
dial telephone number: connection number for data
communication
Each data packet consists of 3-byte header and not more
than 128 bytes of data
Header consists of 12-bit connection number, packet
sequence number, acknowledgement number and 2-3 bits
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Frame Relay

Designed to reduce overhead of X.25

Use simple protocol with no flow control and error
control between point-to-point communication

Use “permanent virtual circuit” (like virtual leased
line) between points of communication

Packet size of 1,600 bytes

Data rate of 1.5 Mbps, which is 23 times faster than
that of X.25
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Frame Relay protocol vs. X.25 protocol

Reliability




Frame relay looses capability in flow control and error control
and error control for point-to-point communication
X.25 has connection protocol for reliability point-to-point
communication
No advantage on this point due to more reliability of
transmission and switching devices
Streamlining


Frame relay reduce user interface connection protocols
requirements and processing requirements within network
Better throughput than that of X.25, at least an order of
magnitude
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Asynchronous Transfer Mode (ATM)

Technology used in B-ISDN network , called cell
relay

faster than X.25

Sending data continuously faster than frame relay
in many order of magnitude

Packets are small, called “cell”

Cell size is 53 bytes (5 byte for header and 48
bytes for data)

No link-by-link error control or flow control
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ATM
Cell
Format
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ATM Virtual Circuits
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Cell-switching vs Circuit switching

Cell-switching is more flexible and effective for
services with both data of almost constant size, e.g.
multimedia data, and variable data such as data
from database

Data transmission at rate of Gbps, using Cellswitching technique, which more efficient than
using multiplexor รื Circuit- switching, especially on
fiber cable

Cell-switching supports information broadcasting,
such as television broadcasting, while circuitswitching does not directly support
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Next Lecture:
Switching and routing
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