what exactly is a timeslot?

Discussion in 'Cisco' started by atv, Apr 16, 2004.

  1. atv

    atv Guest

    Hello,

    I'm trying to understand the pri-group command and the channel group
    commands. Now i know that a T1 is not the same as ISDN PRI (although never
    clearly explained what a T1 exactly is, i'm talking about remote access
    book sybex here) and i know that a t1 is 23,1 and e1 30,1 channels.

    as the book descends into configuring t1/e1 they lost me. what are
    timeslots? and what is a channelgroup?

    my assumptions are that a timeslot is 1 channel from the 24 or 31, and a
    channel, according to the book is one channel as well?

    then they assign timeslots to the D-channel (why?) for example, 11-24 and
    then use 1-6. i don't understand the relevance.

    and while i'm on the topic, what is a fractional T-1? are they referring to
    what i described above? like when a isp has a t1 and you only purchase 6
    channels you have a fractional t1?

    thanks for your reply.
    Alef
     
    atv, Apr 16, 2004
    #1
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  2. atv

    atv Guest

    yes but _why_ is it called a timeslot? and why do i need to use the
    channel-group command?

    is
    channel-group 0 timeslot 1-24 another way of saying that for channel-group 0
    i want 1-24 controlled by the D-channel? if so, my book isn't explaining it
    very well.
     
    atv, Apr 16, 2004
    #2
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  3. Yes, a timeslot is one channel. Here's a typical config
    for a channelized T1. 24 channels (or timeslots). The channel-group
    is the bundle of whatever you've assigned.

    controller T1 3/0
    framing esf
    linecode b8zs
    channel-group 0 timeslots 1-24
    In short, yes. For more:

    http://www.whatis.com

    alan
     
    Alan Strassberg, Apr 16, 2004
    #3
  4. The CCNP Routing book by Cisco Press has a decent primer/intro on T1 and
    related topics (such as what a timeslot is). Go check that out and be
    prepared to have your memory in electricity stirred somewhat.

    CS
     
    cybersoldier01, Apr 16, 2004
    #4
  5. It's a reference to Time Domain Multiplexing, which is one way of
    putting multiple channels on a digital circuit. The transmission is
    broken up into fixed time increments, which iterate through the
    channels: first something from channel 1, then something from channel 2,
    and so on.
     
    Barry Margolin, Apr 17, 2004
    #5
  6. atv

    rowl Guest

    When you purchase a T1 from a service provider, you will most likely
    buy a fractional T1. This allows you to buy multiple 64k channels
    (called DS0s) which can be easily upgraded to higher speeds by adding
    more DS0s. This is more financially affordable besides you do not need
    to buy additional equipment to upgrade your leased line capacity.

    When the individual 64K channels are bundled into a higher bandwidth
    T1, some method is required to distinguish where once channel stops
    and the other begins.If each channel is individually framed with a
    identifying header, the amount of overhead will be too large. Instead
    each of these channels is transmitted in turn in a time-slot.

    So for example, for the 128k T1 leased line you buy, your Telco will
    give you 2 DS0s, which need not be 2 contiguous time-slots of 64k.ie.
    the telco will transmit data only on some of the 24 timeslots in a T1,
    the rest will be inactive. So what you are doing when configuring
    time-slots into a channel group is simply specifying which of the
    time-slots (1-24) are active to form a 128k bundle. or for 256k line,
    which of the 4 time-slots....

    You absolutely need to read the O'Reilly book "T1: Survival guide" for
    a better understanding...

    Rgrds
    Rahul Sawarkar
     
    rowl, Apr 17, 2004
    #6
  7. atv

    Geert Guest

    Interesting discussion.
    What exactly is the difference between a PRI and a E1 ??
    Can I dial in on a E1-module ?
    I don't know if you guys have noted it, but I've seen that Cisco is
    selling PRI only on NM (network modules), not as WICs. Why is this ?

    Regards,
    Geert
     
    Geert, Apr 17, 2004
    #7
  8. A PRI is an E1 (or T1) with ISDN channels and one of the timeslots
    taken over by the signalling D channel. So, at the lowest levels, a
    PRI is an E1 or T1.
    Define dialing in and what kind of module? If you mean dialin on a
    modem, certainly, you could have a chassis with a digital modem card
    and an E1 controller card, and dialin on modems. If you mean dial into
    some sort of Voice application, you can too with a voice-processor
    card and the E1 controller card. Many combinations and options
    available here.
    You can terminate PRI's on WIC modules?? But the right one you get
    depends on your application. Almost a universal option is the
    VWIC-2MFT-T1 or -E1 card. It can do data, or it can do voice with the
    appropriate voice DSP's somewhere in the system. PRI doesn't depend on
    hardware, don't depend on finding something with PRI in its name. Its
    just a method of signalling the telco, and almost any T1/E1 controller
    can deal with PRI depending on your application.
     
    Doug McIntyre, Apr 17, 2004
    #8
  9. atv

    Geert Guest

    [Geert] Yes, but not ISDN BRI dialin, i presume. Then you need ISDN
    signaling, and then you need a PRI....


    [Geert] Well, as a matter of fact, I once tried to configure a
    VWIC-2MFT-E1 card as a PRI for ISDN BRI dialin, and it didn't work
    (worse: it wasn't configurable). I configured the E1 as usual (line code
    hdb3, framing no-crc4), but when i used the command "channel-group 0
    timeslots 0-15" I got the error: "No DSP resources available". Also,
    when I used the command "channel-group 0 timeslots 0-31", i got the
    error "Timeslot 16 cannot be changed while CAR is enabled" or something
    like that .......

    So I presume that when you buy an PRI Network Module, that the DSP
    resources are fitted on the NM, but when you buy a WIC that the DSP
    resources are not on the WIC. I have the impression that you need DSP
    resources for a PRI. Without these DSPs you can't configure a PRI on a
    WIC.................does this make sence ?

    PS. We are not talking about voice here, just want to do data
     
    Geert, Apr 18, 2004
    #9
  10. atv

    rowl Guest

    A PRI would be a channelized E1 but using a separate time-slot as a D
    channel (slot 16?). E1 may use CAS and use all 24 timeslots for data,
    with 8 bits stolen for the robbed-bit signalling when using voice
    calls (resulting in 56k channels). Else you can use HDB8 for
    clear-channel 64k capability.
    VWIC-2MFT-E1 card as a PRI for ISDN BRI dialin, and it didn't work
    (worse: it wasn't configurable).
    The MFT inserted into a NM-HDV and set up to use ISDN PRI signaling,
    can not terminate ISDN 64K or 56K data connections: it only supports
    voice-call termination. I found that while the T1/E1 cards are used
    for Access Aggregation, the MFT, with or without NM-HDV, can not
    terminate modem connection on the router...thus spake cisco Amen!
    (Will someone please clarify - what happens when I don't use a PBX
    with FXO modules?)
    the error: "No DSP resources available". Also, when I used the command
    "channel-group 0 timeslots 0-31", i got the error "Timeslot 16 cannot
    be changed while CAR is enabled" or something like that .......
    The E1 Multiflex Trunk ports are not configured like the T1 CSU/DSU
    The NM-HDV has DSP SIMMs that do the voice compression codecs /
    algorithms like G.711, G.729a/b, G.726, fax etc... the voice network
    modules primary function is to convert telephone voice signals into a
    form that can be transmitted over an IP network... Here is the list of
    cisco NM-HDVs
    1-slot 4-channel high-density voice network module with one digital
    signal processor (DSP) (NM-HD-1V)
    2-slot 8-channel high-density voice network module with one DSP
    (NM-HD-2V)
    2-slot 48-channel high-density enhanced network module with 3 DSPs,
    supporting up to 8 analog or 48 digital channels (NM-HD-2VE)
    The DSPs are available as PVDMX-X or packet fax/voice DSP modules
    thats right physical interfaces are identical, software requirements
    may be different though...
    T1 CAS 12.0(5)XK : 12.0(7)T, 12.1(1), 12.1(1)T
    T1 ISDN PRI Q.SIG and Q.931: 12.1(2)XH, 12.1(3)T
    E1 ISDN PRI Q.SIG 12.0(7)XK: 12.1(2)T
    E1 ISDN Q.931 12.1(2)XH: 12.1(3)T
    E1 R2 12.1(2)XH: 12.1(3)T

    BTW: PRI for BRI dialin you say? BRI uses 16K D channel, a PRI will
    use 64k, ie a separate time-slot for a D channel... I don't know
    what happens at the CO when you place a data call from a BRI ISDN line
    to a PRI line except that it should be seamless to us endusers... Any
    ideas ?

    Rgrds
    Rahul Sawarkar
     
    rowl, Apr 19, 2004
    #10
  11. atv

    rowl Guest

    correction: T1 may use CAS and use all 24..
    Correction: Else you can use HDB3 for
    PS: E1 always uses timeslot 0 for Framing. Slot 1-31 are available
    to transmit data with an aggregate bandwidth of 2.048 Mbps - 64 kbps (TSO)
    = 1.984 Mbps.


    Rgrds
    Rahul Sawarkar
     
    rowl, Apr 19, 2004
    #11
  12. ~ [Geert] Well, as a matter of fact, I once tried to configure a
    ~ VWIC-2MFT-E1 card as a PRI for ISDN BRI dialin, and it didn't work
    ~ (worse: it wasn't configurable). I configured the E1 as usual (line code
    ~ hdb3, framing no-crc4), but when i used the command "channel-group 0
    ~ timeslots 0-15" I got the error: "No DSP resources available". Also,
    ~ when I used the command "channel-group 0 timeslots 0-31", i got the
    ~ error "Timeslot 16 cannot be changed while CAR is enabled" or something
    ~ like that .......
    ~
    ~ So I presume that when you buy an PRI Network Module, that the DSP
    ~ resources are fitted on the NM, but when you buy a WIC that the DSP
    ~ resources are not on the WIC. I have the impression that you need DSP
    ~ resources for a PRI. Without these DSPs you can't configure a PRI on a
    ~ WIC.................does this make sence ?
    ~
    ~ PS. We are not talking about voice here, just want to do data

    This isn't something that we explain too well (at all?)

    In general, to do sync data (like sync PPP) on a channelized
    interface, you need to terminate it in some kind of framer
    hardware. If you want to terminate 30 sync PPP calls in an
    E1, you need 30 hardware framers.

    In most cases, the framer hardware will be onboard the trunk
    (channelized T1/E1) card. For example, an NM-*2CE1* card will
    have enough onboard resources to handle sync PPP calls across
    all 60 timeslots.

    Some channelized T1/E1 hardware does have NOT sufficient
    onboard resources to frame sync calls across all timeslots.
    The MFT VWIC is one such. (The old non-plus 8E1 PA for the
    7200 was another ... it could only handle 128 sync PPP calls.)

    In my limited understanding, the MFT VWIC's data capabilities
    depend on the mother card that it's stuck into. In general, I
    think it can handle unchannelized T1/E1 (channel-group timeslots
    1-24 or 1-30) or possibly two timeslots. For *voice* calls, if
    it's in a NM-HDV, it can handle the full load of calls ... but
    I don't think that the MFT VWIC in any configuration ever supports
    ISDN data calls ... I suppose theoretically the hardware could
    handle the D channel plus a single B channel, but why bother ....

    This explains more about the MFT VWIC:
    http://www.cisco.com/warp/public/788/products/vwicmf_e1.html

    So, on the midrange boxes, if you want to do ISDN data, use
    NM-*CE1*.

    Aaron
     
    Aaron Leonard, Apr 20, 2004
    #12
  13. atv

    Geert Guest

    Thanks Aaron,
    for your clear explanation

    Regards,
    Geert
     
    Geert, Apr 20, 2004
    #13
  14. atv

    rowl Guest

    Hello Aaron
    I think by channelized you mean fractional E1 capability, where
    individual 64k channels can be distinguished... I don't see the need
    for 30 hardware framers - this implies that framing is done
    individually in isolation for each 64k channel - which is not the
    case. A single frame consists of 32 time-slots in E1 (24 in T1).

    With Framed E1, we have option of using either 30 or 31 time-slots.
    Time-slot zero is always used for framing. For voice calls additonal
    signallig bits require a multi-frame structure and time-slot 16 is
    used along with time-slot zero - this is Channel Associated Signaling
    (CAS) in E1 For data calls one can use timeslots 1 through 31.

    Two channel-groups, which is a virtual serial interface that can
    contain 1-31 time-slots
    Heres is selected text copied from a TAC Doc:
    "The Multi-Flex Trunk card does not support ISDN data or modem
    termination in a traditional Network Access Server (NAS) dial
    scenario. Under specific scenarios, ISDN data and modem connectivity
    may be possible when the VWIC is used for packet voice, such as in the
    case of Modem Passthrough and Modem Relay to a peer voice gateway."
    Further
    "It only supports voice-call termination when using ISDN PRI
    signaling. When the Multi-Flex Trunk is used as a Telephony interface
    to terminate voice traffic, each DS0 can *contain* one voice call..."

    Any ideas how a voice call may be terminated without FXO/FXS and
    PBX/Telephones?

    Rgrds
    Rahul Sawarkar
     
    rowl, Apr 21, 2004
    #14
  15. atv

    cracknut

    Joined:
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    Hi ,
    we have an E1 PRI link at our HQ , and I want to share it with our branch offices.I have a router with e1 chanalized card.realy apprciate if anybody can explain me how it works.
     
    cracknut, May 25, 2008
    #15
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