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In the last post on cellular network, you saw that the geographic area of a cell phone service provider is divided into cells - roughly hexagonal shaped areas that serve a particular region. The cell site provides coverage for that particular cell area. This results in the use of quite an interesting technique called handoff.

What happens when the caller (or the called) travels from one cell to the adjacent during a call? How does the network handle it? How does the network know which cellular network tower should be used to keep the connection between the caller and the called in order to continue the call?

The SIM card in a cell phone will be constantly identifying its location back to the tower. It will be keeping track of it and storing it in the LAI. A combination of the location and the usually the strength of the signals from the adjacent towers, helps the network make a handoff of the call. The moment a caller crosses the imaginary boundary of a cell and enters an adjacent one, handoff takes place, and the tower of the entered cell starts to handle the call.
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Ever wondered why mobile phones are sometimes called ‘cell’ phones? What put the ‘cell’ in the cell-phone?

In case you want to find out, read on.

A cell phone works on the back bone called the cellular network. Hence, the name cell phone - a phone that works on a cellular network. Yes - why is a cellular network called a ‘cell’ network? We will get to that, but before that, let us look at how a call you make from a cell phone to another cell phone work.

When you make a call from your cell, the cell phone transmits a request to the base station of the cell site. It requests for a call to be placed to another cell phone. When the called cell phone is with in its range, then it allots a channel and establishes the connection between the caller and called cell phone (number).

The call proceeds and the channel remains alloted to the call until either of the party breaks the call. When the call ends, the channel goes back to the pool from which the network assigns it to other requested calls.

This was a simple straight forward case. Both the caller and the called were with in the same cell site range. The range depends on the kind of network. If it is GSM network, then the range is around 40 km. If it is CDMA network, then the range is around 60 km. Any network may be the case, but the range is limited. Even a 100 km radius is not sufficient to encompass all the subscribers.

So, what happens when the called number is outside the range of a particular transmitter of a cell site?
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I have not used Floppy Disks a lot - but I have had my share as I started of my experience with computers in CLIs. Over the years, the scope of data storage from a consumer perspective has grown in scale - from floppy disks to TeraDisks. Not long back, Vinyas had written about Blu Ray disks, and now Sourjya writes about TeraDisks in the near future.

Let us look at how the storage was used by people, how they are used presently, and what would the TeraDisk mean in that perspective.

Floppy Disks

There were two types of floppy disks. The partially flexible 5.2″ floppy disks that could store 1.2 MB of data. And then, there was the 3.2″ floppy disk that could store 1.4 MB which came in a sturdy plastic casing. This was quite enough during those times (back in late 90s) when the OS could be booted out of a floppy disk. It was quite sufficeint for the restricted consumer market. At most, it was used for storing RTF (Rich Text Formatted) documents, and spread sheets. The more common use was to store documents in vanilla ASCII format. The The Adventures of Sherlock Holmes by Sir Arthur Conan Doyle amounts to just around 576 kB and contains 12 novels. This is not the usual size of the documents that people would work with. It would range around 1 kB to 10 kB. That meant, one could store around 100 documents in a single floppy disk.
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Yes - we are all in a rather excited mood after engadget reported that iPhone could be unlocked and we could use other SIMs than that provided by AT&T. I am not a iPhone user, and I do not much care about it. But it made me remember of the days as communication engineering student, and I thought I would share it here.

What is SIM?

SIM or Subscriber Identity Module is a IC (Intergrated Circuit) chip that is used to identify a GSM subscriber with in a network. GSM is considered the second generation in mobile phone telephony protocol - more on that in a later post.

Pushing aside all these technical lingo aside, what does this mean to us users? SIM allows us to change a phone - the device without changing our number, with out changing our subscription to a particular network. It also means that, it allows us to change our network and still keep using our phone.
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In the last post of the series, we looked at the linked list - the basic form of data structure. In this part, we will look at the specialized type of a linked list - a queue.

The queue is nothing but a linked list, and the protocols to add and remove data from the queue are specified in a certain way. These specific constraints, when applied to a linked list, gives us a queue.

The queue of the software realm acts quite similar to the queue you find in real life. It is quite similar to the queue you find at movie theaters or bank teller counters. When people need to join the queue, they do so from behind - at the end of the queue. When they need to leave the queue, it is done at the starting point of the queue. The protocol is said to be ‘first in, first out’.

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Data structures give order to the abstract world of code (software). There are many types of data structure. Among them linked list is the most fundamental type of data structure. Using one or another form of linked list, other data structures (queues or stacks for example) can be created.

Let us look at the theory behind linked list. I promise to try and make it interesting.

The basic unit of a linked list is what is called a node. A node is something like a Lego brick, many of which you can combine to create something. As with the Lego brick, there are different types of nodes, but the most basic form contains a data element and a reference element.

• Data Element - As the name suggests, the data element holds the data. It can be anything from simple data like numbers or characters, to complex data like a record holding the details of a song in a music album.
• Reference Element - Reference element is of the node can either be empty or be containing the information of where the next node of the list is present. When implemented in machine code, it has the address within the memory where the next node is residing.

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