PhD Position in Information and Communication Technology.

Where: Sweden
Deadline: 2013-05-15.
Qualifications: The applicant is expected to hold before august 2013 an MSc degree in Optics, Photonics, Laser Physics, or similar. Experience with clean-room technology, experimental and programming skills (for numerical simulation) are an advantage.

Changing units from CM to inches in ruler in Ms-word 2007.

Sometimes when you change display option from Office Button >>Word Option>>Advance>>Display, you can’t see any changes in ruler. Mostly it is because of Asian language compatibility. 

If you have this problem, you can correct this by doing following steps. 

• Go to Office Button (Left top corner, round button) 
• Click on WORD OPTION (Near the “Exit word” button)
• Click on ADVANCED in the left panel.
• Go to the DISPLAY.
• Go to the Show measurement in unit of  drop down menu with Inches, Centimeters, Millimeters, Points, or Picas. Selcet one of them (e.g. Inches)
• Click on OK

Now,

• Press ALT+F11. You can see the macro editor. 
  
• Press CTRL+G (or Go to View >> Immediate Window).  
• In this window, type vba code as follows:

          Options.UseCharacterUnit =False

Then press Enter.

Now, it’s done. You can see your ruler in Inches.

Good luck. 

Why does Snoop suppress the duplicate TCP acknowledgements it receives after retransmitting lost packets?

Snoop suppresses the duplicate TCP acknowledgements to avoid triggering end-to-end TCP retransmissions and congestion control. A Snoop agent maintains state for each TCP connection traversing the wireless gateway. TCP data packets sent from the wired to the wireless host are cached locally, until TCP acknowledgments from the wireless host verify that they were received. When duplicate acknowledgments arrive, indicating that a packet was lost, the packet is retransmitted by the agent from its local cache. The duplicate acknowledgments are then suppressed. This avoids conflicting local and TCP retransmissions by suppressing duplicate TCP acknowledgments while local error recovery.

Why TCP without Selective Acknowledgments can only recover from a single packet loss per round trip time?

Even though multiple packets lost in the same transmission window, the sender considers the fist packet that was lost from the duplicate acknowledgement it received. After retransmission of the lost packet, the sender has to wait for the acknowledgements from the receiver for the next lost packet. So, if Selective Acknowledgements is not implemented TCP can only recover from a single loss per RTT. The Selective Acknowledgment option for TCP allows each acknowledgment to specify up to three contiguous blocks of data that have been received beyond the last packet in sequence. The sender can thus infer which packets have been lost and retransmit them without waiting for additional duplicate acknowledgments.

With some Split TCP schemes, a data packet can be acknowledged without first reaching its destination. Why is this a problem?

It is a problem with some split TCP scheme because it is a kind of violation of end-to-end TCP semantics. In fact, an acknowledgment originating from the wireless gateway may reach the sender before the corresponding data packet reaches its destination. In case, if the gateway crashes after the acknowledgment has been returned to the sender but before the data packet has reached the receiver, the sender will incorrectly assume that the packet has reached its destination safely. 
There is another issue with some split schemes is that wireless gateways face significant overhead as packets must undergo TCP processing twice.

Which are the main factors leading to TCP throughput degradations over wireless links?

One of the main reasons of throughput degradation over wireless links is transmission errors. Only few packets are corrected by frame error correction codes but more packet errors are assumed as a corrupted packet and discarded. These packets are considered as a lost packet by TCP and congestion window size is reduced as TCP takes packet loss as a sign of network congestion, in reality it may not be due to the congestion. This unnecessary reduction of congestion window may lead to the throughput degradation. 

Another reason of throughput degradation over wireless links is the Frame Error Rate (FER). It suffers from FER of 1.55% when transmitting 1400 byte frames over an 85 foot distance, with clustered losses. Reducing the frame size by 300 bytes halves the measured FER, but causes framing overhead to consume a larger fraction of the bandwidth. In shared medium WLANs, forward TCP traffic (data) contends with reverse traffic (acknowledgments). In the WaveLAN this can lead to undetected collisions that significantly increase the FER visible to higher layers. File transfer tests over a WaveLAN with a nominal bandwidth of 1.6 Mbps achieved a throughput of only 1.25 Mbps. This 22% throughput reduction due to a FER of only 1.55% is caused by the frequent invocations of congestion control mechanisms which repeatedly reduce TCP’s transmission rate. If errors were uniformly distributed rather than clustered, throughput would increase to 1.51 Mbps. This is consistent with other experiments showing that TCP performs worse with clustered losses.

Why is TCP performance more affected by losses over high-delay and high-speed network paths?

TCP recovers from errors through end to end retransmissions that is the reason why recovery is slower in high delay paths. Further, because it takes longer to reach the peak throughput supported by higher speed links TCP drastically reduces its throughput after each loss.

The CSMA/CA access protocol does not detect collision as CSMA/CD does. How does this limitation affect link-layer and transport-layer performance?

In wired network, when two transmitters start simultaneously collision occurs and that can be detected by transmitters. Then they abort their transmission and retry later. However, in wireless networks transmission and reception is done in the same band that makes collision detection difficult. Hence, the WLAN use CSMA/CA to avoid collision. CSMA/CA, is listen before talk mechanism, where transmitters wait for a random interval after the medium becomes idle before starting transmission. The transmitter that starts first seizes the medium and all others back off, there by reducing the change of collisions. However, there may be collision that is not detected by CSMA/CA and corrupted frames not retransmitted by CSMA/CA are considered by the link-layer and transport-layer. Link layer is responsible for flow control, acknowledgement and error recovery. It also species which mechanism are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the sender and receiver. To recover from wireless losses, including undetected collisions, transmitters may need to optionally ask for acknowledgements to retransmit transparently unacknowledged frames at the link layer. Further, in some cases, wireless transmission errors are not related to network congestion but due to corrupted packets that not handed over to TCP. TCP assumes those packets are lost and reduces its congestion window size that leads to throughput degradation.

New Google

Thesedays, Google is a synonyms for search engine. People say googling for search in the internet. Many new innovative (not sure how innovative they are though) search engines are showing their existence in the market. 

Today, I received an email from SONSIK yahoo group member mentioning that the search engines that we are mostly using Google and Yahoo, there is a new searching engine promoting by Nepalese. The mail requested to try the search engine. I tried it and searched some stuffs too. I read the About Us column. It seems that basically the search engine is operating from US. It has offices in both country US and India. The promoters are seems to be Nepalese and Indian origin. The official blog is written in January 28, 2009, looks pretty new. 

The name of the search engine is given Sagoon. Although the GOO of Sagoon looks like taken from Google, it’s not true. It’s derived from Sanskrit which means Auspicious or ushering good results. In News section there are two language changing options – Nepali and Hindi. It can be assumed that this search engine is targeted to the Nepali and Indian users. History shows that Google and Yahoo are not big success in non-english speaking countries like China and Korea. So this search engine can be very popular as a regional search engine. 

The founder of the company Sagoon Inc. Govinda mentions in the blog that it’s the beginning of their journey, a journey with bigger challenge. Of course, as there are many Big Bosses in search engine industries occupying large market shares already, it might be challenging for Sagoon. 

Anyway, finger crossed to Sagoon team for their success. 

www.sagoon.com

Why TCP modeling?

There are certain things we can achieve with TCP modeling that cannot be achieved with simulation. Some of them are as follows. 

• We can model some uncertainties using stochastic process that cannot be determined by simply simulation. For example, number of current TCP connections, how much bandwidth is occupied by the non TCP transport protocols and what is the effect of those in TCP etc. 
  
• TCP operating scale is very large in terms of geographical scale and the number of inputs/outputs. To simulate the large number of inputs/outputs and geographical scale is not easy due to hardware constraints (some times budget constraints as well).
  
• Mathematical models are required to quantify metrics that define system performance and develop control strategies that result in the optimum being achieved.
• It also helps to determine how close it is to achieving a system optimum, as defined by a specific cost function, and subsequently identify issues that need to be addressed in future control protocol. 
  
• TCP is already mostly used transport layer protocol in the world. To test some optimization in it is not easy to implement in real world scenario and observe its performances. So, before implementation any optimization patches it is very much necessary to model it. Some examples of those patches are Fast Retransmission and Fast Recovery. These were not in the very first version of TCP.
  

The role of the "gateway" option of tcpdump

‘Gateway’ in tcpdump specifies a name of gate that transfers the packet. We can use gateway to monitor traffic from specific TCP/IP applications at strategic locations of a given TCP/IP network. For example to print all ftp traffic through snup gateway the command is as follows:
#tcpdump 'gateway snup and (port ftp or ftp-data)'

To print IP packets longer than 576 bytes sent through gateway snup the command is:
#tcpdump 'gateway snup and ip[2:2] > 576'

Disadvantages of IP fragmentation?

Some disadvantages of IP fragmentation are: 
A. Once packet is fragmented it can be reassembled only at another end. If there is larger bandwidth on the way, it cannot take an advantage of that. 
B. The overhead is large when IP fragmentation is used. IPv4 does not restrict fragmentation and can be performed at any link along the path. This can result in heavy processing demands on the routers in the network. (Not in IPv6).
C. There are some security issues with fragmentation. (e.g. Tiny Fragment Attack and Overlapping Fragment Attack)

Situations when RTO occurs even though packets reach receiver

When the ACK sent by the receiver lost and sender could not get the ACK then the RTO occurs. If there is a delay in arriving ACK to the sender due to some reasons (as described below) than RTO occurs even though packets reach receiver. For example, on a low-bandwidth link the arrival of competing traffic (possibly with higher priority), or some other change in available bandwidth, can cause a sudden increase of the RTT. This may cause a RTO. If the communication takes place from a low latency path to a high latency path, it may suddenly increase the RTT beyond the current RTO value. 

Why is the ISN (initial sequence number) of TCP selected randomly?

TCP layer on the TCP devices exchange Initial Sequence Number (ISN) that is assigned to any device during the startup of the TCP connection. The ISN is always random. There are several reasons to select ISN randomly. One of the reasons is that TCP segments may get mixed up with the different connections if the sequence number is started from the same number by all the devices. For example, a TCP connection is started with ISN 1. Let’s say there are 1 to 50 bytes. Suppose that there is some problem and the TCP connection is terminated. Now a new connection with the same ISN (i.e. 1) is started for the same number of bytes (i.e. 1 to 50 bytes). The device may think that these segments are the part of new connection. So, the data may get collapsed. 

The another reason to select ISN number randomly is any malicious person could write code to predict the ISN of subsequent TCP connection based on the ISNs used in earlier ones, which causes serious security risk. 

How an efficient TCP/IP implementation can extend the battery life of a mobile device?

Efficient TCP/IP implementation can reduce packet loss rate, data transfer time and resource consumption. Retransmissions of the packet eat up most of the battery power unnecessarily. If the packet loss can be reduced by the efficient flow control and congestion control mechanism then the packet retransmissions reduced significantly that conserve battery energy of a mobile device.

Which of the following applications can run satisfactorily ?

One-way delay measurement of a corporate TCP/IP network spanning two distant sites reveals that the mean one way delay is 500 ms. Which of the following applications can run satisfactorily over this network?
(a) Voice-over IP
(b) File Transfer
(c) Web Browsing
(d) E-mail
(e) Video on demand

Answer: 

Following applications can sun satisfactorily over given network: (a) File Transfer (b) Web Browsing (c) e-mail. 
The reason VOIP and VOD may not run satisfactorily is that the tolerable delay of these applications are several milliseconds only. e.g. <= 400 ms for VOIP. 

WiBro vs. WiMAX

WiMAX, is an acronym of Worldwide Interoperability for Microwave Access, is a telecommunications technology provides wireless data over long distance. It is based on the IEEE 802.16 standard, also called WirelessMAN. According to WiMAX forum WiMAX is a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL.  

WiBro (Wireless Broadband) is a wireless broadband Internet technology being developed by the South Korean telecoms industry. WiBro is the South Korean service name for IEEE 802.16e (mobile WiMAX) international standard. Actually, WiBro was developed to overcome the data rate limitation of mobile phones (for example CDMA 1x) and to add mobility to broadband Internet access (e.g. ADSL or Wireless LAN). WiBro adapts TDD for duplexing, OFDMA for multiple access and 8.75 MHz as a channel bandwidth. WiBro base stations offer an aggregate data throughput of 30 to 50 Mbit/s and cover a radius of 1-5 km allowing for the use of portable internet usage. It provides mobility for moving devices up to 120 km/h.

Advantages of WiBro over WiMax:

WiBro provides mobility for moving devices up to 120 km/h compared to WLAN having mobility up to walking speed and Mobile Phone having mobility up to 250 km/h. It also offers QoS. The inclusion of QoS allows for WiBro to stream video content and other loss-sensitive data in a reliable manner. 

Future trends for Cell phones, PDAs and Mobile Internet Devices

These days, our cell phones can take and send pictures/video. We can text message, download ringtones, watch streaming music video and even download and play mobile games. Not only that we can watch DMB and surf internet as well. The services available now couldn't even be imagined by most of us 10 years ago. The rapid advancements in information and communication technology have made it possible to do all these things. Before lunching 2G in Finland for the first time in 1991, today’s revolutionary 3G/4G communication were out of imagination. 

PDA is a handheld computer having many uses: calculation, use as a clock and calendar, accessing the Internet, sending and receiving E-mails, word processing, use as an address book, making and writing on spreadsheets, playing computer games and Global Positioning System (GPS). PDAs can access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). Many PDAs employ touch screen technology. 

Mobile Internet Device (MID) gives an internet experience in our pocket. MID represent a portable and mobile devices category that gives wireless connectivity and long battery life. Samsung Q1, is an example of Mobile Internet Device, comes with a 7" (18 cm) LCD and exists in several different versions.

However, people are beginning to appreciate the value of connecting to the internet from their portable mobile devices. Carrying multiple devices is not convenient, so they prefer single device having versatile capabilities. New generation cell phones (smart phones) are capable of performing PDA’s tasks. Today most smart phones sold worldwide incorporate at least one advanced feature. Competition among phone manufacturers is primarily based upon a race to add additional features within accepted price ranges. The smart phones (with 4G+ or future technological standard) in the near future will bring the complete solution for the user by providing all in one solution.

The iPhone is a one example of the media based mobile device. Mobile companies they are selling iPhones are generating revenue not only by selling a phone plan, but also charging on bandwidth for audio and video downloads. Also, they are making money through mobile marketing.

Technically, in my opinion, in the future cell phones will dominate the future internet. Because IEEE 802.11 is not channelized, so control packets, such as RTS, CTS, and ACK, use the same channel as data packets. This leads to complex arbitration schemes and potentially unfair bandwidth allocation due to hidden and exposed terminals. Cell phones use channelized media, which intrinsically share the wireless medium better and are immune to a variety of hidden terminal problems. Moreover, unlike a WiFi-based PDA, a cell phone cannot be blocked from accessing the channel because the data channel is hogged by another cell phone. For these reasons, it appears that a channelized cell phone may better use wireless spectrum than a WiFi-based laptop or PDA. Incidentally, cell phone spectrum is licensed, so it is also immune to interference from cordless phones and microwave ovens that occupy the unlicensed ISM bands.

Generally, these handheld devices will become powerful computing devices, comparable to desktop systems. But there will be no market for standalone, handheld devices like only PDA, MID or Cell phones. Growing popularity shows all portable devices including cell phones, PDA and Mobile Internet Devices will come in one device.

2G and 3G cell phones and Internet

2G cell Phones: 2G stands for Second Generation Wireless Telephone Technology. 2G networks were analog but its predecessor 1G networks were analog. 2G technology came with text messaging (SMS) capabilities.

3G cell Phones: 3G is the third generation of mobile phone technology provides a wider range of more advanced services while achieving greater network capacity through improved spectral efficiency. This technology also offers wide-area wireless voice telephony, video calls, and broadband wireless data, all in a mobile environment. Also includes HSPA data transmission capabilities able to deliver speeds up to 14.4Mbit/s on the downlink and 5.8Mbit/s on the uplink.

Internet: Internet is a "network of networks" that consists of millions networks, which together carry various information and services, such as electronic mail, online chat, file transfer, and the interlinked web pages and other resources of the World Wide Web. Internet can be accessed in both wired and wireless environment. IEEE 802.11 is a standard for WLAN. The term 802.11b and Wi-Fi are often used interchangeably. IEEE 802.11 (Wi-Fi or WLAN) networks are short range, high-bandwidth networks primarily developed for data communication.

In the following prospective 2G, 3G and Internet are same:

• All are wireless technologies
• Both offer broadband data service

In the following prospective 2G and 3G, and Internet are different:

• From the users prospective 2G is for voice communication with some messaging service, 3G is mostly voice and then data service and Internet is mostly for data communication service with voice capability.
• 2G/3G and Internet’s network structure is different.
• Current business models/deployments are different. 3G services provided by Mobile operators where as WiFi provided by data communications industry or we can say it’s a byproduct of the internet industry.
• Spectrum policy and management: (2G and 3G use licensed spectrum but Internet uses unlicensed shared spectrum. This has important implications for (1) cost of service; (2) quality of service (QoS) and congestion management; and (3) industry structure.)
• 3G offers better support for secure/private communications than others.
• 3G has a relatively small family of internationally sanctioned standards, collectively referred to as IMT-
• 2000.36. In contrast, Wireless Internet is one of the families of continuously evolving 802.11x wireless Ethernet standards, which is itself one of many WLAN technologies that are under development.
• Getting license for 3G is difficult than 2G.
• 3G is more developed than WiFi as a business and service model.

All of them have some advantages and disadvantages. 2G requires lower powered radio signals and consumes less battery power, so phones last much longer. 2G (without GPRS) is not much capable for data service. 3G is very reliable in voice communication with a wide range of more advanced services with improved spectral efficiency. But data communication with 3G phones is expensive comparing to general internet service. It consumes more power than 2G cell phones and it is not fully IP-based integrated system. Internet is basically for data communication with some voice communication capabilities. Of course, it is less expensive than the general telephony but still internet is not preferred for the voice communication.