Common Network Architecture:X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

Common Network Architecture
Chapter No. 2

Connection oriented Vs Connection less network
Connection-Oriented means that when devices
communicate, they perform handshaking to set up
an end-to-end connection.
In connectionless design every packet is addressed
and routed independently.

Connection oriented Protocols
Characteristics:
1.Handshaking (Setting up connection) between
communicating devices. Connections sometimes are
called as sessions, virtual circuits or logical
connections.
2.Acknowledgement procedure. This provides a high
level of network reliability.
3.It provides means of error control. Whenever
receiving station found that received data packet
consist of errors it request sender to retransmit that
packet.
4.It is a uni-cast (point-to-point) operation.

•Connection-oriented
•Setup data transfer ahead of time (through
handshaking)
•Internet’s connection-oriented service is TCP
(Transmission Control Protocol). It provides :
•reliable, in-order byte delivery
•flow control
•congestion control.
•Applications using TCP: Email (SMTP), web
browsing (HTTP), and file transfer (FTP)

Connectionless (Stateless) Protocols
Characteristics:
• It sends data with a source and destination
address without a handshake.
• Do not use any acknowledgment procedure.
• Usually do not support error control.
• Connectionless protocols are more efficient
than that of connection oriented protocols.
• It allows multicast and broadcast operations.

•Connectionless
Internet’s connectionless service is UDP (User
Datagram Protocol) . It provides
unreliable data transfer
no flow control
no congestion control
Applications using UDP: streaming media, video
conferencing, and IP telephony

A comparison of Connection-Oriented and Connectionless Network

Peer-to-Peer Network
• A peer-to-peer network is a distributed network
architecture composed of participants that make a
portion of their resources, such as processing power,
disk storage or network bandwidth directly to network
participants without the need for central coordination
instances.
• Used largely for sharing of content files such as audio,
video, data or anything in a digital format.
• Can be very large

• End-systems (or peers), are capable of behaving
as clients and servers of data, hence system is
scalable and reliable
• Peers participation is voluntary, membership is
dynamic, hence topology keeps changing
• Most popularly used for file sharing, hence peer-
to-peer systems have become synonymous with
peer-to-peer file sharing networks
Peer-to-Peer Network

A Peer
• Peers are both suppliers and consumers
while in the traditional client-server model,
the server supplies while the client only
consumes.

Advantages
• The more nodes that are part of the system,
demand increases and total capacity of the
system also increases. Where in client-server
network architectures as more clients are
added to the system, the system resources
decreases.
• There is no single point of failure, due to
robustness of the system.
• All clients provide to the system

Disadvantages
• Security is a major concern, not all shared files
are from beginning sources. Attackers may add
malware to p2p files as an attempt to take
control of other nodes in the network.
• Heavy bandwidth usage
• ISP speeding/slowing of P2P traffic.
• Potential legal/moral concerns

CO Vs. CL
• In connection oriented service authentication is
needed while connectionless service does not need
any authentication.
• Connection oriented protocol makes a connection
and checks whether message is received or not and
sends again if an error occurs connectionless
service protocol does not guarantees a delivery.
• Connection oriented service is more reliable than
connectionless service.
• Connection oriented service interface is stream
based and connectionless is message based.

Service Primitives
• A service is specified by a set of primitives. A
primitive means operation. To access the
service a user process can access these
primitives.
• These primitives are different for connection
oriented service and connectionless service.

• LISTEN : When a server is ready to accept an
incoming connection it executes the LISTEN primitive.
It blocks waiting for an incoming connection.
• CONNECT : It connects the server by establishing a
connection. Response is awaited.
• RECIEVE: Then the RECIEVE call blocks the server.
• SEND : Then the client executes SEND primitive to
transmit its request followed by the execution of
RECIEVE to get the reply. Send the message.
• DISCONNECT : This primitive is used for
terminating the connection. After this primitive one
can’t send any message. When the client sends
DISCONNECT packet then the server also sends the
DISCONNECT packet to acknowledge the client.
When the server package is received by client then the
process is terminated.
Connection Oriented Service Primitives

Primitives for Connectionless Oriented Service
• UNIDATA - This primitive sends a packet of data
• FACILITY, REPORT - Primitive for enquiring
about the performance of the network, like
delivery statistics.

CO Vs. CL
• In connection oriented service authentication is
needed while connectionless service does not
need any authentication.
• Connection oriented service is more reliable
than connectionless service.
• Connection oriented service interface is stream
(01 format) based and connectionless is
message based.

X.25 Networks
• X.25 is a standard used by many older public
networks specially outside the U.S.
• The packet switching networks use X.25 protocol
.
• X.25 was developed for computer connections,
used for terminal/timesharing connection.

X.25 Networks
• A protocol X.21 which is a physical layer
protocol is used to specify the physical electrical
and procedural interface between the host and
network.

• It allows the user to establish virtual circuits and
send packets on them. These packets are
delivered to the destination reliably and in order.
• X.25 is a connection oriented service.
• It supports switched virtual circuits as well as
the permanent circuits.

• Packets can then be sent over this connection from
sender to receiver.
• X.25 provides the flow control, to avoid a fast
sender overriding a slow or busy receiver.

• A permanent virtual circuit is analogous to-a
leased line. It is set up in advance with a
mutual agreement between the users.
• Since it is always present, no call set up is
required for its use.

• X.25 Most widely used today
• X.25 is an interface between DTE ( Data Terminal
Equipment) and DCE (Data Communication
Equipment) for terminal operation at the packet mode
on public data network.
– A packet switching protocol used in WAN.

• DTE is typically either a dumb terminal or the
serial port on a computer/workstation.
• DCE is typically a modem, or other piece of
data communications equipment, hence the
names.

Three Layers of X.25
The X.25 interface is defined at three levels:
The three levels are:
(i) Physical layer (level 1)
(ii) Data link layer (level 2)
(iii) Packet layer (level 3).

• The three layers of X.25 interface are as
shown in Fig..
• At the physical level X.21 physical interface is
being used which is defined for circuit
switched data network.
• At the data link level, X.25 specifies the link
access procedure-B (LAP-B) protocol.

Common Network Architecture:X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

• At the network level (3rd
level), X.25 defines a
protocol for an access to packet data subnetwork.
• This protocol defines the format, content and
procedures for exchange of control and data
transfer packets. The packet layer provides an
external virtual circuit service.
• Next Fig. shows the relationship between the
levels of X.25. User data is passed down to X.25
level 3.
• ---------------------

• The entire X.25 packet formed at the packet level
is then passed down to the second layer i.e. the
data link layer.
• Information packets and Control packets formed
at packet level and passed to LAP-B frame.
• This frame is then passed to the physical layer
for transmission.

Packet Layer Protocol
• PLP packets : Information packets and Control packets
• Information Packets (I-packets)
– transmit user data
– consists of a header and a user data field
– The last bit in the header = 0 for I-packets

• I-packet fields
– General format identifier (GFI) field: 4 bits
• Q bit : not defined in the standard. Users define
two types of data.
• D bit: for packet sequencing.
– Virtual circuit identification (VCI) fields
• Logical channel group number (LCGN) : 4 bits
• Logical channel number (LCN) : 8 bits
• total 12 bits to identify the virtual circuit for a
transmission

• I-packet fields
• P(S) and P(R) : packet sequence numbers for
flow and error control
• P(S) for packet send, P(R) for packet receive

• Control Packets (C-packets)
~ flow and error control
~ connection, termination and management control
– Category I (for flow and error control)
• RR: Receive Ready - the station is ready to
receive more packets
• RNR: Receive Not Ready
• REJ: Reject - an error in the packet (go-back-n
ARQ)

• Control Packet (C-packet)
– Category II
• for connection, termination and management control
• control packet types

Control Packets
• Category II
– Call Request/Incoming Call
• request the establishment of a connection between
two DTEs
– Call Accepted/Call Connected
• indicate the acceptance of the requested connection
by the called system

Control Packets
• Category II (cont’d)
– Clear Request/Clear Indication
• to disconnect the connection at the end of an exchange
– Clear Confirm
• sent in response to the clear indication
– and More...

Virtual Circuit Service
• With the X.25 packet layer, data are
transmitted in packets over external virtual
circuits, The virtual circuit service of X.25
provides for two types of virtual circuits.
• The virtual circuit service of X.25 provides for
two types of virtual circuits i.e. "virtual call"
and "permanent virtual circuit".

Virtual Circuit Service
• A virtual call is a dynamically established
virtual circuit using a call set up and call
clearing procedure.
• A permanent virtual circuit is a fixed, network
assigned virtual circuit. Data transfer takes
place as with virtual calls, but no call set up or
clearing is required.

Characteristics of X.25
• Multiple logical channels can be set on a single
physical line.
• Terminals of different communication speeds can
communicate.

Wired LANs: Ethernet
.
•The LAN market has several technologies such as
Ethernet, Token Ring, Token Bus and ATM LAN.
• Ethernet is dominant technology.

IEEE STANDARDS
•Computer Society of the IEEE started a project,
called Project 802, to set standards to enable
intercommunication among equipment from a variety
of manufacturers.
•Project 802 is a way of specifying functions of the
physical layer and the data link layer of major LAN
protocols.

•The relationship of the 802 Standard to the
traditional OSI(open source interconnection) model
is shown in Figure .
•The IEEE has subdivided the data link layer into
two sub layers:
 logical link control (LLC)
 Media Access Control (MAC).
•IEEE has also created several physical layer
standards for different LAN protocols.

Figure : IEEE standards for LANs

Data Link Layer:
•Data link control handles framing, flow control, and
error control.
•In IEEE Project 802, flow control, error control, and
part of the framing duties are collected into one sub
layer called the logical link control.
•Framing is handled in both the LLC sub layer and
the MAC sub layer.

Data Link Layer:
•The LLC provides one single data link control
protocol for all IEEE LANs.
•The purpose of the LLC is to provide flow and error
control for the upper-layer protocols(like Network
layers etc.) that actually demand these services.

Data Link Layer:
•IEEE Project 802 has created a sub layer called
Media Access Control (MAC) that defines the
specific access method for each LAN.
•For example, it defines the token passing method
for both Token Ring and Token Bus LANs.

STANDARD ETHERNET
• It has gone through four generations: Standard
Ethernet (10+ Mbps), Fast Ethernet (100
Mbps), Gigabit Ethernet (l Gbps), and Ten-
Gigabit Ethernet (10 Gbps), as shown in
Figure.

Media Access Control (MAC) Sublayer
• Frame Format : The Ethernet frame contains
seven fields: preamble, SFD, DA, SA, length or
type of protocol data unit (PDU), upper-layer
data, and the CRC.
• Ethernet does not provide any mechanism for
acknowledging received frames, making it what
is known as an unreliable medium.

802.3 MAC frame
• Preamble: The first field of the 802.3 frame
contains 7 bytes (56 bits) of alternating 0s and 1s
that alerts the receiving system to the coming frame
and enables it to synchronize its input timing.
• The pattern provides only an alert and a timing
pulse.

802.3 MAC FRAME
START FRAME DELIMITER (SFD).
• The second field (1 byte: 10101011) signals the
beginning of the frame.

DESTINATION ADDRESS (DA)
•The DA field is 6 bytes and contains the physical
address of the destination station or stations to receive
the packet.
SOURCE ADDRESS (SA)
•The SA field is also 6 bytes and contains the physical
address of the sender of the packet.

LENGTH OR TYPE.
•This field is defined as a type field or length field.
DATA.
•This field carries data encapsulated from the upper-
layer protocols. It is a minimum of 46 and a
maximum of 1500 bytes.
CRC.
•The last field contains error detection information,
in this case a CRC-32

Frame Length
• Ethernet has imposed restrictions on both the
minimum and maximum lengths of a frame, as shown
in Figure.

ADDRESSING
• Each station on an Ethernet network (such as a PC,
workstation, or printer) has its own network interface card
(NIC).
• The NIC fits inside the station and provides the station
with a 6-byte physical address. As shown in Figure, the
Ethernet address is 6 bytes (48 bits), normally written in
hexadecimal notation, with a colon between the bytes.
Example of an Ethernet address in hexadecimal notation

Unicast, Multicast, and Broadcast Addresses
• A source address is always a unicast address-the
frame comes from only one station.
• The destination address, however, can be unicast,
multicast, or broadcast.
• Figure shows how to distinguish a unicast address
from a multicast address.

Unicast, Multicast, and Broadcast Addresses
• The least significant bit of the first byte defines the
type of address.
• If the bit is 0, the address is unicast; otherwise, it is
multicast
Unicast and Multicast addresses

• A unicast destination address defines only one
recipient; the relationship between the sender
and the receiver is one-to-one.
• A multicast destination address defines a
group of addresses; the relationship between
the sender and the receivers is one-to-many.

• The broadcast address is a special case of the
multicast address; the recipients are all the
stations on the LAN. A broadcast destination
address is forty-eight 1s.
• The broadcast destination address is a special
case of the multicast address in which all bits
are 1s.

Examples
5=23
+22
+21
+20
=4+1= 5= 0101 A-10, B-11, C-12,D-13,E-14,F-15

Categories of Standard Ethernet

1OBase5: Thick Ethernet
• The first implementation is called 10Base5, thick
Ethernet, or Thicknet.
• 1OBase5 was the first Ethernet specification to
use a bus topology with an external transceiver
(transmitter/receiver) connected via a tap to a
thick coaxial cable.
• Figure shows a schematic diagram of a 1OBase5
implementation.

1OBase5: Thick Ethernet
1OBase5 implementation

• The transceiver is responsible for transmitting,
receiving, and detecting collisions.
• The transceiver is connected to the station via a
transceiver cable that provides separate paths for
sending and receiving. This means that collision
can only happen in the coaxial cable.
• The maximum length of the coaxial cable must
not exceed 500 m, otherwise, there is excessive
degradation of the signal.

10Base2: Thin Ethernet
• The second implementation is called 1OBase2,
thin Ethernet, or Cheapernet.
• 1OBase2 also uses a bus topology, but the cable
is much thinner and more flexible. The cable can
be bent to pass very close to the stations.
• In this case, the transceiver is normally part of the
network interface card (NIC), which is installed
inside the station. Figure shows the schematic
diagram of a 1OBase2 implementation.

• Collision here occurs in the thin coaxial cable.
This implementation is more cost effective
than 10Base5 because thin coaxial cable is less
expensive than thick coaxial and the tee
connections are much cheaper than taps.
• Installation is simpler because the thin coaxial
cable is very flexible. However, the length of
each segment cannot exceed 185 m .

1OBase-T: Twisted-Pair Ethernet
• The third implementation is called 1OBase-T
or twisted-pair Ethernet. 1OBase-T uses a
physical star topology. The stations are
connected to a hub via two pairs of twisted
cable, as shown in Figure.

• Note that two pairs of twisted cable create two
paths (one for sending and one for receiving)
between the station and the hub.
• Any collision here happens in the hub.
• Compared to 1OBase5 or 1OBase2, we can
see that the hub actually replaces the coaxial
cable as far as a collision is concerned.
• The maximum length of the twisted cable here
is 100 m.

1OBase-F: Fiber Ethernet
• Although there are several types of optical fiber
1O-Mbps Ethernet, the most common is called
10Base-F. 1OBase-F uses a star topology to
connect stations to a hub. The stations are
connected to the hub using two fiber-optic cables,
as shown in Figure.

Summary of Standard Ethernet implementations

CHANGES IN THE STANDARD
• The 10-Mbps Standard Ethernet has gone through
several changes before moving to the higher data
rates.
• These changes actually opened the road to the
evolution of the Ethernet to become compatible with
other high-data-rate LANs.

BRIDGED ETHERNET
• Bridges have two effects on an Ethernet LAN:
– They raise the bandwidth
– they separate collision domains.
• In an unbridged Ethernet network, the total capacity (10
Mbps) is shared among all stations with a frame to send;
the stations share the bandwidth of the network.
• If only one station has frames to send, it benefits from
the total capacity (10 Mbps). But if more than one
station needs to use the network, the capacity is shared.
• A bridge divides the network into two or more networks.
Bandwidth-wise, each network is independent.

A network with and without a bridge
•Another advantage of a bridge is the separation of
the collision domain.
• In Bridge the collision domain becomes much
smaller and the probability of collision is reduced .

A network with and without a bridge

Switched Ethernet
• If we can have a multiple-port bridge, why not
have an N-port switch? In this way, the
bandwidth is shared only between the station
and the switch (5 Mbps each). In addition, the
collision domain is divided into N domains.
• A layer 2 ( Data Link Layer) switch is an N-
port bridge with additional sophistication that
allows faster handling of the packets.
• Combination of a bridged Ethernet to a
switched Ethernet gives faster Ethernet. Figure
shows a switched LAN.

Full-Duplex Ethernet
• One of the limitations of 10Base5 and
10Base2 is that communication is half-duplex
(10Base-T is always full-duplex); a station can
either send or receive, but may not do both at
the same time.
• The full-duplex mode increases the capacity of
each domain from 10 to 20 Mbps.
• Instead of using one link between the station
and the switch, the configuration uses two
links: one to transmit and one to receive.

FAST ETHERNET
• Fast Ethernet is backward-compatible with
Standard Ethernet, but it can transmit data 10
times faster at a rate of 100 Mbps. The goals
of Fast Ethernet can be summarized as
follows:
– Upgrade the data rate to 100 Mbps.
– Make it compatible with Standard Ethernet.
– Keep the same 48-bit address.
– Keep the same frame format.

AUTONEGOTIATION
• A new feature added to Fast Ethernet is called
autonegotiation. It allows a station or a hub a
range of capabilities.
• Autonegotiation allows two devices to negotiate
the mode or data rate of operation.

AUTONEGOTIATION
• It is designed particularly for the following
purposes:
• To allow incompatible devices to connect to one
another. For example, a device with a maximum
capacity of 10 Mbps can communicate with a
device with a 100 Mbps capacity (but can work at
a lower rate).
• To allow a station to check a hub's capabilities.

Topology :
•Fast Ethernet is designed to connect two or more
stations together.
• If there are only two stations, they can be connected
point-to-point.
• Three or more stations need to be connected in a
star topology with a hub or a switch at the center, as
shown in Figure.

• Fast Ethernet implementation at the physical layer
can be categorized as either two-wire or four-wire.
• The two-wire implementation can be either
category 5 UTP (100Base-TX) or fiber-optic cable
(100Base-FX).
• The four-wire implementation is designed only for
category 3 UTP (l00Base-T4).

Summary of Fast Ethernet implementations

WIRELESS LANs
• Wireless communication is one of the fastest-
growing technologies. Wireless LANs can be
found on college campuses, in office
buildings, and in many public areas.
• We concentrate on two promising wireless
technologies for LANs:
– IEEE 802.11 wireless LANs, sometimes called
wireless Ethernet
– Bluetooth, a technology for small wireless LANs.

IEEE 802.11
• IEEE has defined the specifications for a wireless
LAN, called IEEE 802.11, which covers the
physical and data link layers.
• Architecture: The standard defines two kinds of
services: the basic service set (BSS) and the
extended service set (ESS).
• Basic Service Set:
• IEEE 802.11 defines the basic service set (BSS) as
the building block of a wireless LAN.
• A basic service set is made of stationary or mobile
wireless stations and an optional central base
station, known as the access point (AP).

• The BSS without an AP is a stand-alone
network and cannot send data to other BSSs.
• It is called an ad-hoc architecture. In this
architecture, stations can form a network
without the need of an AP; they can locate one
another and agree to be part of a BSS.
• A BSS with an AP is sometimes referred to as
an infrastructure network.
• A BSS without an AP is called an ad-hoc
network; a BSS with an AP is called an
infrastructure network.

Extended Service Set
• An extended service set (ESS) is made up of two
or more BSSs with APs. In this case, the BSSs are
connected through a distribution system, which is
usually a wired LAN.
• The distribution system connects the APs in the
BSSs. IEEE 802.11 does not restrict the
distribution system; it can be any IEEE LAN such
as an Ethernet.
• Extended service set uses two types of stations:
mobile and stationary.
• The mobile stations are normal stations inside a
BSS.
• The stationary stations are AP stations that are
part of a wired LAN. Figure shows an ESS.

• When BSSs are connected, the stations within
reach of one another can communicate without
the use of an AP. However, communication
between two stations in two different BSSs
usually occurs via two APs.
• The idea is similar to communication in a
cellular network if we consider each BSS to be
a cell and each AP to be a base station.
• A mobile station can belong to more than one
BSS at the same time.

STATION TYPES
• IEEE 802.11 defines three types of stations based
on their mobility in a wireless LAN: no-
transition, BSS-transition, and ESS-transition
mobility.
• A station with no-transition mobility is either
stationary (not moving) or moving only inside a
BSS.
• A station with BSS-transition mobility can move
from one BSS to another, but the movement is
confined inside one ESS.
• A station with ESS-transition mobility can move
from one ESS to another.

MAC SUBLAYER
• IEEE 802.11 defines two MAC sublayers: the distributed
coordination function (DCF) and point coordination function
(PCF). Figure shows the relationship between the two MAC
sublayers, the LLC sublayer, and the physical layer.

Distributed Coordination Function
• One of the two protocols defined by IEEE at the
MAC sublayer is called the distributed
coordination function (DCF).
• DCF uses CSMA/CA [Carrier Sense Multiple
Access/ Collision Avoidance]as the access
method.
• Wireless LANs cannot implement CSMA/CD
[Carrier Sense Multiple Access/ Collision
Detection] for three reasons:

three reasons:
•For collision detection a station must be able to
send data and receive collision signals at the same
time.
•Collision may not be detected because of the
hidden station problem.
•The distance between stations can be great. Signal
fading could prevent a station at one end from
hearing a collision at the other end.

Figure :
NAV ( Network Allocation Vector) is virtual carrier sensing
mechanism used with wireless network protocol such as IEEE 802.11.

• Before sending a frame, the source station senses
the medium by checking the energy level at the
carrier frequency.
– The channel uses a persistence strategy with back-off
until the channel is idle.
– After the station is found to be idle, the station waits for
a period of time called the distributed inter-frame space
(DIFS); then the station sends a control frame called the
request to send (RTS).

• After receiving the RTS and waiting a period of
time called the short inter-frame space (SIFS), the
destination station sends a control frame, called the
clear to send (CTS), to the source station. This
control frame indicates that the destination station
is ready to receive data.
• The source station sends data after waiting an
amount of time equal to SIFS.

• The destination station, after waiting an amount of
time equal to SIFS, sends an acknowledgment to
show that the frame has been received.
• Acknowledgment is needed in this protocol
because the station does not have any means to
check for the successful arrival of its data at the
destination.
• On the other hand, the lack of collision in
CSMA/CD is a kind of indication to the source that
data have arrived.

Point Coordination Function (PCF)
• The point coordination function (PCF) is an
optional access method that can be implemented in
an infrastructure network (not in an ad-hoc
network).
• It is implemented on top of the DCF and is used
mostly for time-sensitive transmission.
• PCF has a centralized, contention-free polling
access method. The AP performs polling for
stations that are capable of being polled.
• The stations are polled one after another, sending
any data they have to the AP.

Point Coordination Function (PCF)
• To give priority to PCF over DCF, another set of
inter frame spaces has been defined: PIFS and
SIFS.
• The SIFS is the same as that in DCF, but the PIFS
(PCF IFS) is shorter than the DIFS.
• This means that if, at the same time, a station wants
to use only DCF and an AP wants to use PCF, the
AP has priority.

• Due to the priority of PCF over DCF, stations that only
use DCF may not gain access to the medium. To prevent
this, a repetition interval has been designed to cover both
contention-free (PCF) and contention-based (DCF) traffic.
• The repetition interval, which is repeated continuously,
starts with a special control frame, called a beacon frame.
• When the stations hear the beacon frame, they start their
NAV for the duration of the contention-free period of the
repetition interval.
• NAV ( Network Allocation Vector) is virtual carrier
sensing mechanism used with wireless network protocol
such as IEEE 802.11.

• During the repetition interval, the PC (point
controller) can send a poll frame, receive data, send
an ACK, receive an ACK, or do any combination of
these (802.11 uses piggybacking).
• At the end of the contention-free period, the PC
sends a CF end (contention-free end) frame to allow
the contention-based stations to use the medium.
• The wireless environment is very noisy; a corrupt
frame has to be retransmitted. The protocol,
therefore, recommends fragmentation-the division of
a large frame into smaller ones. It is more efficient to
resend a small frame than a large one.

WLAN PROBLEMS
HIDDEN STATION PROBLEMS
• Figure shows an example of the hidden station
problem. Station B has a transmission range shown by
the left oval (sphere in space); every station in this
range can hear any signal transmitted by station B.
• Station C has a transmission range shown by the right
oval (sphere in space); every station located in this
range can hear any signal transmitted by C.
• Station C is outside the transmission range of B;
likewise, station B is outside the transmission range of
C. Station A, however, is in the area covered by both
Band C; it can hear any signal transmitted by B or C.

• Assume that station B is sending data to station A. In the
middle of this transmission, station C also has data to send
to station A. However, station C is out of B's range and
transmissions from B cannot reach C. Therefore C thinks
the medium is free. Station C sends its data to A, which
results in a collision at A because this station is receiving
data from both B and C.
• In this case, we say that stations B and C are hidden from
each other with respect to A. Hidden stations can reduce
the capacity of the network because of the possibility of
collision. The solution to the hidden station problem is the
use of the handshake frames (RTS and CTS)
• The CTS frame in CSMA/CA handshake can prevent
collision from a hidden station.

EXPOSED STATION PROBLEM
• Consider a situation that is the inverse of the previous one: the exposed
station problem. In this problem a station refrains from using a channel when
it is, in fact, available. In Next Figure, station A is transmitting to station B.
• Station C has some data to send to station D, which can be sent without
interfering with the transmission from A to B. However, station C is exposed
to transmission from A; it hears what A is sending and thus refrains from
sending. In other words, C is too conservative and wastes the capacity of the
channel.

• Station C hears the RTS from A, but does not hear the CTS from B.
Station C, after hearing the RTS from A, can wait for a time so that
the CTS from B reaches A; it then sends an RTS to D to show that it
needs to communicate with D. Both stations B and A may hear this
RTS, but station A is in the sending state, not the receiving state.
Station B, however, responds with a CTS.
• The problem is here. If station A has started sending its data, station
C cannot hear the CTS from station D because of the collision; it
cannot send its data to D. It remains exposed until A finishes
sending its data as in next Figure.

BLUETOOTH
• A Bluetooth LAN is an ad hoc network, which
means that the network is formed spontaneously;
the devices, sometimes called gadgets, find each
other and make a network called a piconet
• Bluetooth technology has several applications.
Peripheral devices such as a wireless mouse or
keyboard can communicate with the computer
through this technology.

• Monitoring devices can communicate with
sensor devices in a small health care center.
• Bluetooth technology is the implementation of
a protocol defined by the IEEE 802.15
standard.
• The standard defines a wireless personal-area
network (PAN) operable in an area the size of
a room or a hall.

ARCHITECTURE
• Bluetooth defines two types of networks: piconet
and scatternet.
• Piconets: A Bluetooth network is called a piconet,
or a small net. A piconet can have up to eight
stations, one of which is called the primary; the
rest are called secondaries.
• All the secondary stations synchronize their
clocks and hopping sequence with the primary.
Note that a piconet can have only one primary
station.
• The communication between the primary and the
secondary can be one-to-one or one-to-many.
Figure shows a piconet.

• Although a piconet can have a maximum of seven
secondaries, an additional eight secondaries can be in the
parked state. A secondary in a parked state is
synchronized with the primary, but cannot take part in
communication until it is moved from the parked state.
• Because only eight stations can be active in a piconet,
activating a station from the parked state means that an
active station must go to the parked state.

• Scatternet: Piconets can be combined to form
what is called a scatternet. A secondary station in
one piconet can be the primary in another piconet.
• This station can receive messages from the
primary in the first piconet (as a secondary) and,
acting as a primary, deliver them to secondaries in
the second piconet. A station can be a member of
two piconets. Figure illustrates a scatternet.

Bluetooth Devices
A Bluetooth device has a built-in short-range radio
transmitter. The current data rate is 1 Mbps with a 2.4-
GHz bandwidth.

Bluetooth Layers
• Bluetooth uses several layers that do not exactly
match those of the Internet model.

• Radio Layer: The radio layer is roughly
equivalent to the physical layer of the Internet
model. Bluetooth devices are low-power and have
a range of 10 m.
– Band: Bluetooth uses a 2.4-GHz ISM (Industrial
Scientific and Medical) band divided into 79 channels
of 1 MHz each.
– FHSS: Bluetooth uses the frequency-hopping spread
spectrum (FHSS) method in the physical layer to avoid
interference from other devices or other networks.
– Bluetooth hops 1600 times per second, which means
that each device changes its modulation frequency
1600 times per second.

BASEBAND LAYER:
•The baseband layer is roughly equivalent to the
MAC sublayer in LANs. The access method is
TDMA.
•The primary and secondary communicate with
each other using time slots.
•This means that during the time that one
frequency is used, a sender sends a frame to a
secondary, or a secondary sends a frame to the
primary.
•Note that the communication is only between the
primary and a secondary; secondaries cannot
communicate directly with one another.

• TDMA: Bluetooth uses a form of TDMA that is
called TDD-TDMA (time division duplex TDMA).
• TDD-TDMA is a kind of half-duplex
communication in which the secondary and
receiver send and receive data, but not at the same
time (half duplex); however, the communication for
each direction uses different hops.
• This is similar to walkie-talkies using different
carrier frequencies.

Physical Links
• Two types of links can be created between a
primary and a secondary: SCO links and ACL
links.
• SCO: A synchronous connection-oriented (SCO)
link is used when avoiding latency (delay in data
delivery) is more important than integrity (error-
free delivery).
• In an SCO link, a physical link is created between
the primary and a secondary by reserving specific
slots at regular intervals.

Physical Links
• The basic unit of connection is two slots, one for
each direction.
• If a packet is damaged, it is never retransmitted.
SCO is used for real-time audio where avoiding
delay is all-important.
• A secondary can create up to three SCO links
with the primary, sending digitized audio (PCM)
at 64 kbps in each link.
• ACL :An asynchronous connectionless link
(ACL) is used when data integrity is more
important than avoiding latency.

• In this type of link, if a payload encapsulated in
the frame is corrupted, it is retransmitted.
• A secondary returns an ACL frame in the
available odd-numbered slot if and only if the
previous slot has been addressed to it.
• ACL can use one, three, or more slots and can
achieve a maximum data rate of 721 kbps.

L2CAP
• The Logical Link Control and Adaptation
Protocol, or L2CAP (L2 here means LL), is
roughly equivalent to the LLC sublayer in
LANs.
• It is used for data exchange on an ACL link
(If frame is corrupted, it is retransmitted. );
SCQ channels do not use L2CAP.
• The 16-bit length field defines the size of the
data, in bytes, coming from the upper layers.

• The L2CAP has specific duties:
– Multiplexing,
– segmentation and reassembly,
– quality of service (QoS),
– group management.
Multiplexing:
– The L2CAP can do multiplexing. At the sender site, it
accepts data from one of the upper-layer protocols,
frames them, and delivers them to the baseband layer.
– At the receiver site, it accepts a frame from the
baseband layer, extracts the data, and delivers them to
the appropriate protocol layer.
L2CAP

Segmentation and Reassembly:
– The L2CAP divides these large packets into
segments and adds extra information to define the
location of the segments in the original packet.
– The L2CAP segments the packet at the source and
reassembles them at the destination.
L2CAP

QoS:
– Bluetooth allows the stations to define a quality-
of-service level.
Group Management:
– This is similar to multicasting. For example, two
or three secondary devices can be part of a
multicast group to receive data from the primary.
L2CAP

Common Network Architecture:X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

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