Mobile Networks

GPRS

GPRS General Packet Radio Service

GPRS offers an always-on Internet accessGPRS is a packet-oriented data service:

The TDMA radio slot are used only when transmitting some packetWe do a statistical multiplexing of more users over the same slotsThe fixed part of the network is an IP packet networkWe add new network nodes that are in fact IP routers

GPRS Network architecture

GPRS Protocols

SGSN Serving GPRS Support Node

It is an IP router that mimicks in GPRS the same functions that the MSC has in GSMOf course it adds special capabilities to the ones of an IP router:

It handles BSS interfaces and protocols It manages mobilityIt routes higher level packets to the right GGSN

SGSN Serving GPRS Support Node

Tasks:Authentication and ciphering procedures ; they are the same ones of GSMTogether with GSM RR management it allocates radio resources to each userIt routes the IP packets in the PLMN network up to the GGSN nodeIt encapsulates and tunnels the MS packets:

A GTP ( GPRS Tunneling Protocol) tunnel between SGSN and GGSN carries the GPRS packets ( usually IP, but X.25 too)The tunnel has a TID ( Tunnel Identifier ) for each user, derived from the IMSI

SGSN Serving GPRS Support Node

More tasks:User mobility management:

There is a LR (Location Register) like the VLRLocalization and subscription data are stored in the LR

Setup an LLC (Logical Link Control ) connection with the MS, used for reading and writing informations from the MS

GGSN Gateway GPRS Support NodeIt bridges GPRS and PDN (Packet Data Network)Usually the PDN is a public IP network (Internet) or a private one (Intranet, normally via an IPSEC tunnel)It encapsulates in GTP tunnels with the current SGSN the mobile user dataIt stores in its LR the current SGSN of each user, its profile and current PDP contextOn demand it activates the PDP context:

It describes the connection with an external network, e.g. the assigned IP address, the connection type ( e.g. the Intranet tunnel ), the requested QoS

When connecting to the outside, it behaves like a normal IP router

PCU Packet Control UnitWe must modify the BSC to include GPRS resource handling and implement packet protocols : this is the PCUThe PCU is usually located in the BSC (but it could also be in the BTS)Tasks:

Segmentation/reassembly of LLC level framesScheduling of packet transmissions in the physical channels (PDCH-Packet Data Channel)ARQ ProtocolChannel access control ( resource request and grant)Channel management ( Power control, congestion, control information broadcast, etc)

GPRS

GPRS ProtocolsThe GPRS protocol stack is defined by:

At the radio interface:Management protocols for dynamic resource sharing, as typical for packet networks, at media access level (MAC) and link control level (RLC/LLC)Media access is controlled by request and grantLink control has a retransmission mechanism when errors are detected

On the fixed network:IP transportHigher level encapsulation ( normally IP ) in a tunneling protocol (GTP) that is also used for mobility management

SNDCP Subnetwork Dependent Convergence Protocol

It adapts to the network level tunneled by GPRS (IP or X.25) and makes the transport level independent from the transported protocolIt offers to higher levels a transport service for varying length PDU between SGSN and MSAccording to OSI rules, it identifies the type of the higher level using the communication SAPI (NSAPI – Network Service Access Point Identifier)It offers:

Compression/decompression of higher level headersSegmentation/reassembly of higher level PDUMultiplexing of many NSAPI over the same LLC link

LLC Logical Link ControlIt is a level 2 protocol like the LAPDIt implements a logical ciphered p-to-p connection between SGSN and MSIt can be used in two modes:

Acknowledged – uses ARQ ( the one from the RLC level)Unacknowledged – no error control or only bad frame detection, it is used for signalling and SMS

RLC Radio Link Control

It implements ARQ splitting the information units in RLC blocks with error detection codesThe ARQ is Selective Reject The blocks are numbered modulo 128The transmission window is 64It uses cumulative ACK and NACKIt can also be unacknowledged

MAC Medium Access Control

It is used for dynamic sharing of radio resourcesIt manages downlink information flow multiplexingIt handles access and collisions in uplinkIt uses an uplink mechanism that has requests sent on contention slots and then allocation grants via downlink signalling

Physical LevelGPRS defines 4 channel coding schema up to 20 kb/sEDGE (Enhanced Data Rate for Global Evolution) adds lighter coding and multilevel modulations (8PSK)

Fixed NetworkBSSGP (BSS GPRS Protocol)

It allows signalling and data exchange between SGSN and BSCIt uses the lower level transport network

Network service:It allows data transport between SGSN and BSS that are not usually connected with dedicated linksUsually it is a frame relay circuit ( a PVC between SGSN and BSC)

GTP GPRS Tunneling ProtocolIt allows a transparent transport of network packets (IP or X.25) between GGSN and SGSN over the IP network of the GPRS providerIt is like other tunneling protocols used with IP networks ( e.g. IPSEC ) but uses a transport layer that is:

TCP for more reliable links (usually for X.25)UDP for best effort service (usually for IP)

Physical and logical channelsThe physical channel used by GRPS is the PDCH (Packet Data Channel)It uses a slot each frameThe minimum transmission unit is the Radio BlockThe Radio Block is a sequence of 456 coded bits sent from the MAC/RLC to the physical layer that is sent in 4 normal burstsThe radio resources are assigned dynamically in Radio Blocks and not in physical slots

Logical channelsPPCH – Packet Paging ChannelPRACH – Packet Random Access ChannelPAGCH – Packet Access Grant ChannelPNCH – Packet Notification ChannelThe set of the above channels is the PCCCH (Packet Common Control Channel)PBCCH – Packet Broadcast Control ChannelPDTCH – Packet Data Traffic Channel (Up or Down)PACCH – Packet Associated Control ChannelPTACCH – Packet Timing Advance Control Channel

Logical channels mappingWe use a multiframe built out of 52 framesIn the multiframe 48 frames carry 12 Radio Blocks, 2 are reserved for the PTCCH, 2 are idle onesMapping of logical into physical channels is packet orientedIn the MAC header of each radio block we store the payload type and therefore the logical channel type

Medium Access ControlThe MAC dynamically assigns the resources to the MSThe transmission resources allocated to a terminal are named TBF (Temporary Flow Block)Each MS can use more than a time slot (PDCH) at the same timeThe usable PDCH set is assigned from the network to the MS ( at MAC level) together with an USF (3 bit Uplink State Flag)The real usability of a PDCH by an MS depends from the USF value included in the MAC header of downlink radio blocksUSF=111 means free, that is that the PRACH can be used for TFB requests, that are granted via the downlink PAGCH

GPRS Radio AccessThe slots are sharedThere is explicit addressing in the MAC headerWe can use all slots left free by voice service

Mobility ManagementMobility management changes according to MS status:

Idle It cannot transmit or receive packets and the network considers the MS unreachable

StandbyIt cannot transmit or receive packets but can receive paging informations and signalling ; the MS has been localized in a routing area (RA) associated to an SGSN

ReadyIt can transmit or receive packets and its cell location is known ( we know its CGI – Cell Global Identity )

Mobility ManagementChangeover from idle to ready and viceversa is done via GPRS attach/detach proceduresChangeover from ready to standy happens according to an inactivity timer reset by sending LLC framesChangeover from standby to ready happens by sending LLC frames

GPRS mobility management is like the GSM one, but the reference one is the SGSN, not the MSC/VLR

Session ManagementWe must activate a Packet Data Protocol context before sending packets to and from an external networkIf we activate a session with an external IP network, we must also assign an IP address to the MS, that can be static or dynamic, private or public

GPRS Applications

GPRS Applications