Hannu H. KARI/Helsinki University of Technology (HUT)

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Tik-109/110.300 Fall 2000 © Hannu H. Kari Page 1

Tik-109/110.300

Telecommunications architectures:

Protocols

Hannu H. KARI/Helsinki University of Technology (HUT)

TML-laboratory/CS/HUT

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Agenda

Protocol design

Multivendor interfaces Handling protocol error Handling lost packets

Handling corrupted data

Handling failed network elements Fault tolerant system design

Protocol examples

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Protocol design:

architecture

Local area network

Server

Router

Local area network

Server

Router

Corporate 2

Corporate 1 Serving GPRS

Support Node (SGSN)

Gateway GPRS Support Node (GGSN)

GPRS INFRA- STRUCTURE

HLR MSC/VLR

Other GPRS

operators Border gateway

Gb

Gr Gd

Gi.IP Gi.X.25 Gi.IP

Firewall Firewall

Firewall

Firewall R/S Um

SMS-GMSC

Gr Gd

Gs

Gp Gc

Gn, Gp

EIR Gf

Gf

BTS BSC ^PSTN

GPRS backbone

Data network (Internet)

Data network (X.25)

Data network (Internet) SS7 Network

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Protocol design

• Example protocol stack

Um Gb

MS BSS SGSN

SNDC SMS GMM

LLC

RLC

MAC

GSM RF

Relay RLC

MAC

GSM RF

BSSGP Network

Service

L1

SNDC SMS GMM

LLC

BSSGP Network

Service

L1

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Protocol design

• Basic operation model

4. Confirm

1. Request 3. Response 2. Indication

Level - N+1

Level - N

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Multivendor interfaces

• Proprietary interfaces

• Implementations of different manufacturers won’t necessarily interwork

• Possible manufacturer extensions, interpretations, solutions, combined equipments, ...

• Multivendor interfaces

• Well defined protocols

• Strong interoperability requirements

• Interoperability testing (type approvals)

• Support for multiple versions

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Handling protocol errors

• E.g., in GTP protocol (GPRS Tunneling Protocol), the protocol layer handles:

• Protocol has version numbers

• Handling too short packets

• Unknown and erroneous headers

• Unexpected message

• Missing or invalid mandatory elements

• Unknown information element

• Out of sequence information elements

• Repeated information elements

• Incorrect optional information elements

• TLV, TV, V formatting (T=Type, L=Length, V=Value)

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Handling lost packets

• In principle, data packets at any protocol layer may be lost

• Methods to cope lost packets

• Acknowledges

• Timers

• Retries

• State machines

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Handling corrupted data

• Methods to cope corrupted data

• CRC checks (RLC, LLC, L2 under IP, …)

• Sanity checks of data

• State machines

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Handling failed network elements

• Network elements with permanent data (HLR failure)

• After restart, inform all MSC/VLRs and SGSNs that have MSs of that HLR

• Network elements with non-permanent data (SGSN failure)

• Inform associated neigboring element

• VLRs

• Inform MS next time when it next time sends uplink message

• Inform GGSN next time when GGSN sends MT data. MS may be paged

• MT SMSs are rejected

• CS page is sent to entire MSC/VLR area or to MSC/VLR specified

area

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Handling failed network elements

• Simpler network elements (GGSN failure)

• Inform SGSN when SGSN sends MO data

• Reject data packets MT packets

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Fault tolerant systems

• Fault models

• Only knows faults can be handled (i.e., if you are not prepared to handle the faults, you can’t survive those faults)

• Fault prevention

• Fault detection

• Fault isolation

• Reduced/Degraded service

• Fault recovery

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Example:

Simple procedure, Attach

MS Network

ATTACH REQUEST ATTACH ACCEPT

ATTACH COMPLETE

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Example:

Signaling chart

7d. Cancel Location Ack 7c. Cancel Location 7b. Update Location

7g. Update Location Ack 7e. Insert Subscriber Data 7f. Insert Subscriber Data Ack 6d. Insert Subscriber Data

6c. Cancel Location Ack 6b. Cancel Location 3. Identity Response

2. Identification Response 2. Identification Request 1. Attach Request

5. IMEI Check 3. Identity Request

4. Authentication

6a. Update Location

7a. Location Updating Request

7h. Location Updating Accept 6f. Update Location Ack 6e. Insert Subscriber Data Ack

MS BSS new SGSN old SGSN GGSN EIR HLR

old MSC/VLR new

MSC/VLR

9. Attach Complete 8. Attach Accept

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Example:

Signaling chart

• 1) The MS initiates the attach procedure by the transmission of an Attach Request (IMSI or P TMSI and old RAI, Classmark, CKSN, Attach Type, DRX Parameters, old P TMSI Signature) message to the SGSN.

IMSI shall be included if the MS does not have a valid P TMSI available. If the MS has a valid P TMSI, then P TMSI and the old RAI associated with P TMSI shall be included. Classmark contains the MS's GPRS multislot

capabilities and supported GPRS ciphering algorithms in addition to the existing classmark parameters defined in GSM 04.08. Attach Type indicates which type of attach that is to be performed, i.e., GPRS attach only, GPRS Attach while already IMSI attached, or combined GPRS / IMSI attach. DRX Parameters indicates whether the MS uses discontinuous reception or not.

If the MS uses discontinuous reception, then DRX Parameters also indicate

when the MS is in a non-sleep mode able to receive paging requests and

channel assignments. If the MS uses P TMSI for identifying itself and if it

has also stored its old P TMSI Signature, then the MS shall include the old

P TMSI Signature in the Attach Request message.

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Example:

Signaling chart

• 8) The SGSN sends an Attach Accept (P TMSI, VLR TMSI, P TMSI Signature) to the MS. P TMSI is included if the SGSN allocates a new P TMSI.

• 9) If P TMSI or VLR TMSI was changed, the MS acknowledges the

received TMSI(s) with Attach Complete (P TMSI, VLR TMSI).

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Example:

Time critical approach

MS Network

ATTACH REQUEST Start T3310

ATTACH ACCEPT Stop T3310

ATTACH COMPLETE

Stop T3350

If P-TMSI and/or TMSI allocated Start T3350 If P-TMSI and/or

TMSI allocated

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Sequence diagram

C.1 ATTACH, GPRS only, MS unknown in the network

GMMREG-ATTACH- REQ [Attach type, READY-timer, STANDBY-timer ]

START Tattach LL-UNITDATA-REQ [PDU=Attach Req ( IMSI or TLLIplusRAI), oldTLLI,Cipher=off ]

GRR-DATA-REQ

[LLC-UI, SAPI=signalling]

GRR-DATA-IND

[LLC-UI, TLLI, RAI,CI]

LL-UNITDATA-IND

[PDU=Attach Req ( IMSI or TLLIplusRAI), oldTLLI, CI]

LLGMM-ASSIGN-REQ

[Kc, Algorithm]

START Tauth LL-UNITDATA-REQ

GRR-DATA--REQ

STOP Tauth [ PDU=Auth-Cipher-

Req (Alg, RAND,..), oldTLLI, Cipher=off ] [LLC-UI,

DRX, CLM, SAPI=signalling]

GRR-DATA-IND

[LLC-UI]

LL-UNITDATA-IND

[PDU=Auth-Cipher- Req (Alg., RAND,…) ]

LL-UNITDATA-REQ

[PDU=Auth-Cipher- Resp (SRES), oldTLLI,Cipher=off ]

GRR-DATA-REQ

GRR-DATA-IND

[LLC-UI, oldTLLI]

LL-UNITDATA-IND

[PDU=Auth-Cipher- Resp (SRES), oldTLLI ]

[Kc, Algorithm]

MS NET

GMM

SNDCP SM LLC RR RR BSSGP LLC GMM SM SNDCP

LLGMM-ASSIGN-REQ

START Tident LL-UNITDATA-REQ

[ PDU=

Identification Req, oldTLLI, Cipher=off ] GRR-DATA--REQ

[LLC-UI, DRX, CLM, SAPI=signalling]

GRR-DATA-IND

[LLC-UI]

LL-UNITDATA-IND

[PDU=

Identification Req]

STOP Tident LL-UNITDATA-REQ

[PDU=Identification- Resp (IMSI), oldTLLI,Cipher=off ]

GRR-DATA-REQ

GRR-DATA-IND

[LLC-UI, TLLI]

LL-UNITDATA-IND

[PDU=Identification- Resp (IMSI), oldTLLI ] The oldTLLI may be a

Local TLLI, or a Foreign TLLI, or a Random TLLI

IMSI fetched if

- No GPRS MM Context exists, and - Attach Request without IMSI

The network starts the authentication together with setting the ciphering mode.

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Example:

Service primitives

PRIMITIVE PARAMETER

(message, info elements of message, other parameters)

REFERE

GMMREG-ATTACH-REQ attach-type, READY-timer, STANDBY-timer 6.6.1.1 GMMREG-ATTACH-CNF PLMNs MT-caps, attach-type. 6.6.1.2

GMMREG-ATTACH-REJ cause 6.6.1.3

GMMREG-DETACH-REQ detach-type, power-off/normal-detach 6.6.1.4

GMMREG-DETACH-CNF detach-type 6.6.1.5

GMMREG-DETACH-IND detach-type 6.6.1.6

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Example:

Service primitives

• 6.6.1.1 GMMREG-ATTACH-REQ

• MS initiates the GPRS and/or IMSI attach. GMM is requested to send an ATTACH REQUEST message to the network. The attachment is registration pending in the MS.

• 6.6.1.2 GMMREG-ATTACH-CNF

• The attach (either GPRS-attach or IMSI-attach or both) was successful. The network confirmed the attach, i.e.the ATTACH ACCEPT message was received by the MS. The LLC and RR sublayer will be informed by GMM about the TLLI to be used.

• 6.6.1.3 GMMREG-ATTACH-REJ

• The attach (either GPRS-attach or IMSI-attach or both) has failed. The network rejected the attach attempt, i.e. the

message ATTACH REJECT was received from the network.

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Example:

Information exchange

• Attach request

EI Information Element Type/Reference Presence Format Length Protocol discriminator Protocol discriminator

10.2 M V 1/2

Skip indicator Skip indicator

10.3.1 M V ½

Attach request message identity Message type

10.4 M V 1

MS classmark MS classmark 4

10.5.5.12 M LV 4 - 6

Attach type Attach type

10.5.5.2

M V ½

GPRS ciphering key sequence number

Ciphering key sequence number 10.5.1.2

M V ½

DRX parameter DRX parameter

10.5.5.6

M V 2

Force to standby Force to standby

10.5.5.7 M V ½

Spare half octet Spare half octet

10.5.1.8 M V ½

P-TMSI or IMSI Mobile station identity

10.5.5.13 M LV 6 - 10

X MS Radio Access capability MS RA capability

10.5.5.12.a O TLV 5-12

Old P-TMSI signature P-TMSI signature

10.5.5.8 O TV 4

Old routing area identification Routing area identification 10.5.5.15

O TV 7

Requested READY timer value

Timer 10.5.5.16

O TV 2

Requested STANDBY timer value Timer

10.5.5.16 O TV 2

MS classmark MS classmark 1

10.5.1.5 O TV 2

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Example:

Information exchange

• 9.5.1.0 MS Radio Access capability

• This IE may be included by the MS in order to provide its radio access capabilities.

• 9.5.1.1 Old P-TMSI signature

• This IE is included if a valid P-TMSI and P-TMSI signature are stored in the MS.

• 9.5.1.2 Old routing area identification

• This IE is included if the MS has stored the old RAI and an associated valid P-TMSI.

• 9.5.1.3 Requested READY timer value

• This IE may be included if the MS wants to indicate a preferred value for READY timer.

• 9.5.1.4 Requested STANDBY timer value

• This IE may be included if the MS wants to indicate a preferred value for STANDBY timer.

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Example:

Information exchange

• Attach accept

10.2

M V 1/2

Skip indicator Skip indicator 10.3.1

M V 1/2

Attach accept message identity Message type 10.4

M V 1

Attach result Attach result

10.5.5.1

M V 1/2

Force to standby Force to standby 10.5.5.7

M V 1/2

Routing area identification Routing area identification 10.5.5.15

O TV 7

Periodic RA update timer Timer value 10.5.5.16

M V 1

P-TMSI signature P-TMSI signature 10.5.5.8

O TV 4

Negotiated DRX parameter DRX parameter 10.5.5.6

O TV 3

Negotiated READY timer value

Timer 10.5.5.16

O TV 2

Negotiated STANDBY timer value

Timer 10.5.5.16

O TV 2

Allocated P-TMSI Mobile station identity 10.5.5.13

O TLV 7

MS identity Mobile identity

10.5.1.4

O TLV 6 - 11

GMM cause GMM cause

10.5.5.14

O TV 2

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Example:

Information exchange

• Sample information elements

• attach result

• routing area identifier

8 7 6 5 4 3 2 1

Routing Area Identification IEI octet 1

MCC digit 2 MCC digit 1 octet 2

1 1 1 1 MCC digit 3 octet 3

MNC digit 2 MNC digit 1 octet 4

LAC octet 5

LAC cont’d octet 6

RAC octet 7

8 7 6 5 4 3 2 1 Attach result

IEI

0 spare

Result of attach

octet 1

Result of attach (octet 1) Bits

3 2 1

0 0 1 GPRS only attached

0 1 1 Combined GPRS/IMSI attached All other values are reserved.

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Example:

Information exchange

• Attach complete

10.2

M V 1/2

Skip indicator Skip indicator

10.3.1

M V 1/2

Attach complete message identity

Message type 10.4

M V 1

Force to standby Force to standby 10.5.5.7

M V 1/2

Spare half octet Spare half octet 10.5.1.8

M V 1/2

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Example:

Information exchange

• Attach reject

EI Information Element Type/Reference Presence Format Length

Protocol discriminator Protocol discriminator 10.2

M V 1/2

Skip indicator Skip indicator 10.3.1

M V 1/2

Attach reject message identity Message type 10.4

M V 1

GMM cause GMM cause

10.5.5.14

M V 1

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Example:

Exception cases

• If the attach request cannot be accepted by the network, an ATTACH REJECT message is transferred to the MS. The MS receiving the ATTACH REJECT message stops timer T3310 and takes one of the following actions depending upon the reject cause:

• # 3 (Illegal MS)

# 6 (Illegal ME)

# 7 (GPRS services not allowed)

• The MS shall set the GPRS update status to GU3 ROAMING NOT ALLOWED (and shall store it according to section 4.1.3.2.9) and shall delete any P-TMSI, P-TMSI signature, RAI and GPRS ciphering key sequence number. The SIM shall be considered as invalid until switching off or the SIM is removed. The new state is GMM-DEREGISTERED.

• # 11 (PLMN not allowed)

# 12 (Location area not allowed)

# 13 (Roaming not allowed in this location area)

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Example:

Exception cases

• The MS shall delete any RAI, P-TMSI, P-TMSI signature and GPRS ciphering key sequence number stored, shall set the GPRS update status to GU3

ROAMING NOT ALLOWED (and shall store it according to section 4.1.3.2.9) and shall change to state GMM-DEREGISTERED.

• The MS shall store the LAI or the PLMN identity in the appropriate

forbidden list, i.e. in the “forbidden PLMN list” for cause #11, in the list of

“forbidden location areas for regional provision of service” for cause #12 or

in the list of “forbidden location areas for roaming” for cause #13. In case

cause #11 or #13 was received, the MS shall perform a PLMN selection

instead of a cell selection.

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Example:

Abnormal cases

a) Access barred because of access class control

• The GPRS attach procedure shall not be started. The MS stays in the current serving cell and applies normal cell reselection process. The GPRS attach

procedure is started as soon as possible, i.e. when access is granted or because of a cell change.

b) Lower layer failure before the ATTACH ACCEPT or ATTACH REJECT message is received

• The procedure shall be aborted, the MS shall keep, if any, the stored P-TMSI, RAI, and GPRS ciphering key sequence number and shall proceed as described below.

c) T3310 time-out

• On the first expiry of the timer, the MS reset and restart timer T3310 and shall retransmit the ATTACH REQUEST message. This retransmission is repeated until the timer has expired four times, i.e. on the fiftth expiry of timer T3310, the GPRS attach procedure shall be aborted and the MS shall proceed as described below.

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Example:

Abnormal cases

d) ATTACH REJECT, other causes than those treated in section 4.7.3.1.4 e) Change of cell into a new routing area

• If a cell change into a new routing area occurs before an ATTACH ACCEPT or ATTACH REJECT message has been received, the GPRS attach procedure shall be aborted and re-initiated immediately. If a routing area border is crossed when the ATTACH ACCEPT message is received but before an ATTACH

COMPLETE message is sent, the GPRS attach procedure shall be aborted and the routing area updating procedure shall be initiated. If a P-TMSI was allocated during the GPRS attach procedure, this P-TMSI shall be used in the routing area updating procedure. If a P-TMSI signature was allocated together with the P- TMSI during the GPRS attach procedure, this P-TMSI signature shall be used in the routing area updating procedure.

f) Power off

• If the MS is in state GMM-REGISTERED-INITIATED at power off, the GPRS detach procedure shall be performed.

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Example:

Abnormal cases

g) Procedure collision

• If the MS receives a DETACH REQUEST message from the network in state GMM-REGISTERED-INITIATED, the GPRS detach procedure shall be progressed and the GPRS attach procedure shall be aborted. If the cause IE, in the DETACH REQUEST message, indicated a “reattach request”, the GPRS attach procedure shall be progressed and the DETACH REQUEST message shall be ignored.

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Example:

State machine

• General state machine idle

attached

context active

attach

detach

activate context

deactivate context detach

mobility management

send/receive SMS

mobility management

send/receive SMS

send/receive data

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Example:

State machine

• Simple state machine

IDLE

REGISTRA- TION PEND.

GMMREG- ATTACH-REQ

GMMREG-ATTACH-CFN GMMREG- ATTACH-REJ

GMMREG-DETACH-CFN

REGISTERED

GMMREG-DETACH -IND

GMMREG-DETACH -REQ (power off)

GMMREG-DETACH-REQ (not power off)

DE-

REGISTRA- TION PEND.

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Example:

State machine at MS

NULL RA-UPDATING-

INITIATED

DEREGISTERED Enable

GPRS

DETACH requested (not power off)

REGISTERED

Any state

DEREGISTERED- INITIATED

ATTACH

request ATTACH rejected DETACH

requested (power off)

Network init. DETACH requested

Lower layer failure ATTACH accept RAU rejected

Network init. DETACH implicit DETACHd DETACH accepted Lower layer failure

RAU requested RAU accepted RAU failed

REGISTERED- INITIATED Disable

GPRS

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Example:

State machine at network

DE-REGISTEREDGMM-

- Network initiated DETACH requested

REGISTEREDGMM-

GMM-COMMON- PROCEDURE-INITIATED

GMM- DE-REGISTERED-

INITIATED

- COMMON procedure failed

- Lower layer failure - ATTACH procedure

successful

- MS initiated DETACH requested - RAU rejected

- Iimplicit DETACH

- DETACH accepted - Lower layer failure

- COMMON procedure successful

- ATTACH procedure successful

- COMMON procedure requested

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