ss7 network -=-=[hitman]=-=- -=-=h1tman@anti-social.com=-=- Introduction SS7 Network Components Signaling Network Architecture SS7 Links SS7 Packets (brief) The Telephone Call Ending Introduction Today, most telephone carriers are using CCIS or Common Channel Interoffice signalling. SS7 or Signalling System Seven, is a good example of CCIS. SS7 has three basic components to set up a proper network. SS7 Network Components Signal Switching Points (SSPs) SSPs are are telephone switches with ss7 software installed, they originate,terminate,switch and route calls. Signal Transer Points (STPs) STPs are packet switches that transfer signalling packets from one point to another and also perform specialized routing functions. Signal Control Points (SCPs) SCPs are databases that are responsible for providing data for advanced calling situations like 800/877/888 routing,calling card services, caller id,etc. Also note that i am talking about QAS( Quasi-Associated Siganlling and not Europes ASSociated Signalling. :) Signaling Network Architecture The subcriber telephones are connected to the SSP via their local loops. The SSPs are linked to two STPs,which are linked to two redundant SCPs. 1) STP A and STP B are redundant,a mated pair. 2) SSP C and SSP D are also a mated pair. 3) Each SSP are linked to each STP Here is a sample SS7 network that i drew up.excuse my assci art CO1 [SSP]** * ** * * [SCP2]-*----[STP1++----------+[STP3]--------[SCP3] * ** * |*** **| ** ** * | ** * * | * CO2 [SSP]* * * | **** | *********** CO4 [SSP] * * | ** ** | * ** * [SCP2]------[STP2+**-------**+[STP4]---------[SCP4] * * * * CO3 [SSP]****** * * * * * * * * * * * * * * * * [T1] [T2] [T3] The network is set up like this because if one device were to fail,like perhaps an accident were to take out a ss7 t-1s,then there is a backup. This is the major difference between europes and north america's ss7 networks. SS7 Links There are several types of links in a SS7 network. A - Access Link B - Bridge C - Cross D _ Diagnol E _ Extended F - Fully Associated A links connect STPs to SSPs and SCPs. A links are used to deliver siganlling messages between ss7 packet switches and COs or tandem offices. An A link would be STP1 and SCP2 and STP2 to CO1. B Links connect two STPs to another STP pair. Example of a B link is STP1 to STP3 and STP2 and STP4. C Links are used for packet rerouting just incase on STP pair should fail. Example of a C link are STP1 to STP2 and STP3 to STP4. D Links are almost the same as B links except as you expect connect STPs diagnolly. Example of a D link would be STP1 to STP4 and STP3 to STP2. E Links directly link to STPs together,one of the STPs might be located elsewhere F links are what are used in Associated Siganlling in Europe and connect two close offices directly to each other. SS7 Packets This section i will briefly go over the functions of the three ss7 packets. Next issue i we will take a close look at their structure. Siganl Units or SUs are the ss7 packets that are being sent across the siganling link. These are the three SU packets Message signal units (MSUs) Link status signal units (LSSUs) Fill-in signal units (FISUs) FISUS, or Fill-In Signal Units are sent when no important informations is being sent over the siganl link. They contain no data,they fill up the signalling link until there is a need to send important signalling. They provide a constant signal over the link which is good for monitering and troubleshooting. MSUs are Message Signall Units, send messgases between SSPs and STps and STPs and SCPs. MSUs conatain routing information,trunk data,etc. There are several types of MSUs.. ACM-Adress Complete Message- Indicates that an IAM has been recieved. ANM-Answer Message- Is sent when the calling subscriber picks up his phone, it tells the trunk to open itself in both directions. IAM- Initial Address Message-is used to begin all call REL- Release Message- is sent to tell that one of the subcribers has hung up. RCL- Release Complete Message- confirms that the trunk has been released. LSSUs or Link Status Signal Units provide information on the status of the signal link. The Telephone Call You pick up the phone and your CO gives you dial tone. You punch in some numbers and your CO analyzes it and determines what it needs to send the call to the neighboring CO. Your switch (stp) selects and idle trunk between itself and the other and sends an IAM. The IAM tells the other trunk who is calling who,what trunk is selected and other information. When the other CO recieves this IAm,it sends an ACM to tell the other switch that it got the IAM. When the CO orginitating switch receieves the ACM,it connects to the calling subscribers line to the selecte trunk in the backwards direction,so that the ringing could be heard. When or if the called subscriber picks up,the second switch sends the ANM,which remember tells the trunk to open itself in both directions. Whne the calling party hangs up,his switch sends an REL which tells the other switch to release the line and then an RCL to confirm that the trunk is released and to stop billing. Ending Hope that wasn't too hard. If so just read it over again. And do what i did,just take out a pen and paper and draw your own little ss7 network. In the next issue along with the detailed packet structure and ss7 layers i will go over how an 800 call is routed. I want to give a shout out to mah boi Lupus Gentry for proof reading this and my other article for me and shiz.aight werd. -until next time