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IP/ATM
GGSN
SGSN and GGSN
geographically close Home
Intranet


Figure 4.51 Connection using a leased line or VPN

GTP

leased line
GRX (for Internet
international)
NSS NSS GGSN
ATM backbone
BSS leas
IP over Frame Relay
ed
GGSN
Microwave
Ethernet or line
SGSN
Lease Line
IP/ATM
GGSN
etc.
Home
SGSN and GGSN geographically remote
Intranet
End to End QoS more difficult to implement


Figure 4.52 GPRS roaming con¬guration
152 GENERAL PACKET RADIO SERVICE


connect the SGSN to GGSN but the transfer delays may not be constant and the data
rates attained will generally be low. It is possible for the mobile operator to come to
an arrangement with the ISP whereby the operator™s traf¬c has priority over standard
Internet user traf¬c. This in effect gives the appearance of a leased line but should be
much cheaper. An ATM, frame relay or X.25 network may also be implemented, or a
number of radio towers (which the operator already has) may be used as point-to-point
microwave links.
It can be seen from the above that the core network can get more and more complex,
depending on the actual implementation. The scenario of over-dimensioning the core net-
work is only cost effective when the SGSN and GGSN are physically located in close
proximity. To over-dimension a network covering many kilometres may not be econom-
ically viable, but microwave links using existing infrastructure may be used. Since the
GTP packets are carried between the SGSN and GGSN via UDP and not TCP, reliability
is a concern. Any error checking and retransmission will only be end-to-end (with possi-
bly very high delays) unless a method for doing this is implemented transparently under
the GTP protocol.
For international roaming it is likely that the operator will enter into a roaming agree-
ment, as is currently done with GSM, allowing a user to roam transparently. A GPRS
roaming exchange (GRX) network is used for this inter-PLMN connection.


4.11 OTHER CELLULAR HIGH-SPEED DATA
TECHNOLOGIES
4.11.1 High-speed circuit-switched data (HSCSD)
HSCSD is based around the existing GSM interface, allows a user to occupy multiple time
slots for data transfer and guarantees a certain number of bits per second across the air
interface and through the operator™s mobile network. Currently HSCSD mobile devices
support two or three GSM time slots; each of these time slots will offer 14.4 kbps, giving
a total throughput of around 40 kbps. It is possible to provide up to eight time slots to
a single HSCSD user; however, this will have an adverse effect on other GSM users,
introducing increased call blocking etc. The user is usually billed on time in a similar
method to a normal dial-up Internet connection. If a user downloads a web page in 10
seconds and then spends 5 minutes reading that page while remaining connected, they
will be charged for 5 minutes 10 seconds of connected time. The resources reserved for
the user can be asymmetric, allowing faster transfer in the downlink than in the uplink.
This is because it is anticipated that users will be downloading more information than
they upload. Since it is still based on a circuit switched system the HSCSD system does
not utilize the resources of the operator ef¬ciently for bursty packet data.


4.11.2 Enhanced data rates for global evolution (EDGE)
EDGE was originally seen as an evolutionary step for GSM and the acronym stood for
enhanced data for GSM evolution. However, the TDMA community has since embraced
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 153


EDGE as a 3G solution under the UWC-136 proposal and it now stands for enhanced
data for global evolution. It is seen as both a migratory step towards 3G standards such
as UMTS and also as a complementary technology which will support such networks
in the future. Like GPRS, EDGE uses much of the underlying GSM system, including
the existing frequency band that an operator has been allocated. UMTS, on the other
hand, does require new frequencies, which in many cases have been very expensive for
the operator to purchase. This section describes the new features and attributes that are
gained from EDGE. Figure 4.53 illustrates how EDGE, with its higher transfer rates, can
be used to ef¬ciently free up time slots, which can then be reallocated to other GSM users.
EDGE can be used in conjunction with both HSCSD and GPRS to provide higher data
rates for the subscriber. EDGE introduces nine new modulation and coding schemes over
the air interface, as shown in Table 4.15.
Four of these coding schemes use the standard GSM GMSK modulation technique and
the other ¬ve use 8 PSK. The introduction of the new modulation technique enables data
rates up to three times that possible with standard GMSK. It can theoretically support
384 kbps using all eight time slots. However, to take advantage of this system the BSS has
to be upgraded: generally this requires new transcoders in the BTS and software upgrades
to both the BTS and the BSC. The mobile devices also have to be capable of operating
with the EDGE air interface and signalling. When used with GPRS, EDGE is known as
E-GPRS and when used with HSCSD it is referred to as ECSD. The Abis interface for

GSM and GPRS

(a) VOICE VOICE VOICE VOICE VOICE GPRS 1 GPRS 2 GPRS 3




(b) VOICE VOICE VOICE VOICE VOICE FREE FREE EDGE

EDGE

Figure 4.53 Improved data capacity using EDGE


Table 4.15 EDGE modulation schemes
Coding Modulation Data rate
scheme type (kbps)
MCS-1 GMSK 8.8
MCS-2 GMSK 11.2
MCS-3 GMSK 14.8
MCS-4 GMSK 17.6
MCS-5 8PSK 22.4
MCS-6 8PSK 29.6
MCS-7 8PSK 44.8
MCS-8 8PSK 54.4
MCS-9 8PSK 59.2
154 GENERAL PACKET RADIO SERVICE


0 1 2 3 4 5 6 7


000 Information Training Information 000 8.25

3 bits 58 bits 26 bits 58 bits 3 bits 8.25 bits

156.25 symbols sent in 0.477mS

Figure 4.54 EDGE data burst

GSM consists of n — 64 kbps links that are broken up into 16 kbps time slots, so there is
a one-to-one correspondence with a time slot on the air interface. For an EDGE solution
there may be more data in one air time slot than will ¬t into one 16 kbps slot, therefore
the Abis now needs to allow dynamic allocation of bandwidth to support the air upgrades.
An EDGE burst structure is the same as GSM/GPRS consisting of 200 kHz spacing
and 156.25 bits per burst and taking 0.577 ms. However, one modulated bit (referred to
as a symbol) can actually represent three bits due to the introduction of an enhanced
modulation technique. The data burst is shown in Figure 4.54. A simple GSM burst
consists of 114 bits of information. This can be compared to EDGE, which with the same
burst can transport up to 348 bits due to the 8PSK modulation technique. When compared
to the GSM burst, it can be seen that there are no stealing bits, which are required in
GSM/GPRS to indicate that the burst contains either control or data. This is because the
stealing bits are encoded as part of the 58 symbol information parts. It can therefore be
seen that whereas a GPRS radio block can transfer 456 bits of data, an EDGE radio block
using GMSK (MCS-1, 2, 3 and 4) can support up to 464 bits and EDGE using 8PSK
(MCS-5, 6, 7, 8 and 9) can support up to 1392 bits. This data consists of higher-layer
data after convolution coding, CRC addition etc., and is not pure user data.


4.11.3 Modi¬cation to RLC/MAC
When compared to the GPRS RLC/MAC, it can be seen that there are slight modi¬cations
to the RLC/MAC header. In fact, there are actually three variations to the header format in
both the uplink and downlink for transferring RLC data blocks. However the RLC/MAC
header for control messages is the same as that for GPRS. The three header formats
are referred to as types 1, 2 and 3 and are used for the different coding schemes as
listed below:

• type 1 is used for MCS-7, 8 and 9
• type 2 is used for MCS-5 and 6
• type 3 is used for MCS-1, 2, 3 and 4.

There are a number of reasons for this, one being that with EDGE it is possible to transmit
two RLC blocks within a single radio block using MCS-7, 8 and 9 and the location of
these separate RLC blocks needs to be identi¬ed.
4.11 OTHER CELLULAR HIGH-SPEED DATA TECHNOLOGIES 155


The RLC/MAC window size has also been modi¬ed with EDGE. In a GPRS transfer
the maximum window size is 64 blocks. This means that if a block has been sent in
error then only 63 more blocks can be outstanding before a successful retransmission of
the errored block. This maximum can be easily reached using a mobile with multislot
capability and when the data transfer is stalled. Taking the higher data rates available
with EDGE into consideration this problem would become more acute. The window size
has therefore been increased to a maximum of 1024 blocks depending on the multislot
allocation being used.
By comparing the EGPRS RLC/MAC header formats for data transfer (Figures 4.55
and 4.56) with those of GPRS (Figure 4.24), it can be seen that there are a number of
modi¬cations. The header ¬elds are described below:

MCS-7, 8 and 9
BSN Type 1
BSN2 1
CPS BSN2
TFI RRBP ES/P USF

BSN1 PR TFI MCS-5 and 6
BSN Type 2
BSN1 CPS
1
MCS group specific
MCS-1, 2, 3 and 4
BSN Type 3
SPB CPS
1

Figure 4.55 EDGE downlink RLC/MAC headers


MCS-7, 8 and 9
Type 1
BSN2 BSN1

BSN2
Sp
PI RSB CPS
are
Spare
TFI Countdown value SI R

BSN1 TFI MCS-5 and 6
Type 2
MCS group specific CPS BSN1

Spare PI RSB CPS

Spare


MCS-1, 2, 3 and 4
Type 3
CPS BSN1
Sp
PI RSB SPB CPS
are

Figure 4.56 EDGE uplink RLC/MAC headers
156 GENERAL PACKET RADIO SERVICE


• Temporary ¬‚ow identity (TFI): this 5-bit ¬eld identi¬es the TBF within which this
block belongs.
• Relative reserved block period (RRBP): this 2-bit ¬eld speci¬es a reserved block that
the mobile device may use for a packet control acknowledgement message or a PACCH
block.
• EGPRS supplementary/polling (ES/P): this 2-bit ¬eld indicates whether the RRBP ¬eld
is valid or not. If this is 00 then the RRBP is not valid.
• Uplink status ¬‚ag (USF): the USF consists of 3 bits and is sent in all downlink RLC
blocks to indicate the owner of the next uplink radio block on the same time slot.
• Block sequence numbers 1 and 2 (BSN1 and BSN2): this works in a similar fashion to
GPRS. However, it is extended from 7 bits to be an 11-bit ¬eld in EGPRS, allowing
more blocks to be outstanding, i.e. a larger window size. It consists of the sequence
number of the RLC block within the TBF to identify missing blocks. As discussed and
shown in Table 4.16, it is possible using MCS-7, 8 and 9 for the EGPRS RLC/MAC
frame to carry two RLC blocks. Each of these will have its own individual block
sequence number. This is why the type 1 header format has BSN1 and BSN2.
• Power reduction (PR): this 2-bit ¬eld indicates the power level reduction of the current
RLC block as compared to the power of the BCCH.
• Coding and puncturing (CPS): this indicates the channel coding and puncturing scheme
used. For example, if the type 1 header was employed and the CPS ¬eld had the value
8, this would indicate that MCS-9 was used for both blocks 1 and 2 and that block 1
would use puncturing scheme 3 whereas block 2 would use puncturing scheme 1.
• Split block indicator (SPB): this is only required in header type 3 and is used for
identifying retransmissions.
• Countdown value (CV): this 4-bit ¬eld is sent by the mobile device to allow the network
to calculate the number of RLC blocks remaining in the current uplink TBF.
• Stall indicator (SI): this single-bit ¬eld indicates whether the mobile station™s RLC

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