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G.703 und G.704 -- gesammeltes Wissen :

Es gibt ganz speziell für diese Normen sehr viele gleichlautende Be- und Umschreibungen, die uns manche Nacht gekostet haben.

What is the difference between G.703 and G.704 ?

G.703 is an international standard describing the physical coding of signals. It defines the data rate (2 Mbps(E1), 8.4 Mbps(E2), 45/34 Mbps (E3), 98/139 Mbps (T4/E4) and the coding scheme (HDB3).The most common speeds are 2.048 Mbps (E1).


On top of the G.703 physical definition G.704 defines the format of data (the frames) being transmitted with E1 speeds. pls read multiple E1 with Cisco


The bandwith of 2 Mbps is divided into 32 timeslots of 64 K where timeslot #0 bears all the information concerning the framing.

Any unit supporting G.704 is able to use only certain timeslots of these 32 and leaves the other ones untouched.


Das war eine Interpretation. Die weiteren kommen noch. Siehe auch Ascend.

E1 = PRI = Primary rate interface

The Primary rate interface is defined by physical layer protocol and also by higher protocols included LAPD. The PRI has a full duplex point to point serial ,synchronuos configuration. In classical ISDN there are two channels reserved used for :


1. Signal synchronization and line performing

2. High level protocols for network management


The CCITT recommendation G703,G704 defines the electrical interfaces and the frame formats.

There are two different interfaces:


1. North America T1-1.544Mhz.

2. Europe CEPT E1-2.048Mhz.


for Europe it is CEPT E1-2.048Mhz

The 2.048kbit/s PRI multiplexes 32 channels.Time slot0 is reserved for physical layer synchronization and signaling.Time slots 1 through 15 and 17 through 31 are used for B channels.Time slot 16 is used for D-channels for network signaling. pls read multiple E1 with Cisco

Was bringt G.704 ?

- das sind die verschiedenen Antworten :


Can I have many 64K DDS circuits terminate on a single physical interface of a router ??

Antwort 1 = >> no. -- Antwort 2 = > Incorrect.

That may have been the case long ago, but you can ask for a "Stacked E1" interface. You can have up to 8 separate services terminating on a single G.704 interface, as long as the total number of timeslots is 31 or less.

DDS is carried by 31 slots of 64kbs.

  • each circuit can utilise one or more slots.
  • you can have them terminating onto one E1/G.704 cable.
  • but thats likely to be rather a lot more both upfront and ongoing costs
  • then to have it terminate at a few seperate v35 (or equivalent) "clear channel" cables.

Sein Statement : G.704 may become worthwhile where you need "many" or high performance.

Frage : G703 gives 2048kb but Cisco gives 1984Kb Date: 05/12/2000

Antwort :

Are you implying that the Cisco 72xx 8-port channelized E1 card supports data on timeslot 0 (i.e. no framing)? Our experience does not bear this out (but maybe we are doing something wrong). As far as I know, Cisco has a separate 4-port G.703 interface serial card for the 72xx that lets you do a pure unframed 2048 kb/s.


You're right. There is a E1 designed for channelized E1 interface (where you can get only 31 channels for data, typically used in n*64 aggregation), and there is another that cannot be used for this kind of channelized links, but works for G.703 E1 leased lines.

Frage :

I am working with Digex/Intermedia to switch our telco from AT&T to Southwestern Bell. We have an Ascend Pipeline 220 T1 router and Digex has some kind of Cisco router at their end.


When we move the router to the new line, the connection goes up, then down, then up, then down, etc. Ascend engineers told me that the problem is with the way that Cisco routers handle magic numbers, and that the Cisco router is mistakenly thinking that it is loopback mode when the Ascend router sends the original magic number back as an ACK. Digex believes that the problem is with the Ascend router and they refuse to do anything to fix the problem. Does anyone know what needs to be done to which router in order to fix this problem?


Antwort :

Make sure that the DSU/CSU's are correctly configured to match the line signalling and whatnot. Also make sure that both routers (they were working before, right? you're just switching the T1 circuit?) are set up to use the same protocol (probably PPP) .If it was working before, it should be working now so I suspect the configuration of the DSU/CSU's.


As I remember magic numbers in PPP, you either have the value "0" or the endpoints negotiate different values. During LCP negotiation, if an endpoint sent a conf-req with the same magic number, the response from the other node should be should be a conf-nak - with a suggested "acceptable" alternative magic number. Its a way of detecting a loop on the line. There isnt of course a loop on the line....which would give the symptoms you mention..? If not a loop, then a posted debug of the LCP negotiation may be useful. Nick

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