EVOLVING MOBILE NETWORKS UMTS

EVOLVING MOBILE NETWORKS
While the history of mobile communications is long [1–3], and the background of mo
bile networks therebyx is also long, in this chapter we focus on the historic evolution in
terms of network architecture and services starting with 2nd generation (2G) mobile
systems. In particular we consider the development of the architecture of Global Systems
for Mobile Communications (GSM), since it is by far the most widespread mobile
system in the world today. This will provide the basis to cover the introduction of Universal
Mobile Telecommunication Services (UMTS) in relation to its Core Network
(CN) and radio architectures. The latter will in turn serve as the platform to present
UMTS Radio Access Technology, which is one the aims of this book.
1.1 THE GROWTH OF MOBILE COMMUNICATIONS
Today wireless voice service is one of the most convenient and flexible means of modern
communications. GSM technology has been at the leading edge of this wireless
revolution. It is the technology of choice in over 120 countries and for more than 200
operators worldwide. Current estimates are that by the year 2001 there will be around
600 million wireless subscribers (e.g. mobile telephone users), out of which more than
50% will depend on GSM technology.
As the wireless revolution has been unfolding, the Internet has also shown a phenomenal
growth simultaneously. The advent of the World Wide Web and web browsers has
propelled TCP/IP protocols into the main stream, and the Internet is widespread not
only in the corporate environment but also in households. Large number of consumers
have embraced the Internet and use it today to access information online, for interactive
business transactions, and e-commerce as well as electronic mail. Figure 1.1 illustrates
the growth in mobile and Internet subscribers.
The success of mobile communications, i.e. the ubiquitous presence it has established
and the emergence of the Internet point towards a tremendous opportunity to offer integrated
services through a wireless network.
One of the main market segments for wireless services besides corporate intranet/
internet access is the consumer sector. The availability of intelligent terminals1 or
multipurpose wireless telephones is already ushering a new era of the information age,
where subscribers can receive directly through GSM-SMS: news, sport updates, stock
quotes, etc. However, the progress of audiovisual techniques and the support for a Weblike
interface in a new generation of terminals, will push consumers to a new era of
multimedia communications with a focus on services rather than technology.
To support the growth of Internet type services2 and future demands for wireless services,
ETSI SMG and other standards bodies3 have completed or are now completing
specifications to provide a transition platform or evolution path for wireless networks
like GSM. Figure 1.2 illustrates the wireless data technology options.
The technology options in Figure 1.2 can be summarized as follows:

 14.4 kbits/s allows GSM data calls with a rate of 14.4 kbits/s per time slot, resulting
in a 50% higher data throughput compared to the current maximum speed of 9.6
kbits/s.

 High Speed Circuit Switched Data (HSCSD) aggregates symmetrically or asymmetrically
several circuit channels, e.g. 28.8 kbits/s for two time slots (2 + 2) or
43.2 kbits/s for three time slots (3 + 1).

 General Packet Radio Service (GPRS) enables GSM with Internet access at high
spectrum efficiency by sharing time slots between different users. It affords data
rates of over 100 kbits/s to a single user while offering direct IP connectivity.

 Enhanced Data Rate for GSM Evolution (EDGE) modifies the radio link modulation
scheme from GMSK to 8QPSK. Thereby increasing by three times the GSM
throughput using the same bandwidth. EDGE in combination with GPRS (EGPRS)
will deliver single user data rates of over 300 kbits/s.

 UMTS as 3rd generation wireless technology utilizes a Wideband CDMA or
TD/CDMA transceiver. Starting with channel bandwidths of 5 MHz it will offer
data rates up to 2 Mbits/s. UMTS will use new spectrum and new radio network
configurations while using the GSM core infrastructure.

E1 Link for ATM Physical Interface

E1 ATM Physical Interface


For information on testing E1 links

The E1 interface operates at 2 Mbps over coax cables, compliant with ATM Forum UNI specifications. It supports both PLCP and direct cell mapping and complies with the following standards: G.704, G.706, G.732. The interface has BNC connectors.

The E1 transmission link consists of 32 transmission channels (0-31), each of which is 64 Kbits/sec. The overall transmission rate is 2.048 Mbits/sec. Channels 0 and 16 are reserved for transmission management, while all other channels are used for payload. The payload bandwidth is thus 1.920 Mbits/sec. Since ATM uses 48 out of the possible 53 bytes for payload transmission, the net transmission rate becomes 1.738 Mbits/sec.

Channel 0 carries F3-OAM information, signals loss of frame or synchronization, and is responsible for transferring FERF and LOC messages. Channel 16 is reserved for signalling.

Direct Mapping

The direct mapping of ATM cells onto E1 transmission frames is specified in CCITT recommendation G.804. This specifies that ATM cells are to be carried in bits 9-28 and 137-256 (corresponding to channels 1-15 and 17-31).

The following is an illustration of the E1 frame format when direct mapping of ATM cells is used. The 53 byte ATM cell begins with a header and wraps around consecutive E1 frames.


PLCP Cell Mapping

The PLCP format for E1 is described in ETSI document ETS 300 213, where an E1 PLCP frame is specified as consisting of ten rows of 57 bytes each. Four bytes are added to the cell length of 53 bytes to provide the various overhead functions.

The E1 frame structure with PLCP cell mapping is illustrated in the following diagram:



A-bits
Separator bytes.

P-bits
Path overhead identifier.

C1
Pad bit counter.

M-bits
SIP layer 1 management information.

G1
PLCP path status.

B1
Bit-interleaved parity 8 (BIP-8).

F1
PLCP path user channel.

Z-bits
For future use.

Thirty of the available 32 E1 channels are used for transporting the PLCP frame. The remaining two channels are reserved for E1 framing and signalling functions. The PLCP frame is octet aligned to the channel boundaries in the E1 frame; thus the A1 octet of the first row of the PLCP frame is inserted into time slot 1 of the E1 frame.