Wednesday, February 23, 2011

What's the difference between "dB", "dBm", and "dBi" ?

I keep seeing people using the terms "dB", "dBm", and "dBi" interchangeably, when they actually mean very different things. So, here's a little background on the correct usage of the terms.

A dB is a RELATIVE measure of two different POWER levels. There's also dB relative to VOLTAGE levels, but I won't go into those, as we're mostly concerned with POWER levels in our discussions here. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). Thus, I can say that an amplifier has “30 dB of gain”, or I have “6dB total feedline loss”. I CANNOT say, “My amp puts out 30 dB”, or “I have a 24dB antenna”, as you must state what you're referencing it to, which is where the subscript comes in. The dB by itself is not an absolute number, but a ratio.

For amplifiers, a common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt. How much gain it has is a different matter entirely, and you can have two different amps, each with an output of 30dBm (1Watt), that have different gains, and require different levels of drive power to achieve their outputs. You can also have two different amps with the same gain that have different output powers.

There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with “Big Stuff”, as 30dBW is 1000w, and way beyond what we deal with here!

For antennas, a common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source. An Isotropic source is the perfect omnidirectional radiator, a true “Point Source”, and does not exist in nature. It's useful for comparing antennas, as since it’s theoretical, it’s always the same. It's also 2.41 dB BIGGER than the next common unit of antenna gain, the dBd, and makes your antennas sound better in advertising. The dBd is the amount of gain an antenna has referenced to a DIPOLE antenna. A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd, since we're comparing it to itself. 

If I say I have a “24dB antenna”, it means nothing, as I haven't told you what I referenced it to. It could be a 26.41dBi antenna (also 24dBd), or a 21.59dBd (also 24dBi) antenna, depending on what my original reference was. The difference is 4.81dB, a significant amount. Most antenna manufacturers have gotten away from playing this game, but the reference will be different in different fields. 

Reference Link:

Explain SS7 Protocols Stack ?

SS7 Protocols Stack

The standard SS7 protocol has 4 levels (layers) as defined in the OSI 7 Layer Model. The levels 1 to 3 constitute the Message Transfer Part (MTP) and level 4 is the User Part (Transport Layer in OSI).

  • MTP1 = Message Transfer Part 1
  • MTP2 = Message Transfer Part 2
  • MTP3 = Message Transfer Part 3
  • SCCP = Signaling Connection Control Part
  • TCAP = Transaction Capabilities Application Part
  • MAP = Mobile Application Part
  • INAP = Intelligent Network Application Part
  • ISUP = ISDN User Part

1.) Message Transfer Part (MTP Level 1) Physical
  • Provides an interface to the actual physical channel over which communication takes place
  • CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps
    2.) MTP Level 2 (Data Link)

    • Ensures accurate end-to-end transmission of a message across a signaling link
    • Variable Length Packet Messages are defined here
    • Implements flow control, message sequence validation, error checking and message retransmission
    • Monitor links and reports their status
    • Test links before allowing their use

    3.) MTP Level 3 (Network)
    • Message routing between signaling points in the SS7 network
    • Signaling network management that provides traffic, links and routing management, as well as congestion (flow) control
    • Re-routes traffic away from failed links and signaling points, controls traffic when congestion occurs
      4.) Signaling Connection Control Part (SCCP)
      ·         Provides connectionless and connection-oriented network services
      ·         Provides global title translation (GTT) capabilities above MTP level 3; translates numbers to DPCs and subsystem numbers
      ·         Provides more detailed addressing information than MTPs
      ·         Used as transport layer for TCAP (Transaction capabilities applications part) based services

      5.) Transaction Capabilities Applications Part (TCAP)

      ·         Exchange of non-circuit related data
              Between applications across the SS#7 network
              Using the SCCP service
      ·         Queries and responses sent between Signaling Switching Point (SSPs)  and Signaling Control Point (SCPs)
      ·         Sends and receives database information
              Credit card validation
              Routing information

      6.) Telephone User Part (TUP)
      • Basic call setup and tear down
      • In many countries, ISUP has replaced TUP for call management
        7.) ISDN User Part (ISUP)
        • Necessary messaging for setup and tear down of all circuits (voice and digital)
        • Messages are sent from a switch, to the switch where the next circuit connection is required
        • Call circuits are identified using circuit identification code (CIC)

        Explain SS7 Links ?

        SS7 Links

        The figure shows the relationship between the link names and the link location (type). Signaling links are logically organized by link type ("A" through "F") according to their use in the SS7 signaling network. 
        The "A" (access) links connect the signaling end points (e.g., an SCP or SSP) to the STPs. Only messages originating from or destined to the signaling end point are transmitted on an "A" link. The "B" (bridge) links connect the STP to another STP. The "C" (cross) link connects STPs performing identical functions into a mated pair. "D" (diagonal) links connect the secondary (e.g., local or regional) STP pair to a primary (e.g., inter-network gateway) STP pair in a quad-link configuration. "E" (extended) links connect the SSP to an alternate STP. An "F" (fully associated) link is connected between two signaling end points (i.e., SSPs and SCPs).All SPs (signalling points) are connected using (typically) pairs of Links. 

        All links use the same physical connections (typically DS0A - 56K bit/s or DS1 (T1)).


        A link (access)
        Connects signaling end point (SCP or SSP) to STP
        B link (bridge)
        Connects an STP to another STP; typically, a quad of B links interconnect peer (or primary) STPs (STPs from a network connect to STPs of another network)
        C link (cross)
        Connects STPs performing identical functions, forming a mated pair (for greater reliability)
        D link (diagonal)
        Connects a secondary (local or regional) STP pair to a primary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D links is arbitrary
        E link (extended)
        Connects an SSP to an alternate STP
        F link
        (fully associated)
        Connects two signaling end points (SSPs and SCPs) in the same local network


        Explain SS7 Architecture ?

        SS7 Architecture

        SS7 Architecture consists of three different entities: 
        1. SSP  (Service Switching Point)
        2. STP (Signal Transfer Point)
        3. SCP (Service Control Point)                                                                                                 

        1.    1.) Service Switching Point (SSP)

        The Service Switching Point (SSP) is the local exchange in the telephone network. An SSP can be a combination voice switch and SS7 switch, or an adjunct computer connected to the local exchange's voice switch. The SSP provides the functionality of communicating with the voice switch via the use of primitives and creating the packets, or signal units, needed for transmission in the SS7 network. The SSP function is to use the information provided by the calling party (such as dialed digits) and determine how to connect the call. A routing table will identify which trunk circuit to use to connect the call, and which exchange this trunk terminates at. An SS7 message must be sent to this adjacent exchange requesting a circuit connection on the specified trunk. There are very few features required of an SSP. The ability to send messages using the ISDN User Part (ISUP) protocol and the Transaction Capabilities Application Part (TCAP) protocol is the only requirement, other than the network management.

          • SSPs are switches in SS7 network.
          • SSPs convert a dialed number from a subscriber line to SS7 signaling messages.
          • SSPs setup, manage and release voice circuits required to make a call.
          • SSPs send messages using the ISDN User Part (ISUP) and Transaction Capabilities Application Part (TCAP) protocols 
          • SSP's function is to use a global title to determine how to connect a call using its   routing table.                                                                                                                                           

        2.) Signal Transfer Point (STP)

        STPs are packet switches, and act like routers in the SS7 network. Routes each incoming message to an outgoing signaling link, based on routing information contained in the SS7 messages and a pre-defined route table. It does not offer termination services. STPs are paired to ensure redundancy

        There are three levels of STPs.
        • ·        National Signal Transfer Point
        • ·        International Signal Transfer Point
        • ·        Gateway Signal Transfer Point

          National STP exists within the national network
          ·        Protocol converters often interconnect a National and an International STP by converting from ANSI to ITU-TS.

          International STP functions within an international network.
          ·        All nodes connecting to an International STP must use the ITU-TS protocol standard.

          Gateway STP converts signaling data from one protocol to another.
          ·        Gateway STPs are often used as an access point to the international network.
          ·        Depending on its location, the Gateway STP must be able to use both the International and National protocol standards.

          •         An STP is a router and/or a gateway in the SS7 network.
          •         Messages are not originated by an STP..
          •         If an originating SSP does not know the address of a destination SSP, the STP must    provide it using Global Title Translation.
          •         Gateway STPs serve as the interface into another network and they can provide protocol conversion.
          •         STPs also provide traffic and usage measurements.

           3.) SCP (Service Control Point)

          An SCP is usually a computer used as a front end to a database system. It is an interface to application-specific databases. The address of an SCP is a point code, and the address of the database it interfaces with is a subsystem number. The database is an application entity which is accessed via the TCAP protocol. It accepts a query for information from a subsystem at another node. SCP is used by STP to perform a function called global title translation

            Monday, February 21, 2011

            What is SS7 ?

            SS7 (Common Channel Signaling System No. 7/Short Signalling 7/Signalling System Number 7/ C7)

            1.) SS7 Definition
            2.) SS7 Functionality 
            2.) SS7 Architecture
            3.) SS7 Link Interface
            4.) SS7 Signalling Protocols

            1.) SS7 Definition :  
            Signalling System Number 7 (SS#7 or C7) is the protocol used by the telephone companies for interoffice signallingSS7 is a critical component of modern telecommunications systems. SS7 is a communications protocol that provides signaling and control for various network services and capabilities. Every call in every network is dependent on SS7. Likewise, every mobile phone user is dependent on SS7 to allow inter-network roaming. SS7 is also the "glue" that sticks together circuit switched (traditional) networks with Internet protocol based networks.

            2.) SS7 Functionality:

            The SS7 network and protocol are used for:
            • basic call setup, management, and tear down
            • wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
            • local number portability (LNP)
            • toll-free (800/888) and toll (900) wireline services
            • enhanced call features such as call forwarding, calling party name/number display, and three-way calling
            • efficient and secure worldwide telecommunication

            3.) SS7 Architecture:

            There are three kinds of signaling points in the SS7 network as shown 
            • SSP  (Service Switching Point)
            • STP (Signal Transfer Point)
            • SCP (Service Control Point)

            • Access Link (A Link) 
            • Bridge Link (B Link) 
            • Cross Link (C Link) 
            • Diagonal Link (D Link) 
            • Extended Link (E Link) 
            • Fully Associated Link (F Link)

            This is just an brief overview of the SS7 network layout. 

            Sunday, February 20, 2011

            What is LTE ?

            LTE, or Long Term Evolution, is a a 4th generation (4G) mobile broadband standard and is aimed to be the successor to the 3G technologies GSM/UMTS. It is currently in development and is considered the competitor to WiMAX. Carriers will include Verizon, Vodafone, AT&T, and many more worldwide. Verizon launched their LTE service in 38 markets in late 2010.

            Like WiMAX, this technology provides broadband services wirelessly (like EVDO), but instead of transmitting signals via microwaves, LTE utilizes a radio platform. Users need an LTE modem to access the network, which can be in USB format, ExpressCard, PCMCIA, or embedded in a laptop; it will also likely be featured as the internet connection on PDAs and phones.

            This super-fast network, which is promising theoretical peak download rates of up 100Mbps (real-world speeds will vary, of course), provides an alternative to DSL, cable, satellite, and dial-up internet, which will be a big boon to people living in areas that aren't currently serviced by a high-speed network. It will also free people from the burden of having to find a WiFi hotspot when they are on the road - as long as you have an LTE modem, you can connect to the internet anywhere in the service provider's coverage area!

            Key Details:

            Theoretical downlink peak data rates up to 100 Mbps with 20 MHz bandwidth
            Theoretical uplink peak data rates up to 50 Mbps with 20 MHz bandwidth
            Reduced latency to 10 msec round-trip time between user equipment and base station

            LTE deployment is considerably behind WiMAX, and it likely won't be widely available until about 2012 .

            What Verizon has to say about its LTE technology have a look.

            AT&T iPhone beats Verizon in nationwide 3G speed tests

            'CNN - The AT&T iPhone's average download speed was 1,769 Kbps, and the average upload speed was 730 Kbps. By way of comparison, the Verizon iPhone's average download speed was 848 Kbps, and the average upload speed was 506 Kbps.,
            Well ! what difference does it makes if AT&t beats Verizon 3G Speed test by 300-400Kbs. I mean what difference it would make if you stream video faster than your friend ultimately you both gonna watch the same video. In my opinion the actual test should be of " Network Availability and Accessibility". 
            What I mean by Network Availability and Accessibility ? 
            'CNN- However, the AT&T iPhone sometimes could not complete tests because it did not have a connection, whereas the Verizon iPhone successfully completed every test. In short, I found the Verizon iPhone to be slower with network transfers but more reliable with coverage. Reviewers at other publications had the same results.'
            Now, that is what I mean... Verizon has far more better network coverage than At&T.  If a user is not able to access the network then what is the use of all that high speed. Give it a thought !

            Types Of Interfaces: 2G

            Different Types of interfaces:

            l   A interface
            l   B interface
            l   C interface
            l   D interface
            l   zz interface
            l   39/xx interface
            l   Q interface
            l   T1 interface
            l   ISUP interface
            The A interface exists between the circuit switched domain of the CDMA network subsystem (NSS) and the base station subsystem (BSS). In terms of entity, it is the interface between the MSCe/MGW and BSC.
            The A interface complies with the base station subsystem application part (BSAP) of signaling system SS7. It includes voice channels and signaling channels. The A interface is used to transmit the following messages:
            l   Mobile station (MS) management messages
            l   Base station (BS) management messages
            l   Mobility management messages
            l   Call processing messages

            2. B Interface

            The B interface exists between MSCe and VLR. It adopts an internal protocol without a specific signaling transmission mode. The B interface is used in the following cases:
            l   The MSCe obtains user messages from the VLR.
            l   The MSCe notifies the VLR to record location information when an MS updates its location.
            l   The MSCe notifies the HLR to update data through VLR when an MS activates a supplementary service or modifies the relevant data.

            3. C Interface

            The C interface exists between the MSCe and HLR. It complies with the mobile application part (MAP) of the SS7. The C interface is used in the following cases:
            l   The HLR sends routing information to the MSCe when an MS is called.
            l   The short message service (SMS) is implemented.

            4. D Interface

            The D interface exists between VLR and HLR. It complies with the MAP of the SS7. The D interface is used to transmit the following messages:
            l   Authentication data
            l   Location update messages
            l   User data indexed during call connection
            l   Data of supplementary services
            l   VLR recovery messages

            5. zz Interface

            The zz interface exists between two MSCes. It complies with the SIP with encapsulated ISUP (SIPI).
            The zz interface provides the inter-office call control function for the narrowband circuit-switched domain services that are independent of the user plane bearer technology and the control plane transmission technology.

            6. 39/xx Interface

            The 39/xx interface exists between an MGW and a media resource function processor (MRFP). It complies with the Megaco/H.248. The 39/xx interface is used in the following cases:
            l   The MSCe controls dynamic and static resources of the transmission modes (IP/TDM) in the MGW during call processing, including terminal attributes, terminal connectivity, and media streams.
            l   The MSCe maintains and manages the status of the MGW.

            7. Q Interface

            The Q interface exists between the MSCe and the MC. It complies with the MAP of the SS7 to support the short message service.

            8. T1 Interface

            The T1 interface exists between the MSCe and the SCP. It complies with the WIN of the SS7 to support the intelligent network service
            9. ISUP Interface
            When the MSC serves as a gateway office, it provides the interface between the PSTN and other mobile network devices. It also controls the incoming calls and outgoing calls through ISUP or TUP of the SS7