Data Communications

Subject Code:18CS46

Module-5

Chapter 13: Outline

13.1 ETHERNET PROTOCOL

13.2 STANDARD ETHERNET

13.3 FAST ETHERNET

13.4 GIGABIT ETHERNET

13.5 10 GIGABIT ETHERNET

13-1 ETHERNET PROTOCOL

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• The data-link layer and the physical layer are the territory of the local and wide area networks

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• Discussed LAN whose sole purpose is to share the resources

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• Most of the LANs today is connected to WAN or Internet

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• Ethernet Protocol is most popular LAN technology because it was able to update itself to meet the needs of organization with the demand for higher data transmission.

IEEE Project 802

• In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers.

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• Project 802 does not seek to replace any part of the OSI model or TCP/IP protocol suite

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• Instead, it is a way of specifying functions of the physical layer and the data-link layer of major LAN protocols

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• The relationship of the 802 Standard to the TCP/IP protocol suite is shown in the Figure below

IEEE standard for LANs

IEEE Project 802

• IEEE has subdivided the data link into two sub layers

i) Logical Link Control(LLC)

ii) Media Access Control(MAC)

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• Logical Link Control: In IEEE 802 project, Flow Control, Error Control & part of framing duties are collected into this sub layer

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• Framing is handled in both LLC & MAC sub layers

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• LLC provides a single link layer control protocol for IEEE LANs. i.e LLC protocol can provide inter connectivity between different LANs

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• Media access Control that defines the specific access method for each LAN

Ethernet Evolution

• The Ethernet LAN was developed in the 1970s by Robert Metcalfe and David Boggs

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• It has gone through four generations:

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1)Standard Ethernet (10 Mbps)

2)Fast Ethernet (100 Mbps)

3)Gigabit Ethernet (1 Gbps)

4)10 Gigabit Ethernet (10 Gbps)

Ethernet evolution

13-2 STANDARD ETHERNET

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• Original Ethernet technology with the data rate of 10 Mbps as the Standard Ethernet.
• Although most implementations have moved to other technologies in the Ethernet evolution, there are some features of the Standard Ethernet that have not changed during the evolution.

Characteristics

• Discuss some characteristics of the Standard Ethernet.

Connectionless and unreliable service.

• Ethernet provides connectionless service i.e each frame sent is independent of the previous or next frame.
• Ethernet has no connection establishment or connection termination phases.
• Sender sends a frame whenever it has and receiver may not be ready for it.
• Sender may overwhelm the receiver buffer with frames, which may result in dropping frames.
• IP, connectionless protocol, which is using the services of the Ethernet, which is also connectionless, will not know if frame is dropped. If TL is also connectionless using UDP, frame is lost

Characteristics

• This loss will come to know only from Application Layer

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• If TL is using TCP, Sender TCP segment if does not receive acknowledgment for its segment and sends it again

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• Ethernet if connectionless, if frame is corrupted during transmission, and receiver finds out about the corruption because of CRC-32, the receiver drops the frame silently

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• It is the duty of high level protocols to find out

Ethernet frame

802.3 MAC frame

• Ethernet frame contains 7 fields.
• Ethernet does not provide any mechanism for Ack.ing the received frames.
• Ack. must be implemented at the higher layers.

1. Preamble: contains 7 bytes(56 bits) of alternating 1s and 0s to alert the station that frame is arriving and enables it to synchronize its i/p timing.

🡪Pattern provides only an alert and a timing pulse.

🡪Preamble is actually added at the physical layer and is not part of the frame.

2.Start frame delimiter(SFD):1 byte10101011 signals the beginning of the frame.

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802.3 MAC frame

• Warns the station/stations that this is the last chance of synchronization.
• Last 2 bits is 11 and alerts the receiver that the next field is the destination address.

3.Destination address: is 6 bytes and contains the physical address of the destination station/s to receive the packet.

4.Source address: is 6 bytes and contains the physical address of the destination station/s to receive the packet.

5.Type: This field defines the upper layer protocol whose packet is encapsulated in the frame. The protocols can be IP,ARP,OSPF etc.,

It is used for multiplexing and demultiplexing.

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802.3 MAC frame

6.Data: This field carries data encapsulated from the upper layer protocols. It is minimum of 46 bytes and maximum of 1500 bytes.

If data coming from upper layers is more than 1500 bytes, it should be fragmented and encapsulated in more than one frame. If less than 46 bytes, it should be padded with 0’s.

A padded data frame is delivered to upper layer protocol as it is and it is the responsibility of the upper layer protocol to remove the zeros.

7.CRC: Contains error detection mechanism(CRC-32). The CRC is calculated over address, types and data field.

if receiver calculates CRC and finds that it is not zero, it discards the frame.

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Frame Length

• Ethernet imposed min and max lengths of a frame. The minimum length restriction is required for the correct operation of CSMA/CD.

Frame Length

• Ethernet frame needs to have a minimum length of 512 bits or 64 bytes.
• Part of this length is header and the trailer.
• 18 bytes of header and trailer, length,CRC, then minimum length of data from upper layer protocols is 64-18=46 bytes.
• If upper layer packet is less than 46 bytes, padding is added to make up the difference.
• The standard defines maximum length of a frame without preamble and SFD is 1518 bytes.
• Max. length has two historical reasons

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Frame Length

1) Memory was expensive when Ethernet was designed. Max. length restriction helped to reduce the size of the buffer.

2) Max. length restriction prevents one station from monopolizing the shared medium blocking other stations that have data to send.

Minimum Frame length:64 bytes (512 bits)

Minimum data length:46 bytes (368bits)

Maximum frame length: 1518 bytes (12,144 bits)

Maximum data length : 1500 bytes(12000 bits)

Addressing

• Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network interface card (NIC)

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• The NIC fits inside the station and provides the station with a link-layer address

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• The Ethernet address is 6 bytes (48 bits), normally written in hexadecimal notation, with a colon between the bytes

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• For example, the following shows an Ethernet MAC address:

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• The transmission is left to right, byte by byte. For each byte, the LSB is sent first & MSB is sent last.

Show how the address 47:20:1B:2E:08:EE is sent out online.

Solution

The address is sent left to right, byte by byte; for each byte, it is sent right to left, bit by bit, as shown below:

Unicast and multicast addresses

Continued..

• Source address is always Uni cast address- frame comes from only one station
• Destination can be Uni cast, Multi cast or Broad cast
• 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
• The broadcast destination address is a special case of the multicast address in which all bits are 1s.
• The recipients are all stations on the LAN. Broad cast destination address is forty eight 1s.

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Define the type of the following destination addresses:

1. 4A:30:10:21:10:1A
2. 47:20:1B:2E:08:EE
3. FF:FF:FF:FF:FF:FF

Solution

To find the type of the address,

look at the second hexadecimal digit from the left. If it is even, the address is unicast.

If it is odd, the address is multicast.

If all digits are Fs, the address is broadcast. Therefore, we have the following:

a. This is a unicast address because A in binary is 1010 � (even).

b. This is a multicast address because 7 in binary is 0111 � (odd).

c. This is a broadcast address because all digits are Fs in

hexadecimal.

Distinguish between Unicast, multicast & Broadcast transmission

• Std. Ethernet uses coaxial cable(bus) or a set of twisted pair cables with hub topology
• Transmission in standard Ethernet is always broadcast, even if the intention is multicast, unicast or broadcast
• In bus topology, when station A sends frame to B, all stations will receive it
• In star topology, when station sends frame to B, hub receives it, since hub is a passive element, it does not check the destination address of the frame, it regenerates the bits and sends to all stations except A.
• Question is how unicast, broadcast and multicast transmission are distinguished from each other????

Distinguish between Unicast, multicast & Broadcast transmission

• In Unicast transmission, all stations will receive a frame, only the intended station will keep the frame rest will discard

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• In Multi cast transmission, all stations will receive a frame, the stations that are part of group will keep the frame rest will discard

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• In broad cast transmission, all stations will receive a frame except the sender and all stations will keep the frame

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Implementation of standard Ethernet

Access Method

• Network that uses the standard Ethernet protocol is a broadcast network, need to use an access method to control access to the sharing medium
• The standard Ethernet choose CSMA/CD with 1-persistent method
• Let us use a scenario to see how this method works for the Ethernet protocol

Scenario –I

• Assume station A has a frame to send to station D
• Station A first should sense(carrier) if any other station is sending. It measures the energy level of the medium for a short period of time Eg:100microsec.

🡪 If no signal energy on the medium, it means no station is sending data, station A interprets the medium as idle and starts sending the frame

Access Method(Continued..)

• If signal energy on the medium is not zero, it means medium is not idle. station A continuously monitors the medium until it becomes idle for 100 microsecs and starts sending the frame.
• Station A needs to keep a copy of the frame in its buffer to make sure that there is no collision.

Efficiency of Standard Ethernet

• The efficiency of the Ethernet is defined as

the ratio of the time used by a station to send data to the time the medium is occupied by this station.

• The practical efficiency of standard Ethernet has been measured to be

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Where a is number of frames that can fit on the medium.

It can be calculated as

a=(Propagation delay/ Transmission delay)

PD🡪Time taken to reach end of medium

TD🡪Time taken to sent out an average size frame,

NOTE: If value of a decreases, efficiency increases.

Means if the length of the medium is shorter or frame length is longer, the efficiency increases.

In the Standard Ethernet with the transmission rate of 10 Mbps, we assume that the length of the medium is 2500 m and the size of the frame is 512 bits. The propagation speed of a signal in a cable is normally 2 × 10⁸ m/s.

The example shows that a = 0.24, which means only 0.24 of a frame occupies the whole medium in this case. The efficiency is 39 percent, which is considered moderate; it means that only 61 percent of the time the medium is occupied but not used by a station.

Implementation

• The Standard Ethernet defined several implementations, but only four of them became popular during the 1980s

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• Table below shows a summary of Standard Ethernet implementations

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• In the nomenclature, 10 BaseX, number defines the data rate 10Mbps, Base means base band signal, X defines either maximum size of the cable in 100 mts or the type of the cable

Table : Summary of Standard Ethernet implementations

Encoding in a Standard Ethernet

Encoding in a Standard Ethernet�

• All standard implementations use digital signaling at 10 Mbps

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• At the sender site, data is converted to a digital signal using Manchester scheme and at the receiver site, received signal is decoded into data

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• Manchester encoding is self synchronous, providing a transition at each bit interval

10Base5 implementation

10Base5 implementation

• It is called 10Base5, thick Ethernet or Thick net
• Nick name derives from the size of the cable
• First Ethernet specification to use bus topology with an external trans receiver connected via tap to a thick coaxial cable
• Trans receiver is responsible for transmitting, receiving & detecting collision
• Trans receiver is connected to the station via a trans receiver cable that provides separate paths for sending and receiving
• Maximum length of co axial cable should not exceed 500 mts. If greater than 500mts, there will be excessive degradation of the signal. If length > 500mts is needed, five segments with a maximum of 500 mts can be connected using repeaters

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10Base2 implementation

10Base2-Thin Ethernet

• It is called 10Base2, thin Ethernet or Cheap net
• Nick name derives from the size of the cable
• Uses bus topology , but the cable is much thinner and more flexible
• The cable can be bent to pass very close to stations
• Trans receiver is part of the NIC which is installed inside the station
• As Thin co axial cable is very flexible, installation is flexible.
• Length of segment can not exceed 185 mts due to high level of attention in thin coaxial cable

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10Base-T implementation

10Base-T Ethernet

• It is called 10BaseT, twisted pair Ethernet.
• Uses Star topology.
• Stations are connected to a hub via two pair of twisted cables.
• Two pairs creates two paths between station and the hub.
• Any collision happens in the hub.
• Maximum length of twisted pair cable here is defined as 10mts to minimize the effect of attenuation in the cable.

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10Base-F implementation

10Base-F Ethernet

• It is called 10Base-F, Optical fiber 10Mbps Ethernet.
• Uses Star topology.
• Stations are connected to a hub via two pair of fiber-optic cables.
• Two pairs creates two paths between station and the hub.

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Changes in the Standard

• The changes that occurred to the 10-Mbps Standard Ethernet.

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• These changes actually opened the road to the evolution of the Ethernet to become compatible with other high-data-rate LANs.

Ethernet Evolution:

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1. Division of LAN by bridges:
1. Bridges has two Effects on an Ethernet LANs

i) Raise the Bandwidth

ii) Separate Collision Domains.

• In an un bridged Ether N/w, total of 10 Mbps capacity is shared among all stations with a frame to send.

Sharing bandwidth

Raising the Bandwidth

• The bridge divides the network into two or more networks.

Bandwidth of each network is independent

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• Fig. below a network with 12 stations is divided into two networks each with 6 stations.

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• Each network has a capacity of 10 Mbps which is shared between 6 stations and the bridge.

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• In a network with a heavy load, each station theoretically is offered 10/7 Mbps instead of 10/12 devices.

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• If four port bridge is used, each station is offered bandwidth of 10/4.

A network with and without bridging

Separating collision domain

• Another advantage of bridge is the separation of collision domain

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• From the figure below, it is observed that the probability of collision is reduced

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• With bridging 3 stations contend for the access for the medium.

Collision domains

Switched Ethernet

• The idea of bridged LAN can be extended to switched LAN

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• In Switched LAN , instead of 4 networks, there will be N networks where N is the number of stations

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• Here N port switch is used and the bandwidth is shared between the station and the switch

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• Collision domain is divided in to N domains

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• Evolution from a bridged Ethernet to switched Ethernet was a big step that opened the way to an even faster Ethernet

Switched Ethernet

Full duplex Ethernet

• One of the limitations of 10Base5 and 10Base2 is that communication is half duplex

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• Full duplex mode increases the capacity of each domain from 10 to 20 Mbps

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• In Full duplex switched Ethernet, each station is connected to a switch via two separate links, so there is no need for CSMA/CD

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• Each link has a point-to-point dedicated path between the station and switch

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• Job of MAC layer is easy as carrier sensing & collision detection functionalities can be turned off

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• Std. Ethernet was designed as a connectionless protocol at MAC sub layer. To Provide FC & EC in full duplex switched Ethernet, a new sub layer, MAC control, is added between LLC & MAC

Full – duplex switched Ethernet

13-3 FAST ETHERNET(100 Mbps)

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• In the 1990s, Ethernet made a big jump by increasing the transmission rate to 100 Mbps, and the new generation was called the Fast Ethernet

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• The designers of the Fast Ethernet needed to make it compatible with the Standard Ethernet

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• The MAC sub layer was left unchanged, means frame format, min & max size of frame remain unchanged

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• By increasing the transmission rate, the features of the Standard Ethernet that depend on the transmission rate, access method, implementation had to be changed.

Goals of Fast Ethernet

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• Upgrade the data rate to 100 Mbps.
• Make it compatible with standard Ethernet.
• Keep the same 48 bit address.
• Keep the same frame format.

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Access Method

• The proper operation of the CSMA/CD depends on the transmission rate, the minimum size of the frame, and the maximum network length.
• If we want to keep the minimum size of the frame, the maximum length of the network should be changed.
• In other words, if the minimum frame size is still 512 bits, and it is transmitted 10 times faster, the collision needs to be detected 10 times sooner, which means the maximum length of the network should be 10 times shorter (the propagation speed does not change).

Access Method

• The Fast Ethernet came with two solutions

i)Drop bus topology & use a passive hub or a star topology but make the maximum size of Ethernet 250 meters instead of 2500 meters.

ii) Use Link layer switch with a buffer to store frames and full duplex connection to each host to make the transmission medium private for each host.

No need of CSMA/CD.

Since the connection to switch is full duplex, the destination address can even send a frame to another station at the same time.

• New feature called Auto negotiation is added. Auto negotiation allows two devices to negotiate the mode or data rate of operation.

13.3.2 Physical Layer

• To be able to handle a 100 Mbps data rate, several changes need to be made at the physical layer.

Encoding for fast Ethernet

Table Summary of Fast Ethernet implementations

13-4 GIGABIT ETHERNET

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• The need for an even higher data rate resulted in the design of the Gigabit Ethernet Protocol (1000 Mbps).
• The IEEE committee calls it the Standard 802.3z.
• The goals of the Gigabit Ethernet were to upgrade the data rate to 1 Gbps, but keep the address length, the frame format, and the maximum and minimum frame length the same.

MAC Sublayer

• A main consideration in the evolution of Ethernet was to keep the MAC sub layer untouched

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• However, to achieve a data rate of 1 Gbps, this was no longer possible

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• Gigabit Ethernet has two distinctive approaches for medium access:

Half-duplex and Full-duplex

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• Almost all implementations of Gigabit Ethernet follow the full-duplex approach.

Physical Layer

• The physical layer in Gigabit Ethernet is more complicated than that in Standard or Fast Ethernet

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• Discuss some features of this layer.

Encoding in Gigabit Ethernet

Table : Summary of Gigabit Ethernet implementations

13-5 10-GIGABIT EHTERNET

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• In recent years, there has been another look into the Ethernet for use in metropolitan areas

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• The idea is to extend the technology, the data rate, and the coverage distance so that the Ethernet can be used as LAN and MAN (metropolitan area network)

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• The IEEE committee created 10 Gigabit Ethernet and called it Standard 802.3ae

Implementation

• 10 Gigabit Ethernet operates only in full-duplex mode, which means there is no need for contention; CSMA/CD is not used in 10 Gigabit Ethernet

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Four implementations are the most common: 10GBase-SR, 10GBase-LR, 10GBase-EW, and 10GBase-X4.

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Table Summary of 10-Gigabit Ethernet implementations

Chapter 15: Outline

15.1 INTRODUCTION

15.2 IEEE 802.11 PROJECT

15.3 BLUETOOTH

15-1 INTRODUCTION

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• Wireless communication is one of the fastest-

growing technologies

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• The demand for connecting devices without the use

of cables is increasing everywhere

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• Wireless LANs can be found on college campuses,

in office buildings, and in many public areas

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Architectural Comparison

• Let us first compare the architecture of wired and wireless LANs

1. Medium:

In wired LAN, communication takes through the cables(full duplex)

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In wireless LAN, medium is air, signal is generally broadcast and the medium is shared

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2. Hosts:

In wired LAN, host is always connected to its N/W at a point with a fixed Link layer address.

In wireless LAN, host is not physically connected to the N/W. It can move freely & can use the service provided by the N/W.

Isolated LANs: wired versus wireless

Architectural Comparison(continued)

3. Isolated LANs:

A wired isolated LAN is a set of hosts connected via link layer switch

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A wireless isolated LAN, called an ad hoc network, is a set of hosts that communicate freely with each other.

Concept of LL switch does not exist.

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4. Connection to other networks:

A wired LAN can be connected to another network or an inter/Intra network using a router.

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A wireless LAN may be connected to a wired infrastructure net work, wire less infrastructure N/W or to another wireless LAN.

Figure shows the two environments

Connection of a wired LAN and a wireless LAN to other

networks

Architectural Comparison(continued)

The wireless LAN is referred to as an infrastructure network and the connection to the wired infrastructure, such an Internet, is done via a device called an Access Point.

An Access point connects two different environments together wired and wireless.

Communication between AP and wireless hosts occurs in a wire less environment ;

Communication between AP and infrastructure occurs in a wired environment .

5. Moving between Environments:

Wired or wireless LAN operates only in lower two layers of TCP/IP protocol suite. As data moves from wired to wireless network, need to change the NIC address designed for wired to wireless and replace the LL switch with an AP. LL address will change but N/W address remains same.

Characteristics

• There are several characteristics of wireless LANs that either do not apply to wired LANs or the existence of which is negligible and can be ignored

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• Characteristics of wireless LAN protocols

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1. Attenuation: As signal disperses in all directions, the strength of electro magnetic signal decreases. Only small portion of it reaches the receiver

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2. Interference: If same frequency band is used by the other senders, receiver may receive from intended sender and others also

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3. Multipath Propagation: As electromagnetic waves can be reflected back from obstacles such as

Characteristics(Continued..)

• walls/ground/objects, the receiver may receive from same sender more than one signal at different phases

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4.Error:

With these characteristics, error & error detection are more serious in wireless networks

Error level is measurement of SNR

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If SNR is high, signal is stronger than noise and the signal can be converted back to data

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If SNR is low, means signal is corrupted by the noise and the data cannot be recovered

Access Control

• Most important issue that need to be discussed in a wireless LAN is access control—how a wireless host can get access to the shared medium (air)

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• The CSMA/CD algorithm does not work in wireless LANs for three reasons:

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1. Wireless hosts do not have enough power to send and receive at the same time.
2. The hidden station problem prevents collision detection.
3. The distance between stations can be great.

To overcome all these three problems, CSMA/CA was invented for wireless LANs.

Hidden station problem

15-2 IEEE 802.11 PROJECT

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• IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data-link layers

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• It is sometimes called wireless Ethernet

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• In some countries, including the United States, the public uses the term WiFi (short for wireless fidelity) as a synonym for wireless LAN. WiFi, however, is a wireless LAN that is certified by the WiFi Alliance, a global non profit industry association of more than 300 member companies devoted to the growth of Wireless LANs.

Architecture

• The standard defines two kinds of services:

i) The basic service set (BSS)

ii) The extended service set (ESS)

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Basic Service Set:

• BSS is a building block of wireless LAN.
• It is made of stationary or mobile wireless stations and optional central base station, known as Access point(AP).
• BSS without an AP is a stand alone network and can not send data to other BSSs (Called Adhoc Architecture).
• Here Stations can form a network without the need of an AP. They can communicate with one another & agreed to be part of BSS.
• A BSS with an AP is referred to as Infrastructure BSS.

: Basic service sets (BSSs)

Extended Service Set

• ESS is made up of two or more BSSs with APs.
• Here BSSs are connected through a distribution system which is wired /wireless network.
• The distribution systems connects the APs in BSS.
• The distribution system can be any IEEE LAN such as an Ethernet.
• Extended service set uses two types of stations:
• Mobile 2. Stationary
• Mobile stations are normal stations inside a BSS.
• The Stationary stations are AP stations that are part of the wired LAN.
• When BSSs are connected, the stations within the reach of one another can communicate without the use of an AP.
• Communication between a station in a BSS & outside BSS occurs via AP.

Extended service set (ESS)

Station types

• IEEE 802.11 defines three types of stations based on mobility in a wireless LAN
• No-transition
• BSS-transition
• ESS-transition mobility

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• A station with no-transition mobility is either stationary or moving only inside a BSS

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• A station with BSS-transition mobility can move from one BSS to another, but movement is confined within the ESS

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• A station with ESS-transition mobility can move from one ESS to another

MAC Sub layer

• IEEE 802.11 defines two MAC sub layers:

i) Distributed coordination function (DCF)

ii) Point coordination function (PCF)

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• Figure below shows the relationship between the two MAC sub layers, the LLC sub layer, and the physical layer.

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Distributed coordination Function:

• Is the protocol defined by IEEE at the MAC sub layer.
• Uses CSMA/CA as the access method.
• Frame Exchange Time Line:
• Before sending a frame, the source station senses the medium by checking energy level at carrier frequency

MAC layers in IEEE 802.11 standard

CSMA/CA and NAV

MAC Sub layer(Continued..)

1. The channel uses a persistence strategy with back off until the channel is idle.
2. After station is found idle, the station waits for a period of time called Distributed inter frame space(DIFS), then station sends a control frame called request to send(RTS).
3. After receiving the RTS and waiting period of time called the short inter frame space(SIFS), the destination send a control frame, called clear to send(CTS) to the source station & CTS indicates that receiver is ready to receive data.
4. The source sends the data after waiting for an amount of time called SIFS.
5. The destination, after waiting an amount of time equal to SIFS, sends an ACK to show that frame has been received.

ACK is needed in this protocol to check the successful arrival of its data at the destination.

Network Allocation Vector

• If one station acquires access, how do stations defer sending their data??????(How Collision avoidance aspect of this protocol accomplished??)
• When station sends RTS, it includes the duration of time that it needs to occupy the channel.
• The stations that are affected by this transmission creates a timer called Network Allocation Vector(NAV).
• It shows how much time must pass before these stations are allowed to check the channel for idleness.
• Each time station access the system & sends RTS frame, other stations start their NAV.
• Collision during handshaking.
• Hidden station problem.

Point Coordination function

• Is an optional access that that can be implemented in an infrastructure network.
• Implemented on top of DCF and used for time sensitive transmission.
• PCF has a centralized, contention free polling access method.
• AP performs polling for stations that are capable of being polled.
• The stations are polled one after another, sending any data they have to AP.
• PIFS(PCF IFS) has been defined to give priority to PCF over DCF and is shorter than DIFS i.e if at the same time station wants to use only DCF & an AP wants use PCF, the AP has priority.

Point Coordination function

• Due to priority of PCF over DCF, stations that use only DCF may not gain access to the medium.
• To prevent this, A repetition interval has been designed to cover the contention free PCF and contention based free traffic.
• Repetition interval, which is repeated continuously starts with a special control frame called beacon frame.
• When stations hears the beacon frame, it starts their NAV for the duration of the contention-free period of the repetition interval.

Example of repetition interval

Point Coordination function

• During the repetition interval, the PC(Point controller) can send the poll frame, receive data, send & receive an ACK or do any combination of these.
• At the end of the contention free period, the PC sends a CF(Contention free end) end frame to allow the contention based stations to use the medium.

Fragmentation:

• Since wireless environment is very noisy, the frames are often corrupted.
• The corrupted frame has to be retransmitted so protocol recommends fragmentation and resending smaller size frame than larger one.

Frame format

Frame format (Continued..)

• MAC sub layer consists of nine fields.
• FC(Frame controller): Is 2 bytes long and defines type of frame and some control information. Defines sub fields.
• D: defines duration of the transmission that is used to set the value of NAV.
• Addresses: There are 4 fields each 6 bytes long. Meaning of each address field depends on the value of To DS & From DS sub fields.
• Sequence Control: Is 16 bit SC field. The first four bits define fragment number, last 12 bits define the sequence number, which is same in all segments.

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Table: Subfields in FC field

Frame format (Continued..)

5.Frame body: Contains the information based on type and sub type defined in FC field.

6.FCS: Is four byte long and contains CRC-32 error detection sequence.

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Frame Types

1. Management frames: used for initial communication between the AP & stations.
2. Control frames: Used for accessing the channel & acknowledging frames.
3. Data frames: Used for carrying data and control information.

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Control frames

Table: Values of subfields in control frames

Addressing Mechanism

• The IEEE 802.11 addressing mechanism specifies four cases, defined by the value of the two flags in the FC field, To DS and From DS.
• Each flag can be either 0 or 1, resulting in four different situations.
• The interpretation of the four addresses (address 1 to address 4) in the MAC frame depends on the value of these flags, as shown in Table .

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Table : Addresses

Addressing Mechanism(Continued..)

• Address 1 is always the address of the next device that the frame will visit.
• Address 2 is always the address of the previous device that the frame has left.
• Address 3 is always the address of the final destination.
• Address 4 is original source when the distribution system is also wireless.

Case 1:00:To DS=0 & From DS=0, means frame is not going to distribution system & is not coming from the distribution system.

The frame is going from one station in BSS to another without passing through the distributed system.

Addressing mechanisms

Addressing Mechanism(Continued..)

Case 2:01:To DS=0 & From DS=1, means frame is not going to distribution system & is coming from the distribution system.

The frame is coming from an AP & going to a station. Address 3 contains the original sender of the frame.

Case 3:10:To DS=1 & From DS=0, means frame is going to distribution system.

The frame is going from a station to an AP & ACK is sent to the original station. Address 3 contains the Final destination of the frame

Case 4:11:To DS=1 & From DS=1, means distribution system is also wireless.

The frame is going from one AP to another AP in wireless distribution system. Four Addresses to define original sender, final destination & two intermediate APs.

Exposed station problem

Physical Layer

• Discuss six specifications, as shown in Table below.
• All implementations, except the infrared, operate in the industrial, scientific, and medical (ISM) band, which defines three unlicensed bands in the three ranges 902–928 MHz, 2.400–4.835 GHz, and 5.725–5.850 GHz.

Table : Specifications

Physical layer of IEEE 802.11 FHSS

Physical layer of IEEE 802.11 DSSS

Physical layer of IEEE 802.11 infrared

Physical layer of IEEE 802.11b

BLUETOOTH

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• Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, note books, computers, cameras and printers when they are at a short distance from each other

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• A Bluetooth LAN is an ad hoc network. The devices, sometimes called gadgets, find each other and make a network called a piconet

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• Bluetooth was originally started as a project by the Ericsson Company

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• Today, 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.

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Applications

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• Wireless mouse & keyboard can communicate with the computer

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• Monitoring devices can communicate with the sensor devices in a small health care center

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• Security devices can use this technology to connect different sensors to main security controller

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• Conference attendees can synchronize their laptop computers at a conference.

Architecture

• Bluetooth defines two types of networks:

i) Piconet

ii) Scatternet

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PICONET:

• A Bluetooth network is called PICONET or a small network
• It can have up to 8 stations & one will be the primary & others stations secondary.
• All secondary stations synchronize their clock & hopping sequence with primary.
• A PICONET can have only one primary.
• Communication between the primary & secondary stations can be one-to-one or one-to-many.
• Additional secondaries in PICONET can be in parked state.

Piconet

PICONET

• Secondary in parked state is synchronized with the primary but cannot take part until it moves to active state

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• Only 8 stations can be part of PICONET, activating from parked state to active means, active station must go to parked state

ScatterNET

• Piconets can be combined to form scatternet

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• Secondary station in one piconet can be primary in another piconet

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• This station can receive message from the primary in the first piconet and act as a primary, delivers them to stations in second piconet.

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Scatternet

Bluetooth Devices

• A Bluetooth device has a built-in short-range radio transmitter

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• The current data rate is 1 Mbps with a 2.4-GHz bandwidth

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• This means that there is a possibility of interference between the IEEE 802.11b wireless LANs and Bluetooth LANs

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15.3.2 Bluetooth Layers

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

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• L2CAP(Logical link control & Adaptation Protocol)

used for exchange of data on an ACL(Asynchronous connectionless) link.

Bluetooth layers

L2CAP data packet format

L2CAP Layer

Length field: Size of this field is 16 bits which defines the size of the data , in bytes from the upper layers.

Data can be up 65,535 bytes

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Channel ID: defines unique identifier for the virtual channel created at this level

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• L2CAP performs multiplexing, segmentation & reassembly, QOS and group management

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• At the sender site, it accepts data from one of the upper layer protocol, frames them and delivers them to baseband layer.
• At the receiver site, it accepts data from baseband layer extracts the data and delivers them to appropriate protocol layer.

Segmentation & Reassembly

• Max. payload length in baseband layer is 2774 bits or 343 bytes which includes 4 bytes to define packet & packet length

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• Size of the data from upper layer protocol should be 339 bytes

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• If application layer need to send data that can be up to 65,535 bytes, L2CAP divides data into segments at the source and reassembles them at the destination

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• Bluetooth allows station to define QOS. Provides best-effort service

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• L2CAP layer allows devices to create a type of logical addressing between themselves.(Group management)

Baseband layer

• Similar to MAC sub layers in the LANs.
• Access method is TDMA.
• Primary & secondary stations can communicate with each other using time slots.
• Length of time slot is625 micro second.
• Communication is only between primary & a Secondary.
• Only in half-duplex mode.
• In slot0, primary sends & secondary receives
• In slot1, secondary sends & primary receives.
• Two type of links defined

i)Synchronous connection oriented link(SCO link)

ii)Asynchronous connectionless link(ACL)

Single-secondary communication

Multiple-secondary communication

Frame format types

Frame format types

• A frame in baseband layer can be one-slot, three-slot, five-slot

A slot is 625 microsecond.

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• Access code: 72 bit code contains synchronization bits and the identifier of the primary to distinguish the frame of one piconet from that of another

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• Header: 54 bit field is a repeated 18 bit pattern

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Each pattern has the following subfields

1. Address: 3bit sub field can define up to seven secondaries. If address is 0, it is used for broadcast communication from primary to all secondaries.
2. Type: defines type of data coming from upper layers.

Frame format types

3. F: 1 bit field for flow control. If F=1, it indicates that the device is unable to receive more frames.

4. A:1 bit field for acknowledgment . Bluetooth uses stop-and wait protocol.

5. S:1 bit field for sequence number. Bluetooth uses stop-and wait protocol.

6. HEC:8 bit error correction subfields to detect error in 18 bit header section

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Payload: This field can be 0 to 2740 bits long. It contains data and control information coming from upper layers.

Frame format types

RADIO Layer: Similar to physical layer of Internet model

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• Bluetooth devices are low power & have a range of 10mts uses FHHS to avoid interference from other devices or other network

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• Hops 1600 times per second

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• To transmit bits to a signal, Bluetooth uses GFSK(Gaussian bandwidth filtering.)

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16-2 Cellular Telephony

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• Cellular telephony is designed to provide communications between two moving units, called mobile stations (MSs) or between one mobile unit and one stationary unit, often called a land unit

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• A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the channel from base station to base station as the caller moves out of range.

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Figure 16.6: Cellular system

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Operation

Let us first briefly discuss the operation of the cellular telephony

Figure 16.7: Frequency reuse patterns

Frequency-Reuse Principle

• In general, neighboring cells cannot use the same set of frequencies for communication because doing so may create interference for the users located near the cell boundaries

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• The set of frequencies available is limited, and frequencies need to be reuse

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• A frequency reuse pattern is a configuration of N cells, N being the reuse factor, in which each cell uses a unique set of frequencies. When the pattern is repeated, the frequencies can be reused

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• There are several different patterns. Figure 16.7 shows two of them.

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Frequency-Reuse Principle ( Contd)

• The numbers in the cells define the pattern. The cells with the same number in a pattern can use the same set of frequencies. We call these cells the reusing cells

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• As Figure 16.7 shows, in a pattern with reuse factor 4, only one cell separates the cells using the same set of frequencies

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• In a pattern with reuse factor 7, two cells separate the reusing cells.

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Transmitting

• To place a call from a mobile station, the caller enters a code of 7 or 10 digits (a phone number) and presses the send button

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• The mobile station then scans the band, seeking a setup channel with a strong signal, and sends the data (phone number) to the closest base station using that channel. The base station relays the data to the MSC

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• The MSC sends the data on to the telephone central office. If the called party is available, a connection is made and the result is relayed back to the MSC

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• At this point, the MSC assigns an unused voice channel to the call, and a connection is established. The mobile station automatically adjusts its tuning to the new channel, and communication can begin.

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Receiving

• When a mobile phone is called, the telephone central office sends the number to the MSC

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• The MSC searches for the location of the mobile station by sending query signals to each cell in a process called paging

• Once the mobile station is found, the MSC transmits a ringing signal

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• When the mobile station answers, assigns a voice channel to the call, allowing voice communication to begin.

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Handoff

• It may happen that, during a conversation, the mobile station moves from one cell to another. The signal may become weak

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• To solve this problem, the MSC monitors the level of the signal every few seconds. If the strength of the signal diminishes, the MSC seeks a new cell that can better accommodate the communication

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• The MSC then changes the channel carrying the call (hands the signal off from the old channel to a new one).

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Types of handoffs

• Hard Handoff

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• Soft Handoff

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Hard Handoff �

• Early systems used a hard handoff

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• In a hard handoff, a mobile station only communicates with one base station

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• When the mobile station moves from one cell to another, communication must first be broken with the previous base station before communication can be established with the new one

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• This may create a rough transition

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Soft Handoff

• New systems use a soft handoff

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• In this case, a mobile station can communicate with two base stations at the same time

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• During handoff, a mobile station may continue with the new base station before breaking off from the old one

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Roaming

• One feature of cellular telephony is called roaming

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• Roaming means, in principle, that a user can have access to communication or can be reached where there is coverage. A service provider usually has limited coverage

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• Neighboring service providers can provide extended coverage through a roaming contract

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First Generation (1G)

• Cellular telephony is now in its fourth generation

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• The first generation was designed for voice communication using analog signals

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• We discuss one first-generation mobile system used in North America, AMPS.

AMPS

• Advanced Mobile Phone System (AMPS) is one of the leading analog cellular systems in North America
• It uses FDMA

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Bands

AMPS operates in the ISM 800-MHz band

The system uses two separate analog channels, one for forward (base station to mobile station) communication and one for reverse (mobile station to base station) communication

The band between 824 and 849 MHz carries reverse communication; the band between 869 and 894 MHz carries forward communication

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Figure 16.8: Cellular bands for AMPS

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Figure 616.9: AMPS reverse communication band

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Second Generation (2G)

• To provide higher-quality (less noise-prone) mobile voice communications, the second generation of the cellular phone network was developed

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• The second generation was mainly designed for digitized voice

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• Three major systems evolved in the second generation: D-AMPS, GSM, and IS-95

D-AMPS

D-AMPS was designed to be backward-compatible with AMPS. This means that in a cell, one telephone can use AMPS and another D-AMPS

D-AMPS was first defined by IS-54 (Interim Standard 54) and later revised by IS-136

Band

D-AMPS uses the same bands and channels as AMPS Transmission

Each voice channel is digitized using a very complex PCM and compression technique. A voice channel is digitized to 7.95 kbps. Three 7.95-kbps digital voice channels are combined using TDMA. The result is 48.6 kbps of digital data

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Figure 16.10: D-AMPS

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Figure 616.11: GSM bands

GSM

• The Global System for Mobile Communication (GSM) is a European standard that was developed to provide a common second-generation technology for all Europe
• The aim was to replace a number of incompatible first-generation technologies

Bands

• GSM uses two bands for duplex communication. Each band is 25 MHz in width, shifted toward 900 MHz
• Each band is divided into 124 channels of 200 kHz separated by guard bands

Transmission

• Each voice channel is digitized and compressed to a 13-kbps digital signal. Each slot carries 156.25 bits. Eight slots share a frame (TDMA).

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Figure 16.12: GSM

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Figure 16.13: Multiframe components

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Figure 16.14: IS-95 forward transmission

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Figure 16.15: S-95 reverse transmission

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Third Generation (3G)

• The third generation of cellular telephony refers to a combination of technologies that provide both digital data and voice communication

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• Using a small portable device, a person is able to talk to anyone else in the world with a voice quality similar to that of the existing fixed telephone network

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• A person can download and watch a movie and listen to music, surf the Internet or play games

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• The third-generation concept started in 1992, when ITU issued a blueprint called the Internet Mobile Communication 2000 (IMT-2000)

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Figure 16.16: IMT-2000 radio interfaces

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16.2.5 Fourth Generation (4G)

The fourth generation of cellular telephony is expected to be a complete evolution in wireless communications

• Some of the objectives defined by the 4G working group are as follows:

a. A spectrally efficient system.

b. High network capacity.

c. Data rate of 100 Mbit/s for access in a moving car and 1 Gbit/s for stationary users.

d. Data rate of at least 100 Mbit/s between any two points in the world.

e. Smooth handoff across heterogeneous networks.

f. Seamless connectivity and global roaming across multiple networks. g. High quality of service for next generation multimedia support

h. Interoperability with existing wireless standards.

i. All IP, packet-switched, networks

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The fourth generation is only packet-based (unlike 3G) and supports IPv6. This provides better multicast, security, and route optimization capabilities.

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• Access Scheme

To increase efficiency, capacity, and scalability, new access techniques are being considered for 4G. For example, orthogonal FDMA (OFDMA) and interleaved FDMA (IFDMA) are being considered respectively for the downlink and uplink of the next generation Universal Mobile Telecommunications System (UMTS). Similarly, multicarrier code division multiple access (MC-CDMA) is proposed for the IEEE 802.20 standard. Modulation More efficient quadrature amplitude modulation (64-QAM) is being proposed for use with the Long Term Evolution (LTE) standards.

Radio System The fourth generation uses a Software Defined Radio (SDR) system. Unlike a common radio, which uses hardware, the components of an SDR are pieces of software and thus flexible. The SDR can change its program to shift its frequencies to mitigate frequency interference

Antenna The multiple-input multiple-output (MIMO) and multiuser MIMO (MU-MIMO) antenna system, a branch of intelligent antenna, is proposed for 4G. Using this antenna system together with special multiplexing, 4G allows independent streams to be transmitted simultaneously from all the antennas to increase the data rate into multiple folds. MIMO also allows the transmitter and receiver coordinates to move to an open frequency when interference occurs.

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Thank You