Network Models

1. COMPUTER COMMUNICATION
NETWORKS

2. UNIT 1
COMMUNICATION - Sharing Information
• Data communications are the exchange of data
between two devices via some form of transmission
medium such as a wire cable.
• This sharing Information can be local or remote.
• Local communication usually occurs face to face,
while remote communication takes place over
distance.
• The term telecommunication, which includes
telephony, telegraphy, and television, means
communication at a distance (tele is Greek for "far ).

3. • The effectiveness of a data communications system depends
on four fundamental characteristics:
 Delivery
 Accuracy
 Timeliness
 Jitter
• Delivery: The system must deliver data to the correct
destination.
• Accuracy: The system must deliver the data accurately.
• Timeliness: The system must deliver data in a timely manner.
• Jitter: Jitter refers to the variation in the packet arrival time.

4. A data communications system has five
components (see Figure 1.1).

5. • Message: The message is the information (data) to be
communicated. Popular forms of information include text,
numbers, pictures, audio, and video.
• Sender: The sender is the device that sends the data
message. It can be a computer, workstation, telephone
handset, video camera, and so on.
• Receiver: The receiver is the device that receives the
message. It can be a computer, workstation, telephone
handset, television, and so on.
• Transmission medium: The transmission medium is the
physical path by which a message travels from sender to
receiver. Some examples of transmission media include
twisted-pair wire, coaxial cable, fiber-optic cable, and
radio waves.
• Protocol: A protocol is a set of rules that govern data
communications. It represents an agreement between the
communicating devices.

6. Communication between two devices can be simplex, half-
duplex, or full-duplex as shown in Figure 1.2.

7. NETWORKS
• A network is a set of devices (often referred to as nodes)
connected by communication links.
• A node can be a computer, printer, or any other device
capable of sending and/or receiving data generated
by other nodes on the network
• The most important of network are
a) Performance
b) Reliability
c) security

8. Performance
• Performance can be measured in many ways, including
transit time and response time.
• Transit time is the amount of time required for a message
to travel from one device to another device
• Response time is the elapsed time between an investigation
and a response.
• Performance is often evaluated by two networking
metrics:
 Throughput
 Delay

9. Reliability
In addition to accuracy of delivery, network reliability is
measured by the frequency of failure, the time it takes a link to
recover from a failure
Security
Network security issues include protecting data from
unauthorized access, protecting data from damage and
development and implementing policies and procedures for
recovery from data losses.

10. TYPE OF CONNECTION
• A network is two or more devices connected through links.
A link is a communications pathway that transfers data from
one device to another
• There are two possible types of connections: point-to-point
and multipoint.
• Point-to-Point A point-to-point connection provides a
dedicated link between two devices.
• Multipoint A multipoint (also called multidrop) connection
is one in which more than two specific devices share a
single link

12. There are four basic topologies possible:
 Mesh
 Star
 Bus
 Ring

13. • In a mesh topology, every device has a dedicated
point-to-point link to every other device.
• The term dedicated means that the link carries traffic
only between the two devices it connects.
• To find the number of physical links in a fully connected
mesh network with n nodes
Mesh

14. Star Topology
• In a star topology, each device has a dedicated point-
to-point link only to a central controller, usually called a
hub. The devices are not directly linked to one another.

15. Bus Topology
 A bus topology, is multipoint. One long cable acts as a
backbone to link all the devices in a network (see Figure 1.7)

16. Ring Topology
 In a ring topology, each device has a dedicated point-to-point
connection with only the two devices on either side of it.

17. Hybrid Topology

18. Categories of Networks
Two primary categories:
a) Local-area Networks
b) Metropolitan Area Networks
c) Wide-area Networks.

19. Classification of interconnected by scale

20. Local Area Networks
• Local area networks, generally called LANs, are networks
within a single building or campus of up to a few kilometers in
size.
• They are widely used to connect personal computers and
workstations in company offices and factories to share resources
(e.g., printers) and exchange information.
• LANs are distinguished from other kinds of networks by three
characteristics: (1) their size, (2) their transmission
technology, and (3) their topology.

21. Local Area Networks

22. Metropolitan Area Networks

23. Wide area network
• A wide area network (WAN) is a network that covers a broad
area (i.e., any telecommunications network that links across
metropolitan, regional, or national boundaries) using private
or public network transports.

24. Internet, Intranet and
Extranets

25. What is Internet ?
• It is a Global network of computers, to exchange
information(servers or clients).
• It is a "network of networks" that includes millions of
private and public, academic, business, and
government networks (local or Global), linked by
copper wires, wireless connections, and other
technologies.
• It is collection of networks runs through a common
TCP/IP protocol

26. Applications Of Internet
• Download programs and files
• E-Mail
• Voice and Video Conferencing
• File Sharing
• Information browsing
• Search the web addresses for access through
search engine
• Chatting and many more…

27. Disadvantages of Internet
• Thefting personal information such as name,
address, credit card number etc.
• Virus threats nothing but a program which
disrupts the normal functioning of your system.
• Spamming refers to receiving unwanted e-mails
in bulk, which provide no purpose and
needlessly obstruct the entire system.

28. What is Intranet ?
• Internal company network that uses Internet
standards (HTML, HTTP & TCP/IP protocols) & software.
• Accessed only by authorized persons, especially
members or employees of the organization

29. Applications of Intranet
• Sharing of company policies/rules &
regulations
• Access employee database
• Distribution of circulars/Office Orders
• Sharing of information of common interest
• Submission of reports
• Corporate telephone directories

30. Disadvantages
Management Problem
 A company may not have person to update
their Intranet on a routine basis
 Fear of sharing information and the loss of
control Limited bandwidth for the business
 Unauthorized access

31. What is Extranet ?
• Extranet is an Intranet for outside authorized
users using same internet technology.
• Enable outsiders to work together with
company‟s employees.
• open to selected suppliers, customers & other
business partners

32. Examples..
• Dealers/distributors have access to product
files such as :-
product specification,
pictures,
images, etc.

33. Benefits of Extranet
• Improved Quality.
• Lower Travel Costs.
• Lower Administrative & Other Overhead Costs.
• Reduction In Paperwork.
• Delivery Of Accurate Information On Time.
• Improved Customer Service.
• Better Communication.
• Overall Improvement In Business effectiveness.

34. Disadvantages
• Faceless contact.
• Information can be misused by other
competitors.
• Technical Employees are required.

35. Clients, partners, customers
Layered System View
Intranet
Extranet
Internet
Corporate members
Global society

36. PROTOCOLS AND STANDARDS
• The two widely used terms: protocols and standards. First,
protocols is nothing but set of rule. Then standards, which are
agreed-upon rules.
• Protocols: A protocol is a set of rules that govern for the
data communications.
• A protocol defines what to communicated, how to
communicated, and when to communicated.
• The key elements of a protocol are
 Syntax
 Semantics
 Timing

37. • Syntax. The term syntax refers to the structure or format of
the data.
For example, a simple protocol might expect the first 8 bits
of data to be the address of the sender, the second 8
bits to be the address of the receiver, and the rest of the
stream to be the message itself.
• Semantics: The semantics refers to the meaning of each
section of bits.
• Timing. The term timing refers to two characteristics: when
data should be sent and how fast they can be sent.

38. Standards
• Standards are essential in creating and maintaining an open
competitive market for manufacturers and in national and
international interoperability of data and
telecommunications technology and processes.
• Standards provide guidelines manufacturers, vendors,
government agencies, and other service providers to ensure
to interconnectivity with today's marketplace and in
international co
• Data communication standards fall into two categories: de
facto (meaning "by fact") and de jure (meaning "by law").

39. • by fact: Standards that have not been approved by
an organized body but standards are often
established by manufacturers
• by law: Those standards that have been established
by an officially recognized body

40. Standards Organizations
• International Organization for Standardization (ISO).
• International Telecommunication Union
Telecommunication Standards Sector (ITU-T).
• Institute of Electrical and Electronics Engineers (IEEE).
• American National Standards Institute (ANSI).
• Electronic Industries Association (EIA).
• Federal Communications Commission (FCC)

41. 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.

42. Sender, Receiver and Carrier

43. Hierarchy
 Higher Layer
 Middle Layer
 Lower Layer
 Services
 The Each layer uses the services of the layer immediately
below it.

44. 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.
ISO is the organization.
OSI is the model.

45. The OSI model is composed of seven layers
Physical (layer1), Data link (layer2), Network (layer3)
Transport (layer4), Session (layer5), Presentation (layer6)
Application (layer7)
Layer
 Designer identified which networking functions had related
uses and collected those functions into discrete groups that
became the layers.
 The OSI model allows complete interoperability between
otherwise incompatible systems.

46. Seven layers of the OSI model
Layered Architecture

47. Peer-to-Peer Processes
Layer x on one machine communicates with layer x on
another machine - called Peer-to-Peer Processes.
Interfaces between Layers
Each interface defines what information and services a layer
must provide for the layer above it.
Organizations of the layers
 Network support layers : Layers 1, 2, 3
 User support layer : Layer 5, 6, 7
 Transport layer (Layer 4)

48. Peer-to-peer Processes (cont’d)
Figure 2.3 The interaction between layers in the OSI model

49. PHYSICAL LAYER
 Physical layer coordinates the functions required to
transmit a bit stream over a physical medium.
 The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

50. PHYSICAL LAYER
 Physical layer is concerned with the following: (deal with
the mechanical and electrical specification of the
primary connections: cable, connector)
 Physical characteristics of interfaces between the
device and medium
 Representation of bits
 Data rate : Transmission rate
 Synchronization of bits
 Line configuration
 Physical topology
 Transmission mode

51. DATA LINK LAYER
 The data link layer is responsible for moving
frames from one hop to the next.

52.  Major duties
 Framing
 Physical addressing
 Flow control
 Error control
 Access control
DATA LINK LAYER

53. Hop-to-hop (node-to-node) delivery
DATA LINK LAYER

54. Network Layer
• The network layer is responsible for the delivery of
individual packets from the source host to the
destination host.

55. Network Layer
• Routing
• Logical addressing

56. Transport Layer
 The transport layer is responsible for the delivery of a
message from one process to another.

57. Transport Layer
• Service port addressing
• Segmentation and reassembly
• Connection control
• Flow control
• Error control

58. Session Layer
• The session layer is responsible for dialog control
and synchronization.

59. Presentation Layer
• The presentation layer is responsible for translation,
compression, and encryption

60. Application Layer
• The application layer is responsible for providing
services to the user.

61. Application Layer
 The major duties of the application
 Network virtual terminal
 File transfer, access, and management
 Mail services
 Directory services

62. Summary of Layers

63.  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-to-network, 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.
TCP/IP PROTOCOL SUITE

64. TCP/IP and OSI model

65. Physical and Data Link Layers
• At the physical and data link layers, TCP/IP does
not define any specific protocol.
• It supports all the standard and registered
protocols.
• A network in a TCP/IP internetwork can be a
local-area network or a wide-area network.

66. Network Layer
• TCP/IP supports the Internetworking Protocol.
• IP uses four supporting protocols: ARP, RARP, ICMP,
and IGMP.
 IP (Internetworking Protocol)
 ARP (Address Resolution Protocol)
 RARP (Reverse Address Resolution Protocol)
 ICMP (Internet Control Message Protocol)
 IGMP (Internet Group Message Protocol)

67. Transport Layer
• The transport layer was represented in TCP/IP by
two protocols : TCP and UDP.
IP is a host-to-host protocol
TCP and UDP are transport level protocols
responsible for delivery of a message from a
process to another process.
• UDP (User Datagram Protocol)
• TCP (Transmission Control Protocol)
• SCTP (Stream Control Transmission Protocol)

68. Application Layer
• The application layer in TCP/IP is equivalent to the
combined session, presentation, and application
layers in the OSI model.
• Many protocols are defined at this layer.

69. ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical, logical, port,
and specific.

70. Addresses in TCP/IP

71. Relationship of layers and addresses in TCP/IP
Physical Addresses

72. Physical Addresses
• The physical address, also known as the link
address, is the address of a node as defined by its
LAN or WAN.
• It is included in the frame used by the data link
layer.
The physical addresses have authority over the
network (LAN or WAN).
The size and format of these addresses vary
depending on the network.

73. 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.
Physical Addresses (cont’d)

74. Figure 2.19 Physical addresses
Physical Addresses

75. 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:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
Physical Addresses (cont’d)

76. Logical Addresses
• Logical addresses are necessary for universal
communications that are independent of
underlying physical networks.
 Physical addresses are not acceptable in an
internetwork environment where different
networks can have different address formats.
 A universal addressing system is needed in
which host can be identified uniquely, regardless
of the underlying physical network.

77. 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.
Example 2.3
Logical Addresses

78. The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Logical Addresses (cont’d)

79. Port Addresses
• The IP and the physical address are necessary for
a quantity of data to travel from a source to the
destination host.
• The end object of Internet communication is a
process communicating with another process.
• For these processes to receive data simultaneously,
we need a method to label assigned to a process
is called a port address.
• A port address in TCP/IP is 16 bits in length.

80. 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.
Example 2.4
Port Addresses (cont’d)

81. Figure 2.21 Port addresses
The physical addresses will change from hop to hop,
but the logical and port addresses usually remain the same.
Port Addresses (cont’d)

82. Example 2.5
a port address is a 16-bit address represented by one
decimal number as shown.
753
A 16-bit port address represented
as one single number.
Port Addresses (cont’d)

83. Specific Addresses
• Some applications have user-friendly addresses
that are designed for that specific address.
 E-mail address
 URL (Universal Resource Locator)

84. Broadband Integrated Services Digital
Networks(B-ISDN)
 In the mid-1980s, the ITU-T initiated a standardization effort
to merge voice, video and data on a single network
 The goal was to replace all existing networks (telephony
networks, Cable TV network, data networks) with a single
network infrastructure. The effort was called B-ISDN
(Broadband Integrated Services Digital Networks)
 The technology selected for B-ISDN was Asynchronous
Transfer Mode (ATM) and SONET/SDH (Synchronous
Optical Network/Synchronous Digital Hierarchy)

85. Traditional Network Infrastructure
Company
A
Company
B
Telephone network
Data network
Residential
user
x
Video network

86. B-ISDN
Company
A
Company
B
Residential
user
x
Broadband
Integrated Services
Network
(B-ISDN)

87. ATM(Asynchronous transfer mode)
ATM is a transfer mode in which the information is
organized into cells;
It is asynchronous in the sense that the duplication
of cells containing information from a particular
user is not necessarily periodic.

88. Synchronous transfer mode
 Synchronous transfer mode (= Time division multiplexing)
 Each source gets periodic assignment of bandwidth
 Adv. Fixed Delays, No Overhead
 Dis adv. Poor Utilization for Sources

89. Asynchronous transfer mode
 Asynchronous transfer mode (= Statistical multiplexing)
Sources packetize data. Packets are sent only if there is data
 Adv. : no bandwidth use when source is idle
 Dis adv. packet headers, buffering, multiplexing delay

90. ATM’s Key Concepts
• ATM uses Virtual-Circuit Packet Switching
o ATM can reserve capacity for a virtual circuit. This
is useful for voice and video, which require a
minimum level of service
o Overhead for setting up a connection is expensive
if data transmission is short (e.g., web browsing)
• ATM packets are small and have a fixed sized
o Packets in ATM are called cells
o Small packets are good for voice and video
transmissions
Header
(5 byte)
Data (48 byte)
Cell is 53 byte long

91. The ATM Reference Model
• ATM technology has its own protocol architecture
Physical Layer
ATM Layer
ATM Adaptation Layer (AAL)
Upper Layer Upper Layer
Control Plane User Plane
Transmission of
Bits
Transfer of Cells
End-to-end layer

92. The ATM Reference Model

93. Functions of ATM

94. Layers of ATM
AAL
ATM Layer
Physical
Layer
Physical
Layer
Physical
Layer
ATM Layer
AAL
ATM Layer
Physical
Layer
AAL Protocol
l
Upper
Layers
Upper
Layers
Upper Layer Protocol
Host A
ATM
Switch
Host B

95. ATM Adaptation Layer (AAL)
• AAL encapsulates user-level data
• Performs segmentation and reassembly of user-level
messages
Data
AAL
Data
AAL
Cells Cells
ATM Network
segmentation reassembly

96. AAL TYPES

97. AAL 1

98. AAL 2
variable bit rate data from upper layers
…….0000111011001100…..110101010000111……..

99. AAL3/4

100. AAL 5

101. AAL SERVICE CLASSES & TYPES

102. ATM Layer

103. Architecture of an ATM network

104. ATM Interfaces
UNI – USER NETWORK INTERFACE
NNI- NETWORK NETWORK INTERFACE

105. ATM Cells
• 4-bit Generic flow control
• 8/12 bit Virtual Path Identifier
• 16 bit Virtual Channel Identifier
• 3 bit Payload Type
• 1 bit Cell Loss Priority
• 8 bit Header Error Control
• 48 byte payload
• GFC field only in UNI cells
VCI
8 bits
GFC VPI
VPI VCI
VCI PT
C
L
P
HEC
1
2
3
4
5
Payload6- 53
UNI Cell

106. ATM Cells
• 4-bit Generic flow control
• 8/12 bit Virtual Path Identifier
• 16 bit Virtual Channel Identifier
• 3 bit Payload Type
• 1 bit Cell Loss Priority
• 8 bit Header Error Control
• 48 byte payload
• At NNI: GFC byte is used for
additional VPI
VCI
8 bits
VPI VCI
VCI PT
C
L
P
HEC
1
2
3
4
5
Payload6- 53
VPI
NNI Cell

107. Virtual Channels
• The virtual channel (VC) is the fundamental unit of transport in
a B-ISDN. Each ATM cell contains an explicit label in its header
to identify the virtual channel.
o a Virtual Channel Identifier (VCI)
o a Virtual Path Identifier (VPI)
• A virtual channel (VC) is a communication channel that
provides for the transport of ATM cells between two or more
endpoints for information transfer.
• A Virtual Channel Identifier (VCI) identifies a particular VC
within a particular VP over a UNI or NNI.

108. TP, VP and VC
Transmission path(TP)
Virtual Path (VP)
Virtual Channel (VC)

110. Routing with a switch

111. ATM layers in endpoint
devices and switches

112. ATM Services
The ATM Layer can provide a variety of services for
cells from an ATM virtual connection:
• Constant Bit Rate (CBR)
o guarantees a fixed capacity, similar to circuit switching
o guarantees a maximum delay for cells
• Variable Bit Rate (VBR)
o guarantees an average throughput and maximum delay
• Available Bit Rate (ABR)
o guarantees „fairness” with respect to other traffic
• Unspecified Bit Rate (UBR)
o service is on a “best effort” basis

113. Constant Bit Rate (CBR)
• For applications with constant rate requirements:
video and audio
• Very sensitive to delay
and delay variations
• Adaptation Layer: AAL1
time
rate
peak rate

114. Variable Bit Rate (rt-VBR, nrt-VBR)
• For applications with variable rate requirements:
compressed audio and video (Real Time-VBR), data
applications (Non Real Time-VBR).
• Adaptation Layer: AAL2, AAL 3 /4, AAL5
0
2000
4000
6000
8000
10000
12000
14000
16000
0 100 200 300 400 500 600 700 800 900 1000
Frame number
Traffic

115. Available Bit Rate (ABR)
• For applications that can tolerate changes to rate
eg. Interconnection of LANs
• Transmission rate (ACR) changes between
Min.CR and Peak CR
• Adaptation Layer: AAL 5
MCR
PCR
time
ACR

116. Unspecified Bit Rate (UBR)
• “Best effort service”
o No bandwidth, loss, or delay guarantees
o UBR gets the bandwidth that is not used by CBR, VBR, ABR
• Applications: Non-critical data applications (file
transfer, web access, etc.)
• Adaptation Layer: AAL5

117. SONET was developed by ANSI;
SDH was developed by ITU-T.
Note
SONET/SDH

118. SONET/SDH rates

119. Let us first introduce the architecture of a SONET
system
 Signals
 SONET Devices
 Connections
SONET ARCHITECTURE

120. SONET System

121. SONET LAYERS
The SONET standard includes four functional layers:
the photonic, the section, the line, and the path layer.
They correspond to both the physical and the data link
layers.
SONET defines four layers:
Path Layer
Line Layer
Section Layer
Photonic Layer

122. SONET Layers

123. Device-Layer Relationship

124. Data Encapsulation

125. SONET FRAMES
Each synchronous transfer signal STS-n is composed of
8000 frames. Each frame is a two-dimensional matrix of
bytes with 9 rows by 90 × n columns.
 A SONET STS-n signal is transmitted at 8000 frames per second.
 Each byte in a SONET frame can carry a digitized voice channel.

126. An STS-1 and an STS-n frame

127. STS-1 Frames in Transmission

128. STS-1 Overhead

129. Section Overhead

130. Line Overhead

131. Section overhead is recalculated for each SONET
device(regenerators and multiplexers).
Note

132. Path Overhead

133. Path overhead is only calculated for end-to-end (at STS
multiplexers).
Note

134. Virtual Tributaries

135. VT Types

136. STS-n

137. STS Multiplexing

138. STS-3 Envelope

139. 17.139
Overhead bytes

140. STS multiplexing/demultiplexing

141. SONET NETWORKS
Using SONET equipment, we can create a SONET
network that can be used as a high-speed backbone
carrying loads from other networks.
SONET networks into three categories: linear, ring,
and mesh networks.

142. Taxonomy of SONET networks

143. 17.143
A point-to-point SONET network

144. A multipoint SONET network

145. • DQDB (Distributed Queues, Dual Bus)- IEEE 802.6
• DQDB consists of two unidirectional Dual Bus using
queuing mechanism
DQDB (Distributed Queues Dual Bus)

146. DQDB Buses and Nodes

147. DQDB Data Transmission

148. Queues

149. Distributed Queues

150. Reservation Token

151. DQDB Rings

152. DQDB Rings

153. DQDB Layers

154. Connecting LANs using T-Lines

155. FDDI
(Fiber Distributed Data Interface)
Standardized by ASI and ITU-T (ITU-T X.3

156. FDDI (Fiber Distributed Data Interface)
Data rate – 100 Mbps
Access method – token passing
CDDI – copper version
S-frames – synchronous (real time data)
A-frame – asynchronous (not real time)

157. Access Method
• Access is limited by time
• Priority – real time data
• Steps
o A station captures the token
o Send S-frames first
o Any remaining time may then be
used to send A-frames
• TIME REGISTERS
o Synchronous allocation (SA)
o Target token rotation time (TTRT)
o Absolute maximum time (AMT)

158. Time Registers
Synchronous Allocation (SA)
o Indicates length of time allowed for different for each
station
Target Token Rotation Time (TTRT)
o Indicates Average time required for a token to
circulate around the ring exactly once
Absolute Maximum Time (AMT)
– Twice the TTRT
– To avoid monopolizing the network

159. Timers
Token Rotation Timer (TRT)
o Runs continuously
o Measures the actual time taken by the token to complete a cycle
o Incrementing or (decrementing) TRT
Token Holding Timer (THT)
o Begins running as soon as the token is received
o Shows how much time remains for sending asynchronous frames
o Decrementing or (incrementing) THT

160. Station Procedure
1. Set the values of timers
a. THT = TTRT – TRT
b. TRT = 0
2. Sends synchronous data
3. Sends asynchronous data as long as the value of THT
is positive

161. FDDI Layers

162. FDDI Frames
SD – J and K control codes
FC – Identify the frame type
FS – same to that of token ring

163. Physical Medium Dependent Layer
Dual Ring
– secondary ring make s FDDI self-healing

164. FDDI Ring Failure

165. FDDI Nodes
MIC – Media Interface Connector
Three Types of Nodes
SAS – Single Attachment Station
DAS – Dual Attachment Station
DAC – Dual Attachment Concentrator