FROM GSM TO LTE-ADVANCED: AN INTRODUCTION TO MOBILE NETWORKS AND MOBILE BROADBAND 2. GENERAL PACKET RADIO SERVICE (GPRS) AND EDGE ➤ GPRS (General Packet Radio Service) ➤ Enhance GSM to transport data in an efficient manner ➤ Enable wireless devices to access Internet ➤ EDGE (Enhanced Datarates for GSM Evolution) ➤ Further improve speed and latency 2.1 CIRCUIT-SWITCHED DATA TRANSMISSION OVER GSM ➤ GSM network was initially designed as a circuit-switched network ➤ All resources for a voice or data session are set up at the beginning of the call and are reserved for the user until the end of the call ➤ The dedicated resources assure a constant bandwidth and end-to-end delay time Figure 2.1 Exclusive connections of a circuit-switched system GSM NETWORK ARCHITECTURE ➤ Advantages for subscriber ➤ Data that is sent does not need to contain any signaling information such as information about the destination ➤ Every bit simply passes through the established channel to the receiver ➤ Once the connection is established, no overhead, e.g., addressing information, is necessary to send and receive the information ➤ As the circuit-switched channel has a constant bandwidth, the sender does not have to worry about a permanent or temporary bottleneck in the communication path ➤ Especially important for a voice call ➤ Circuit-switched connections have a constant delay time ➤ This makes a circuit-switched connection ideal for voice applications as they are extremely sensitive to a variable delay time 2.2 PACKET-SWITCHED DATA TRANSMISSION OVER GPRS ➤ For bursty data applications, it would be far better to request for resources to send and receive data and release them again after the transmission ➤ Done by collecting data in packets before it is sent over the network ➤ The method of sending data is called ‘packet switching’ Figure 2.2 Packet-switched data transmission ➤ As there is no longer a logical end-to-end connection, every packet has to contain a header ➤ The header, e.g., contains information about the sender (source address) and the receiver (destination address) of the packet ➤ This information is used to route the packets through different network elements ➤ In the Internet, e.g., the source and destination addresses are the IP addresses of the sender and receiver GPRS NETWORK ARCHITECTURE GSM BSS MAP Um ISUP A A-bis MAP R MAP ISUP Gb Gs PSTN/ISDN Gf Gr Gc PDN GPRS Gi Gn Private Gn Backbone PDN Gp ➤ GPRS was designed as a packet-switched addition to circuit-switched GSM network ➤ IP packets can be sent over a circuit-switched GSM data connection as well ➤ However, until they reach the ISP they are transmitted in a circuit-switched channel ➤ GPRS is an end-to-end packet-switched network and IP packets are sent packet switched from end-to-end ➤ Packet-switched nature of GPRS offers a number of other advantages for bursty applications over GSM circuit-switched data transmission ➤ By flexibly allocating bandwidth on the air interface, GPRS exceeds the slow datarates of GSM circuit-switched connections of 9.6 or 14.4 kbps ➤ Datarates of up to 170 kbps are theoretically possible ➤ Multislot class 10 mobile devices reach speeds of about 85 kbps and are thus in the range of fixed-line analog modems ➤ The enhancements of EDGE for GPRS are called EGPRS in the standards or EDGE in practice ➤ With an EDGE class 32 mobile device, it is possible to reach transmission speeds of up to 270 kbps ➤ A speed comparison of the different technologies is shown Figure 2.3 GSM, GPRS and EDGE data transmission speed comparison ➤ GPRS is usually charged by volume and not by time ➤ For the operator of a wireless network it offers the advantage that the scarce resources on the air interface are not wasted by ‘idle’ data calls because they can be used for other subscribers Figure 2.4 Billing based on volume ➤ GPRS significantly reduces the call set-up time ➤ GSM circuit-switched data call took about 20 sec to establish a connection with ISP ➤ GPRS accomplishes the same in less than 5 sec ➤ The call does not have to be disconnected for subscriber to save costs ➤ This is called ‘always-on’ and enables applications like e-mail programs ➤ When the subscriber is moving, the network coverage frequently becomes very bad or is even lost completely for some time ➤ Circuit-switched connections ➤ Disconnected and have to be reestablished manually once network coverage is available again ➤ GPRS connections ➤ Not dropped as the logical GPRS connection is independent of the physical connection to the network ➤ After regaining coverage the interrupted data transfer simply resumes 2.3 GPRS AIR INTERFACE 2.3.1 GPRS VS. GSM TIMESLOT USAGE ON AIR INTERFACE ➤ Circuit-Switched TCH vs. Packet-Switched PDTCH (Packet Data Traffic Channel) ➤ GSM ➤ Uses timeslots on air interface to transfer data between subscribers and network ➤ During a circuit-switched call, a subscriber is assigned exactly one traffic channel (TCH) that is mapped to a single timeslot ➤ This timeslot remains allocated for the duration of the call and cannot be used for other subscribers even if there is no data transfer for some time ➤ GPRS ➤ The smallest unit that can be assigned is a block that consists of four bursts of a PDTCH ➤ A PDTCH is similar to a TCH in that it also uses one physical timeslot ➤ If the subscriber has more data to transfer, the network can assign more blocks on the same PDTCH right away ➤ Network can also assign block(s) to other subscribers or for logical GPRS signaling channels block 2.5 Simplified visualization of PDTCH assignment and timeslot aggregation ➤ Timeslot aggregation ➤ To increase the transmission speed, a subscriber is no longer bound to a single TCH as in circuit-switched GSM ➤ If more than one timeslot is available when a subscriber wants to transmit or receive data, the network can allocate several timeslots (multislot) to a single subscriber ➤ Multislot classes ➤ Depending on the multislot class of the mobile device, three, four or even five timeslots can be aggregated for a subscriber at the same time Table 2.1 Selected GPRS multislot classes from 3GPP (3rd Generation Partnership Project) TS 45.002 Annex B1 ➤ Most mobile devices support multislot class 10, 12 or 32 ➤ Multislot class 10 supports 4 timeslots in downlink direction and 2 in uplink direction ➤ The speed in uplink direction is significantly less than in downlink direction ➤ Web browsing benefits from the higher datarates in downlink direction and does not suffer very much from the limited uplink speed ➤ Sending e-mails with file attachments or multimedia messaging server (MMS) messages with large pictures or video content, 2 timeslots in the uplink direction are a clear limitation and increase the transmission time considerably 2.3.2 MIXED GSM/GPRS TIMESLOT USAGE IN A BASE STATION ➤ GPRS is an addition to GSM network, the eight timeslots available per carrier frequency on the interface can be shared between GSM and GPRS ➤ The max GPRS data rate decreases as more GSM voice/ data connections are needed ➤ Network operator can choose how to use the timeslots Figure 2.6 Shared use of the timeslots of a cell for GSM and GPRS ➤ Timeslots can be assigned statically, which means that some timeslots are reserved for GSM and some for GPRS ➤ Operator also has the option of dynamically assigning timeslots to GSM or GPRS ➤ If there is a high amount of GSM voice traffic, more timeslots can be used for GSM ➤ If voice traffic decreases, more timeslots can be given to GPRS ➤ It is also possible to assign a min number of timeslots for GPRS and dynamically add and remove timeslots depending on voice traffic 2.3.3 CODING SCHEMES ➤ Another way to increase the data transfer speed is to use different coding schemes ➤ If the user is at close range to a BS, the data transmitted over air is less likely to be corrupted during transmission than if the user is farther away and the reception is weak ➤ BS adds error detection and correction to the data before it is sent over the air (coding) ➤ In GPRS, four different coding schemes (CS-1 to 4) can be used to add redundancy to the user data depending on the quality of the channel Table 2.2 GPRS coding schemes ➤ CS-4 ➤ Does not add any redundancy to the data ➤ It can only be used when the signal quality between network and mobile device is very good ➤ The following figure shows how CS-2 and CS-3 encode data before it is transmitted over air interface USF：Uplink State Flag Figure 2.7 CS-2 and CS-3 channel coder output=2, input=1, memory=3 output memory memory memory input output http://wireless.ece.ufl.edu/eel6509/lectures/ConvCodes.pdf ➤ GPRS uses the same 1/2-rate convolutional coder as already used for GSM voice traffic ➤ The use of convolutional coding in CS-2 and CS-3 results in more coded bits than can be transmitted over a radio block ➤ To compensate for this, some of the bits are simply not transmitted (puncturing) ➤ As the receiver knows which bits are punctured, it can insert 0 bits at the correct positions and then use the convolutional decoder to recreate the original data stream 2.3.4 ENHANCED DATARATES FOR GSM EVOLUTION (EDGE) ➤ EDGE ➤ Introduce an additional Modulation and Coding Scheme (MCS), 8 Phase Shift Keying (8 PSK), to enhance datarates for GSM ➤ EGPRS ➤ The packet-switched part of EDGE ➤ Data transmission ➤ GSM and GPRS ➤ Use Gaussian Minimum Shift Keying (GMSK) modulation, which transmits only a single bit per transmission step ➤ Two possibilities 0 and 1 (1 bit) are coded as two positions in the I/Q space (In-phase axis, Quadrature axis) ➤ EDGE ➤ Use 8 PSK modulation