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

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➤ 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

2.5 Simplified visualization of PDTCH assignment and timeslot aggregation

block

➤ 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

Figure 2.7 CS-2 and CS-3 channel coder

USF：Uplink State Flag

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➤ 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 to transmit three bits in a single transmission step

➤ 3 bits in eight different positions in the I/Q space

➤ EDGE transmits data up to three times faster than GPRS

Figure 2.8 GMSK (GPRS) and 8PSK (EDGE) modulation

➤ The following table gives an overview of the possible modulation and coding schemes and the datarates that can be achieved per timeslot

Table 2.3 EDGE modulation and coding schemes (MCS)

2.3.5 MOBILE DEVICE CLASSES

➤ Class A

➤ Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time

➤ Class B

➤ Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time

➤ During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded

➤ Most GPRS mobile devices are Class B

➤ Class C

➤ Are connected to either GPRS service or GSM service (voice, SMS)

➤ Must be switched manually between one or the other service

2.3.7 GPRS LOGICAL CHANNELS ON AIR INTERFACE

➤ Logical channels on air interface are used for transmitting user data and signaling data in uplink and downlink direction

➤ GPRS mandatory logical channels

➤ PDTCH (Packet Data Traffic Channel)

➤ A bidirectional channel, which means that it exists in uplink and downlink direction

➤ Used to send user data across air interface

➤ PDTCH is carried over timeslots that are dedicated for GPRS in a 52-multiframe structure

➤ PACCH (Packet-Associated Control Channel)

➤ A bidirectional channel and is used to send control messages

➤ When a mobile device receives data packets from network via a downlink PDTCH, it has to acknowledge them via uplink PACCH

➤ PACCH is also carried over GPRS dedicated timeslots in blocks of 52-multiframe structure

➤ In addition, PACCH is used for signaling messages that assign uplink and downlink resources

➤ For mobile device and network to distinguish between PDTCH and PACCH that are carried over the same physical resource, the header of each block contains a logical channel information field

Figure 2.12 GPRS logical channels

➤ PTCCH (Packet Timing Advance Control Channel)

➤ Used for timing advance estimation and control of active mobile devices

➤ To calculate the timing advance, the network can instruct an active mobile device to send a short burst at regular intervals on PTCCH

➤ The network then calculates the timing advance and sends the result back in the downlink direction of PTCCH

➤ In addition, GPRS shares a number of channels with GSM to initially request for assignment of resources

➤ Random Access Channel (RACH)

➤ When a mobile device wants to transmit data blocks to the network, it has to request for uplink resources

➤ The message asks for packet resources on air interface

➤ AGCH (Access Grant Channel)

➤ The network answers a channel request on RACH with an immediate packet assignment message on AGCH that contains information about the PDTCH timeslot the mobile device is allowed to use in uplink

➤ The first uplink transmissions have to contain the Temporary Logical Link Identifier (TLLI, also known as Packet-Temporary Mobile Subscriber Identity, P-TMSI) the mobile device was assigned when it attached to the network

➤ All further GPRS signaling messages are then transmitted over PACCH, which shares the dedicated GPRS timeslots with PDTCH

➤ Once data is available for the mobile device in downlink direction, the network needs to assign timeslots in downlink direction

Figure 2.13 Packet resources: requests and assignments

TLLI：Temporary Logical Link Identifier

USF：Uplink State Flag

➤ PCH (Paging Channel)

➤ In case the mobile device is in standby state, only the location area of a subscriber is known

➤ As the cell itself is not known, resources cannot be assigned right away and the subscriber has to be paged first

➤ GPRS uses GSM PCH to do this

➤ BCCH (Broadcast Common Control Channel)

➤ A new system information message (SYS_INFO 13) has been defined on BCCH to inform mobile devices about GPRS parameters of the cell

➤ This is necessary to let mobile devices know

➤ if GPRS is available in a cell

➤ which NOM is used

➤ if EDGE is available

➤ and so on

NOM：Network Operation Modes

NOM I：PCH available, PPCH (Packet Paging Channel) optional, Gs interface is available

NOM II：No PPCH available, No Gs interface

NOM III：PCH and PPCH available, No Gs interface

Gs interface connects the SGSN and the MSC/VLR

2.4 GPRS STATE MODEL

➤ The state model addresses the needs of a packet-switched connection for GPRS

➤ Idle State

➤ Mobile device is not attached to GPRS network at all

➤ SGSN is not aware of user’s location, no Packet Data Protocol (PDP) context is established and the network cannot forward any packets to user

➤ Ready State

➤ When a user wants to attach to GPRS network, the mobile device enters ready state as soon as the first packet is sent

➤ While in ready state, the mobile device has to report every cell reselection to network so that SGSN can update the user’s position in its database

➤ This process is called ‘cell update’

➤ It enables the network to send any incoming data for a user directly to the mobile device instead of having to page the mobile device first to locate the user’s serving cell

➤ The mobile device will remain in ready state while signaling or user data is transferred for a certain time afterward

➤ The timer that controls how long the mobile device will remain in this state after the last block of data was transferred is called T3314

➤ The value of this timer is broadcast on BCCH or PBCCH as part of GPRS system information

➤ A typical value for this timer used in many networks is 44 seconds

➤ The timer is reset to its initial value in both mobile device and SGSN whenever data is transmitted

➤ When the timer reaches 0 the logical connection between mobile device and network automatically falls back into standby state

➤ Standby State

➤ In case no data is transferred for some time, the ready timer expires and the mobile device changes into standby state

➤ In this state, the mobile device only informs the network of a cell change if the new cell belongs to a different routing area than the previous one

➤ If data arrives in the network for the mobile device after it has entered the standby state, the data needs to be buffered and the network has to page the subscriber in the complete routing area to get the current location

➤ A routing area (RA) is a part of a location area (LA) and thus also consists of a number of cells

➤ In GPRS, it was decided that splitting location areas into smaller routing areas would enable operators to better fine-tune their networks by being able to control GSM and GPRS signaling messages independently

2.5 GPRS NETWORK ELEMENTS

➤ Three new network components were introduced into mobile network and software updates were made for some of the existing components

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2.5.1 PACKET CONTROL UNIT (PCU)

➤ BSC has been designed to switch 16 kbps circuit-switched channels between MSC and subscribers

➤ BSC is also responsible for the handover decisions for those calls

➤ BSC and its switching matrix are not suited to handle packet-switched GPRS traffic

➤ PCU is the packet-switched counterpart of BSC and fulfills the following tasks

➤ Assignment of timeslots to subscribers in uplink direction when requested by mobile device via RACH or PRACH

➤ Assignment of timeslots to subscribers in downlink direction for data arriving from core network

➤ Flow control of data in uplink and downlink directions and prioritization of traffic

➤ Error checking and retransmission of lost or faulty frames

➤ Subscriber paging

➤ Supervising entity for subscriber timing advance during data transmission

2.5.2 SERVING GPRS SUPPORT NODE (SGSN)

➤ SGSN can be seen as the packet-switched counterpart to MSC in circuit-switched core network

➤ It lies between radio access network and core network

➤ It is responsible for user plane management and signaling plane management

➤ User Plane Management

➤ User plane combines all protocols and procedures for the transmission of user data frames between subscriber and external networks like Internet or a company intranet

➤ All frames that arrive for a subscriber at SGSN are forwarded to PCU, which is responsible for the current cell of subscriber

➤ In the reverse direction the PCU delivers data frames of a subscriber to SGSN, which in turn will forward them to the next network node, which is called gateway GPRS support node (GGSN)

➤ IP is used as the transport protocol in GPRS core network between SGSN and GGSN

➤ Signaling Plane Management

➤ SGSN is also responsible for the management of all subscribers in its area

➤ All protocols and procedures for user management are handled on signaling plane

➤ To be able to exchange data with Internet, it is necessary to establish a data session with GPRS network

➤ This procedure is called PDP context activation and is part of the session management (SM) tasks of SGSN

➤ From user point of view, this procedure is invoked to get an IP address from network

➤ Subscribers can change their location in a mobile network frequently

➤ When this happens the SGSN needs to change its routing of packets to the radio network accordingly

➤ This task is done by GPRS mobility management (GMM) sublayer

➤ When a subscriber leaves the area of the current SGSN, GMM also contains procedures to change the routing for a subscriber in the core network to new SGSN

➤ This procedure is called inter-SGSN routing area update (IRAU)

➤ To charge the subscriber for usage of GPRS network, SGSN and GGSN collect billing information in so-called call detail records (CDRs)

➤ These are forwarded to billing server, which collects all CDRs and generates an invoice for each subscriber once a month

➤ The CDRs of SGSN are especially important for subscribers that roam in a foreign network

➤ SGSN is the only network node in foreign network that can generate a CDR for a GPRS session of a roaming subscriber for foreign operator

➤ For roaming subscribers the CDRs of SGSN are then used by foreign operator to charge home operator for the data traffic the subscriber has generated

2.5.3 GATEWAY GPRS SUPPORT NODE (GGSN)

➤ SGSN routes user data packets between radio access network and core network

➤ GGSN connects GPRS network to external data network

➤ The external data network will in most cases be Internet

➤ For business applications, GGSN can also be the gateway to a company intranet

➤ GGSN is also involved in setting up a PDP context by assigning a dynamic or static IP address to users

➤ GGSN is the anchor point for a PDP context and hides the mobility of user to Internet

➤ When a subscriber moves to a new location, a new SGSN might become responsible and data packets are sent to the new SGSN (IRAU)

2.8 GPRS MOBILITY MANAGEMENT AND SESSION MANAGEMENT (GMM/SM)

➤ GPRS network is also responsible for mobility management (MM) of subscribers and session management (SM) to control individual connections between subscribers and Internet

➤ For this purpose, signaling messages and signaling flows have been defined that are part of GMM/SM protocol

2.8.1 MOBILITY MANAGEMENT TASKS

➤ When a subscriber wants to attach, the network usually starts an authentication procedure, which is similar to GSM authentication procedure

➤ If successful, SGSN sends an Location Update (LU message) to HLR to update the location information of that subscriber in network’s database Figure 2.28 GPRS attach message flow

➤ HLR acknowledges this operation by sending an ‘insert subscriber data’ message back to SGSN

➤ It not only acknowledges the LU but also returns the subscription information of the user to SGSN so that no further communication with HLR is necessary as long as the subscriber does not change the location

➤ SGSN, subsequently, will send an attach accept message to the subscriber

➤ The attach procedure is complete when the subscriber returns an attach complete message to SGSN Figure 2.28 GPRS attach message flow

2.8.2 GPRS SESSION MANAGEMENT

➤ To communicate with Internet, a PDP context has to be requested to use after attach procedure

➤ For end user, this in effect means getting an IP address from network

➤ It is sometimes also referred to as ‘establishing a packet call’

➤ When a GPRS packet call is established there are no resources dedicated to the PDP context

➤ Resources on various interfaces are used only during the time that data is transmitted

➤ Once the transmission is complete (e.g. after the web page has been downloaded), the resources are used for other subscribers

➤ PDP context represents only a logical connection with Internet

➤ It remains active even if no data is transferred for a prolonged length of time

➤ For this reason a packet call can remain established without blocking resources

➤ This is sometimes referred to as ‘always on’

➤ The following figure shows the PDP context activation procedure

➤ Initially, the subscriber sends a PDP context activation request message to SGSN

➤ The APN (Access Point Name) is the reference that GGSN uses as a gateway to an external network. Network operator could have

➤ one APN to connect to Internet transparently

➤ one to offer Wireless Application Protocol (WAP) services

➤ several other APNs to connect to corporate intranets, etc.

➤ SGSN compares the requested APN with the list of allowed APNs for the subscriber that has been received from HLR during attach procedure

➤ APN is a fully qualified domain name like ‘internet.t-mobile.com’ or simply ‘Internet’ or ‘wap’

➤ SGSN uses APN to locate the IP address of the GGSN that will be used as a gateway

➤ SGSN performs a domain name service (DNS) lookup with the APN as the domain name to be queried

➤ To get an internationally unique qualified domain name, the SGSN adds the mobile country code (MCC) and mobile network code (MNC) to APN, which is deduced from the subscriber’s IMSI

➤ As a top level domain, ‘.gprs’ is added to form a complete domain name

➤ An example of domain name for the DNS query is ‘internet.t-mobile.com.026.350.gprs’

➤ Adding MCC and MNC to APN by SGSN enables the subscriber to roam in any country that has a GPRS roaming agreement with the subscriber’s home network and use the service without having to modify any parameters

➤ The foreign SGSN will always receive the IP address of the home GGSN from DNS server, and all packets will be routed to and from the home GGSN and from there to external network

➤ After the DNS server has returned the GGSN’s IP address, the SGSN can then forward the request to the correct GGSN

➤ The APN and the user’s IMSI are included in the message as mandatory parameters

➤ To tunnel user data packets through GPRS network later on, SGSN assigns a so-called tunnel identifier (TID) for this virtual connection that is also part of the message

➤ TID consists of user’s IMSI and a two-digit network service access point identifier (NSAPI)

➤ This allows a mobile device to have more than one active PDP context at a time

➤ This is quite useful, e.g., to separate Internet access from network operator internal services such as MMS

➤ If GGSN grants access to external network (e.g. Internet) it will assign an IP address out of an address pool for the subscriber

➤ Subsequently, GGSN responds to SGSN with a PDP context activation response message that contains the IP address of the subscriber

➤ Furthermore, GGSN stores the TID and subscriber’s IP address in its PDP context database

➤ This information is needed later on to forward packets between subscriber and Internet and, of course, for billing purposes

➤ Once the SGSN receives PDP context activation response message from GGSN, it also stores the context information in its database and forwards the result to the subscriber

➤ The subscriber then uses the IP address to communicate with the external network

➤ Different IDs are used for packets of a certain user on each network interface due to different nature of protocols and due to different packet sizes

➤ On the GPRS air interface, with its small data frames of only 456 bits or 57 bytes, which even includes the overhead for error detection and correction, the three-bit TFI (Temporary Flow Identifier) is used to route the frame to the correct mobile device

➤ In the radio network the P-TMSI/TLLI (Temporary Logical Link Identifier) is used to identify the packets of a user

➤ In the core network, the GPRS TID is used as identification