Computer Networks (CS425)

Introduction

Why to use Wireless?

1. They are ubiquitous networks. As the do not require messy wires as a medium of communication, they can be used to connect far-off places.
2. They are cheaper than wired networks specially in the case of long-distance communication.
3. They are pretty effective and fast, especially with the modern advancements in this field.

Some Terms and Technologies:

ATM-Asynchronous Transfer Mode:
ATM is a connection-oriented switching technology. It was built to support ISDN (Integrated Services Digital Network). ISDN required high speed cables for both its narrow band (64 Kbps) and broad band (155 Mbps) transmission. There were two technologies available for transmitting data-

1. Circuit Switching: In this technology, when a user makes a call, the resources are reserved for him.  The advantage of this technology is that it prevents collisions among various users. But the disadvantage is that it leads to inefficient utilization of bandwidth-- if the user fails to send data or if the transmission speed is faster than the speed of sending data. then most of the bandwidth is wasted.
2. Packet Switching: In this technology, resources are never reserved for any particular user. The advantage of this technology is that it leads to efficient utilization of bandwidth i.e. the channel is never free until & unless there are no users, But the disadvantage is that it causes many collision.

1. CBR-Constant Bit Rate: Jobs that can tolerate no delay are assigned the CBR priority.  These jobs are provided same number of bits every frame..   For example, viewing a video reel definitely requires some blocks in every frame.
2. VBR-Variable Bit Rate: Jobs that may produce different sized packets at different times are assigned VBR priority. They are provided with a variable number of bits varying between  a maximum and a minimum in different frames. e.g.. a document may be compressed differently by different machines. Transmitting it will be a variable transmission.
3. ABR-Available Bit Rate: This is the same as VBR except that it has only the minimum fixed. If there are no CBR or VBR jobs left, it can use the entire frame,
4. UBR-Unavailable Bit Rate: These jobs are the least priority jobs. The network does not promise anything but simply tries its best  to transmit it.

Coupling of Networks:
The alternatives are:

1. WLAN              LAN
2. WATM             LAN
3. WLAN             ATM
4. WATM            ATM

1. WLAN-LAN is the simplest of the above. According to the IEEE standards, the IEEE 802.11 (WLAN)  can be used with IEEE 802.x (LAN) as follows:
2. WLAN-ATM- NOT FEASIBLE.
3. WATM-LAN- NOT FEASIBLE because WATM requires an infrastructure of the type ATM
4. WATM-ATM-this is also a simple scheme because WATM can run on ATM.

Issues involved in Wireless Networks

• Cost and Speed: As it is being considered as an alternative to wired networks, it should be faster and cheaper.
• Quality of Transmission: It gives a higher BER (Bit Error Rate). The BER is greater than 10  ^-6. This is caused because transmission quality depends highly on the physical media including landscape, weather etc.
• RayLeigh Fading: The data has to travel the distance through a medium like air. Several rays of the same stream cause Rayleigh fading due to interference. This causes poor transmission.
• Multipath Propagation: Similarly, due to multipath propagation, the signal received at the destination may be garbled.
• Hand-Offs: If hand-offs are used i.e., hexagonal cells each having a base station and many mobile terminals, two Mobile terminals that are far enough can use the same bandwidth. This reuse of bandwidth is helpful.
• Dynamic Physical Characteristics: The terminal may be mobile and constantly moving. Thus the distance between the base station and any active terminal may be constantly changing. This has to be taken into account while designing.
• Practical Implementation: The practical implementation of any wireless network requires CSMA/CD for proper transmission. The range of any terminal is fixed. So, there may be two terminals that are out of range of each other. These are called HIDDEN TERMINALS. Collisions may be caused due to simultaneous sending of data from any two hidden terminals.  The HIDDEN TERMINAL PROBLEM should be overcome with the help of Base Station.
• .Mobility and Network Topologies: Wireless networks should be effective enough to overcome the problems caused by the topology of the area and the mobility of the terminals
• Frequency Allocation: Licensed & Unlicensed: For licensed networks, permission has to be taken from the authorities that grant you a fixed bandwidth which is not used by anybody else while unlicensed networking does not require any such permissions. It just provides with some unlicensed bands which can be used by anybody. Unlicensed bands may thus, cause collisions.
• Capture Effect: If there are more than one terminals requiring the attention of the Base Station, the one nearer to the base station may capture it. This unfair access to the base station should be prevented.
• Power Requirements and Battery: This problem arises for the Mobile Terminals that run battery or cells. Much dissipation of power  is caused when switching from receiving mode to sending mode and vice versa.
• Human Safety: Not all bandwidths can be used . Also, the intensity should not be very high as it may lead to several complications in human body e.g.. cataract.

Wireless Physical Media

1. Transmission at Infrared frequency: This is easier to build and set-up. It is mainly used for indoor purposes because the beam has to be focussed and can't cross opaque media like walls etc.
2. Transmission through Microwave: This is preferred as it requires low power consumption. (the bandwidth is fixed)  But the basic problem is that it requires Line-of-Sight. Also, it requires license.
3. Transmission at Radio Frequency:  This is the one that is most familiar to us. The bandwidth is pretty large.

Integrity and Security of the signal

Spread Spectrum: To reduce the effect of noise signals, the bandwidth of the signal is increased tremendously. This is costly but assures better transmission. This is called SPREAD-SPECTRUM. This is used in two ways:

• FHSS (Frequency hopping spread spectrum): The entire packet is not sent at the same bandwidth. Say, it is sent at frequency range A for  time T1, frequency range B for time T2, A for T1, B for T2 and so on. The receiver also knows this sequence and so, looks at A for time T1, then at B for time T2 and so on. Thus this sort of understanding between the sender   and receiver prevents the signal from being completely garbled .
• DSSS (Direct Sequence Spread Spectrum): This involves sending of coded data  instead of the actual data. This code is known to the destination only which can decipher the data now.

MEDIUM ACCESS CONTROL

1. QoS Requirements:  It provides Quality of Service according to the priority of jobs.
2. Error Control: Error handling is done using some codes.
3. Frame Size: To transmit maximum data, we want the frame-size to be maximum but at the same time, large frame-size highly increases the probability of errors. So, MAC provides a tradeoff between the above two factors determining the size of the frame.
4. Secure Transmission:  The data meant for a particular receiver is secured from others.
5. Reasonable Transmission:  If the number of users increases, each should get reasonable service. MAC prevents unfair access to channel.
6. Efficient utilization of Power:  If a transmitter is always on, it is continuously using power even if there is no data on the channel for it. This is reduced by sending the transmitter to "sleep mode" whenever the battery is going down. In this mode, the transmitter is unable to receive any data.

Architecture for Wireless Network

1. AD-HOC NETWORK
2. INFRASTRUCTURE NETWORK

THE MAC PROTOCOL

1. Fixed Assignment Policy
2. Random Assignment Policy
3. Centrally Controlled Policy
4. Distributed Controlled Policy
5. Hybrid Controlled Policy

1. TDMA (TIME DIVISION MULTIPLE ACCESS) :  Each user is given a fixed time to speak., after which the chance goes to another user. This cycle continues indefinitely.
2. FDMA (FREQUENCY DIVISION MULTIPLE ACCESS): Each user is given a fixed bandwidth in which he can speak at all times.        
3. CDMA (CODIVISION MULTIPLE ACCESS): Each user is given different frequencies at different times. This ensures that each user gets a fair amount of channel each time.

Also, sometimes, statistical multiple access is used in which a slot is assigned to a user only if it has data to send.
 

Random Assignment Policy
In this  policy, contention slots are provided to all the users. Problem may arise if the number of users increase drastically. The number of contention slots should be variable. This may cause some limiting of data slots but is necessary to prevent the derailment of the service .
 

Centrally Controlled Policy:
This is used in an infrastructure architecture.  It involves the participation of a Base Station which may assign slots and priorities(CNBR,VBR etc.) to all the users.
 

Distributed Controlled Policy:
This is used in Ad-Hoc architecture. The control is among the terminals which decide among themselves about who is going to speak first.
 

Hybrid Controlled Policy:
This combines the best features of centrally controlled and distributed controlled policies.
 

KINDS OF MAC PROTOCOLS:

1. FDD (Frequency Division Duplex) This provides two separate bandwidths for uplink and downlink transmission. This leads to inefficient utilization of bandwidth as there is  more  traffic on downlink than uplink
2. TDD (Time Division Duplex) This provides an adoptive boundary between the uplink and downlink frequency which depends on the what is being used at that particular time. It works as follows:

1. uplink-MT1 sends a random-access request to BS to communicate with MT2
2. downlink: BS sends a b-bit access id to MT2
3. uplink:  MT1 sends the packet
4. downlink: BS sends the packet to MT2