Traffic Link Redundancy

Transcription

1 Traffic Link Redundancy 80% of the traffic saved if one link goes down 2 separated routes 3 separated routes eg 10E per link then: 80*(10+10)=16E 80*( )/2=16E The redundancy factor becomes 1.6 and 1.2 respectively

2 C7 Signaling Concept in the GSM Network Maximum signaling load per signaling link 30 % under normal conditions 60 % under overload conditions 64 kbit/s = 8000 octets/s (1 octet = 8 bits) Normal load = 0.3 x 64 = 19.2 kbit/s or 0.3 x 8000 = 2400 octets/s Overload = 0.6 x 64 = 38.4 kbit/s or 0.6 x 8000 = 4800 octets/s

3 A widely used dimension rule, based on No. 7 signaling link dimensioning for plain PSTN with TUP, is to allow 30% load on links in normal operation and 60% in failure situations. In GSM networks 20% load in normal operation is often used. With MAP MSUs instead of TUP the same signaling volume is generated by fewer and longer MSUs that implies a more bursty load requiring more margin to achieve the same quality.

4 Signaling Volumes Signaling is required not only for setting up of call connections, but also for finding and upgrading the present location of the subscriber. Enhanced security including both authentication and equipment identity control require No. 7 signaling. Estimates of the signaling generated by different events in the network can be used to calculate the approximate signaling load.

5 Signaling Calculation Model: The main input parameters are: Traffic per subscriber Mean Call holding Time Percentage MT traffic Location Updates per subscriber and hour Inter MSC handovers per call IMSI attach per subscriber and hour Number of authentication triplets fetched at a time short messages per subscriber and hour

6 signaling Volume Example Model 1 Model 2 Traffic per sub 0.030E 0.025E Mean holding time 100s 120s MT Percentage 33% 25% Location Updates new VLR / sub&hour Inter MSC Ho/call SM / sub&hour MSC - HLR kb/s per ksub MSC - MSC kb/s per ksub MSC -EIR kb/s per ksub

7 There is a different possibilities for the operator to influence the signaling volumes per subscriber: Placing of MSC borders as well as LA borders impact the mobility experienced in the network. (it reduces the Location Area update signaling) Parameter settings in the AUC for use of selective authentication Parameter settings in the EIR for IMEI check

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9 C7 Routing Strategies In order to meet the need for extended services, i.e. communication with databases without speech connections, the SCCP is used. SCCP maintains connection oriented (CO), connectionless (CL) network services, circuit related and non-circuit related signaling. Connection-oriented signaling: used when many messages to transfer between two signaling points (SP) and when messages are so long that segmenting is needed.

10 Connectionless signaling is used for MAP. In connectionless signaling all message signaling units contain all information required to route each message unit to the correct destination. Circuit related signaling is signaling related to a specific speech or data connection Non circuit related signaling is signaling not connected to any speech or data connection, i.e. roaming signaling in mobile application. SCCP make possible routing of the message on a higher level (Global Title Translation (GTT), SCCP rerouting), i.e. handle the logical signaling connection, and MTP is responsible for transporting the message through the network in a reliable manner.

11 SCCP Routing Section 7 Dimensioning

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13 The SSN indicates the subsystem so the message is distributed to the right software in the terminating node. SSN points out MAP HLR, MAP VLR, MAP MSC/GMSC, BSSAP, MAP EIR, MAP AUC, MAP SC, and ISUP.

14 MTP Routing Section 7 Dimensioning The routing procedure as well as the load sharing between link sets and within link sets is done using: Network Indicator (NI), Destination Point Code, an Originating Point Code (OPC) and a four bit signaling Link Selection code (SLS). NI identifies a No.7 Network. DPC and OPC are the signaling Point Code (SPC) that uniquely defines a signaling Point (SP) in the No.7 signaling network.

15 MTP signaling route could either be one signaling link set or load sharing over signaling link sets.

16 Signaling route alternatives with different priorities can be defined and the routing alternative with lower priority will not be set into action until the alternative with the higher priority is totally blocked. Signaling routing in the GSM can be understand by the example of the network having three HLRs in three different zones along with STPs.

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18 Routing principles for No. 7 signaling: Western MSC load-share signaling to HLRs over Western STP to East HLR and East STP to East HLR. Second choice, if both link sets are out of order, signaling is routed over Central STP to East HLR. Similar is the case for other two HLRs. HLRs are connected to all three STP. Routing of signaling depends on destined MSC group: signaling towards western MSCs is routed in load-share over W E and E E. Second choice, if both link sets are out of order, signaling is routed over C E.

19 signaling towards central MSCs routed in load-share over W E and C E. Second choice, if both link sets are out of order, signaling is routed over E Tr. signaling towards eastern MSCs routed in load-share over C E and E E. Second choice, if both link sets are out of order, signaling is routed over C E.

20 Signaling Load Sharing For load sharing both between link-sets and between the links on the link-sets the signaling Link Selection code is used. This is a four-bit code that is set by the MTP user. Which bit to be used as the load sharing bit for load sharing between the link sets is set by the LSHB-parameter (Load sharing Bit) in the exchange data. If all links get the same number of SLS codes they will all carry the same load, i.e. the load is evenly distributed. If all the links do not get the same number of SLS codes then the load will not be evenly distributed.

21 The maximum load on the link set is limited by the signaling links carrying most of the signaling load

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23 C is the maximum load in normal operation for one link. For example, if we allow 30% maximum load on each 64kb/s link and we have 8 signaling links in a link-set. Then, assuming that we do not load share with another link-set (i.e. four bit load share within the link-set) the capacity of link set is 8*30%*64kb/s=153.6 Kbps.

24 MTP Changeover in case of link failure Section 7 Dimensioning

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26 Traffic Models Two commonly used models are Erlang B and Erlang C:

27 Erlang B This is a loss model, in that blocked calls are simply lost rather than being held in some form of queuing system. It assumes that call arrivals follow a Poisson process, that the number of users is much greater than the number of channels. From the Erlang-B table, 7 channels and a GoS of 0.02 (2%) corresponds to A= Erl of offered traffic.

28 Therefore, carried traffic = A (1- GoS) = (1-0.02) = Erl Channel Utilization: This is the ratio of carried traffic to number of channels Therefore, Channel Utilization = /7 = 0.41 or 41%

29 Erlang C Calls that cannot be handled are put in a queue until a channel becomes available. The queuing delay is a function of the offered packet traffic, the maximum number of links available and the mean holding time of each call. The Erlang C formulas are used to determine the probability of a delay occurring, the probability of such a delay being larger than a certain time and also the mean delay.

30 Example: As compared with circuit switched traffic with a blocking probability of 2% 17.5 Erlangs corresponds to 22 Erlang in C table. This suggests that there is a gain in trunking efficiency offered by tolerating a 10 ms delay in transmission. Mean delay depends on the mean holding time, which in turn is proportional to the packet size. Packet size can be reduce in order to reduce the holding time but it increase the signaling overheads.

31 Processor Load Section 7 Dimensioning

32 Definitions The processor load is the proportion of time that the processor executes instructions having real time requirements. It is normally expressed in percentage of its full capacity. It has following components: Idle load: This component depends on the functionality and to some extent on the size of the exchange. The idle load is not dependent on the traffic or other external activities but varies from processor to processor. Continued..

33 Usage load: This component is caused by operation and maintenance activities such as data dumps, commands, traffic measurements and printout of statistics. Traffic load: This component is used for traffic handling. Loadability: The loadability is the upper limit for the allowed processor load. It depends on the processor but also on the job lengths and delay requirements. Continued..

34 Load per call: This is the amount of execution time that the processor has to spend in setting up and disconnecting a call. Load per call is normally expressed in milliseconds (ms), but is sometimes expressed as the number of ASA (assembler) instructions necessary to fulfill the task. Traffic peak margin: Is sometimes referred to as Safety margin. The traffic peak margin is normally 20-35% of the available traffic load. This is needed to allow for unpredictable traffic peaks.

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