Answers to Questions in Data Communications

Posted under Technology by Roman

These are basic questions thats most people should know before going into a meeting on the subject.

Compare guided and unguided mediums with examples.

Guided mediums are used for point-to-point connections between two devices while unguided mediums allow signals to be sent out uniformly across multiple areas reaching a wide variety of systems.

Compare and contrast half-duplex and full-duplex communications.

Full-duplex is when multiple systems can send or receive data at one time. Half-duplex only allows one system to either send or receive at one time.

Define a digital signal

Digital signals use bits of data represented by 0s and 1s to transmit data across a medium. They are cheap, easy to implement, and less susceptible to noise, unlike their analog counterpart. They are susceptible to attenuation, however.

Define peak amplitude

It is the maximum displacement of a sine wave in an analog signal from the median. On a sine wave graph, it is the wave with the maximum displacement.

What is the frequency range for audio signals?

Typically from 20Hz to 20kHz.

Define Shannon capacity and what is it a function of

Shannon capacity shows the relationship between the rate of data transmission and the amount of noise that can affect transmission. It states that the faster data is transmitted, the more susceptible it is to noise. It is a function of channel capacity.

Give the mathematical representation for NYQUIST bandwidth

C = 2Blog2M

Give the mathematical representation for the theoretical maximum capacity of a channel

C(apacity) = B log2( 1+ SNR)

Explain frequency shift keying (FSK)

This converts digital data into analog data. It maintains the same amplitude but changes the frequency.

How do you convert analog data into a digital signal?

Sampling

Explain sampling theorem and what is the sampling frequency of the telephone system?

It states that the sampling frequency must be twice as high as the highest frequency in the signal in order to create a sample where the original signal can be recovered from. The telephone system has a frequency range between 300Hz and 3.2kHz.

How do you convert analog signal to analog data (A to A)

Modulation

What is the need for protocols

Protocols are needed for standardization and to ensure a reliable connection between systems with minimal confusion. It allows for easy implementation.

Explain the principle of layering

Layering separates tasks into different layers allowing protocols to be much more scalable and easier to operate. It also simplifies the data transmission process.

Explain the TCP/IP architecture in detail and with a diagram

The layers are application, transport, internet, network, and physical. It is a standard developed by DARPA utilizing layering and a standardized protocol architecture.

Explain the 7 layer OSI model and give the chief functions of each layer

Application – interfaces directly with the application in use

Session – provides directory assistance and helps transport layer communications

Presentation – encrypts and compresses data

Transport – ensures data integrity and splits data into packets

Data Link – provides error detection and correction service

Network – provides routing information for packets

Physical – allows communication between the system’s physical interface and the network

Why do we need LANs?

LANs are needed to enable the sharing of data and resources over a small area, such as an office.

Explain the bus LAN topology

Bus – systems are linked together and all systems hear messages being sent out; only take what belongs to them and discards the rest

Explain the star LAN topology

Star – systems are connected to a single node and communicate to other systems through it

Explain the MAC in the LAN protocol architecture

MAC in LAN protocols handles the assembly and disassembly of packets. They also give access to the transmission medium.

Who invented Ethernet?

Xerox

Explain the concept of random backoff and retransmission over an Ethernet connection

When two packet collide along a medium, the packets are bounced back to their original senders and will be retransmitted at a random interval to avoid another collision.

Explain CSMA/CD

Carrier Sense Multiple Access/Collision Detection – allows for packets to back off and retransmit after a collision. Not scalable or deterministic.

Explain the concept of bridges

Bridges allow a single LAN to be connected to other LANs or WANs. It takes any packets designated for other LANs and transfers the packets to the other LAN.

What is the main advantage of WLAN over fixed connections?

WLAN allows for a significant amount of mobility for connected systems since it does not require wires to be connected to a system.

Compare and contrast infrared and WLAN

Infrared uses an invisible part of the light spectrum to transmit data. It requires line of sight and good working conditions, unlike WLAN. WLAN uses RF and is much more robust.

Explain the (basic service set) BSS architecture

Infrastructure – allows for communication through an access point only

Ad-Hoc – allows peer to peer communication without an AP

Explain the concept of a wireless router

It functions as a MAC layer switch on the internal network and helps this internal network interface with external networks like the Internet. It also acts as an access point with its wireless capabilities.

Why is CSMA/CD inadequate and explain the possible solutions?

CSMA/CD doesn’t work well because it is difficult to detect collisions in a wireless environment. CSMA/CA is a possible solution because it aims to prevent collisions altogether. If it senses that the medium is busy, it retries at a random time interval to prevent packet collisions.

Explain Bluetooth technology

Bluetooth is a wireless standard used in many consumer electronics for short range communication using the 2.45GHz band.

What are the main advantages of using WiMAX?

WiMAX can be used over very long distances with a good data rate unlike current wireless standards. It has the potential to displace traditional Wi-Fi and cellular networks.

What are the chief characteristics of routing in packet switched networks?

Routes are usually selected using several performance criteria such as minimum hop or least cost. Stability, correctness, and robustness are also preferred.

Explain the various performance criteria for routing

Minimum hop - fewest required connections

Least cost – high speed links, more bandwidth, stability

Explain fixed routing

Uses a single permanent direct connection between source and receiver. Simple but inflexible.

What are the properties of flooding?

Flooding is when packets are sent to every computer connected in a network. Eventually the correct recipient will get the message or multiple messages. The duplicates are discarded and other computers ignore the packets.

What are the main advantages of adaptive routing?

Routing decisions can be made on the fly and en route so that a packet can get to its destination despite problems that may crop up on the transmission route. It is useful for avoiding network failures or congestion.

Explain the least cost algorithm

Minimize hops and overall link costs. Calculates the cost as the paths between two nodes and sums it up to the destination.

Explain survivable routing

Switch routes of packet around network congestion or network failures to ensure data gets to its destination.

Explain internet working before the concept of IP

There were many different networks running different types of protocols. There was no global addressability or application layer gateways. Data had a hard time going from one place to another.

Explain Class B IP addressing with the header

1-0 class identifier. 14 bits for the network and 16 bits for the host. 2^45 network addresses and 2^16 hosts.

Define subnet

Subnets are mini-networks that allow connected devices within a network to interact with each other without going through a router.

What are the various ways the host machine can get an IP address?

IP addresses can be hard coded into a file located on a central server by an administrator. This is rarely used anymore and has been displaced by DHCP (Dynamic Host Configuration Protocol). This allows connected systems to dynamically retrieve IP addresses from a central DHCP server.

Explain the concept of DNS

DNS (Domain Name Service) is an application-level protocol that helps translate domain names into computer-readable IP addresses. It uses a distributed, hierarchical database with 13 root servers worldwide.

List the main steps that occur when a host machine looks up www.poly.edu

First the client would query the root DNS server to get to the .edu DNS server. The .edu DNS server would rout the client to the poly.edu DNS server where it can retrieve the appropriate IP address.

What is the main problem with the internet today?

It is difficult to deploy internet-wide applications, hard to remedy end-to-end problems, and there is no global addressability. We are also running out of IP addresses to give out to devices.

How is IPv6 and improvement over IPv4?

IPv6 offers and expanded 128-bit address space so there will be more than enough addresses for new devices. It is more flexible and scalable, better resource allocation capabilities, and improved auto-configuration.

Explain the role of users for enabling the internet as a platform for community building

The internet allows users to better interact with themselves easily and efficiently. Users are also able to develop innovative new ways to work and network with each other as well. Social networking was developed by an internet entrepreneur and allows users to network, meet each other, and maintain friendships easily. The internet is an infinitely flexible platform for users to find new ways to interact with each other.

In your own words, describe the implications of social networks for managers of big and/or small organizations.

Managers will need to rethink the way firms are structured and how innovation is fostered. Managers need to realize that much of this innovative spirit is taking place in social networks rather than within the company. This is the shift of the locus of innovation away from the firm.

Explain the key functionalities of any one social network you choose

I don’t want to type this up. Roman, you can make up a buttload of BS about MySpace here.

What are the key business opportunities offered by social networks?

Social networks represent and opportunity for companies to tap into a market that is usually regarded as being very fickle. Marketers need to understand social networks in order to better target their advertising and products to the primary users of social networks, the Gen Y crowd.

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