Introduction [Under GPRS]
Both ISUP (seen in ISUP article) and GSM signaling (seen in GSM Voice call articles) sets up transmission channel through which then voice data is transmitted and received. For the period of connection, the channel remain dedicated for the connection. This makes sense for voice call where data keep getting transmitted. But what if we want to use this transmission channel for purposes other than voice call, say for accessing internet ? To answer it, we must look at differences between two types of data (or bearers).
First and foremost: voice data comes constantly and with known data rate (e.g. 64 kbps for best reproduction of human voice as per Nyquist criterion). Internet data is not necessarily constant. In fact, it is bursty in nature. When you access a website, the page content size is known and available for transmission all at once (unlike voice call). After current page is transmitted and till the next page is accessed, no transmission is required.
Second is quality of transmission. In case of internet data, every character is to be transmitted with no error. If error is found, re-transmission may be needed if the error can not be corrected at receiving side. Due to no limitation of constant bit rate, in fact, re-transmission is possible. For voice data, this is not the case. Voice data need not be so much error free. Few bits lost or corrupted do not cause considerable damage to voice reproduced at receiving end (depending on modulation or compression techniques applied). And also with retransmission, constant bit rate may not be maintained.
Looking at it, it seems we can still use dedicated transmission channel for internet data if we put error (and flow) control at both sides. But there are disadvantages:
Internet data is bursty in nature, so transmission will not be needed at all times. In other words, more wastage or less efficient utilisation of transmission media if we use dedicated channels.
Also, the data rate will be limited to maximum possible to that of transmission channel. Only way to increase the date rate would be to setup additional transmission channels.
In conclusion, we need another way of using radio resources best suited to internet traffic.
Not only radio resources (access stratum), network side (non-access stratum) too need to be enhanced to tunnel these data to Internet (in both signaling and transmission planes).
Note on bursty traffic: The best way to transfer bursty data is in terms of packets. Internet protocol, TCP/IP has been designed for bursty traffic and operates in terms of packets. Another example of bursty traffic is signaling data.
Diagram below depicts overall our requirements.
Access network will target at access and management of radio resources. Core network will provide connectivity with Internet, other IP or packet based networks.
References: GPRS in Practice by McGuiggan, GPRS whitepaper from Christian Bettstetter et al.
© Copyright Samir Amberkar 2010
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