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Hi -
Can anyone tell me whether Switched Multimegabit Data Service is a Cell
Switched or Packet Switched WAN link please?
In my Cisco notes it says it is Cell Switched along with ATM,however in
the glossary it states...
A high-speed, packet-switched, datagram-based WAN networking technology
offered by the telephone companies.

Which is correct?


I think it's more cell than packet.
If I remember correctly there are two buses running in opposite directions
forming a pseudo-ring ( Token-ring like ); in each bus there is a continuous
stream of empty cells inserted by the Telco; a station then can "book"
service by monitoring this stream of cells ( either in clockwise or counter
clock wide direction ) so that if no other station has requested cells then
this station can use them all. However SMDS is somewhat complex, not very
efficient and cell structure is incompatible with ATM so I think it's not
used anylonger now.


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Switched Multimegabit Data Service (SMDS) is a telecommunications
service that provides connectionless, high- performance,
packet-switched data transport.

It is neither a PROTOCOL nor a TECHNOLOGY.

Rather, it supports standard protocols and communications interfaces
using current (and future) technology.

SMDS allows users to transparently extend their data communications
capabilities over a wider geographical area.

Since it is a SERVICE offered by the telephone companies, SMDS permits
this expansion using existing Customer Premises Equipment (CPE) and
protocols, with minimal investment in dedicated leased lines as the
number of line terminations increases.

SMDS has been defined by the IEEE 802.6 Metropolitan Area Network (MAN)
standard, as implemented by Bellcore.

It can use a variety of technologies, including Broadband ISDN (B-ISDN)
and Distributed Queue Dual Bus (DQDB).

Current North American implementations utilize DQDB with DS1 (1.5
Mbit/s) or DS3 (45 Mbit/s) lines.

Other implementations utilize E1 lines at speeds in excess of 1.9
Mbit/s or E3 lines.

Future SMDS networks will couple B-ISDN with SONET OC3 at 155 Mbit/s.

The development of this service has paralleled the emerging
Asynchronous Transfer Mode (ATM) standards.

Like ATM, SMDS uses cell relay TRANSPORT.

Both services use 53 octet cells for transport and can accommodate
packet lengths of 9188 octets (However, the maximum length for SMDS is
9188 octets and the maximum length for ATM is 65535 octets.)

Because of this, SMDS is considered to be an intermediate between the
packet-switched services offered today and the ATM service of the


Brad Reese

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Both - although since it is more or less dead it probably doesnt matter....

The "hand off" from a router to the CPE is normally HSSI for high speed, and
packets flow across the interface. Most services stopped at 34 to 50 Mbps

Within the WAN transmission - the UK service which was run by BT was cell
based, even for links as slow as 2 Mbps.

And just to confuse the issue, BT also provided interworking with both ATM
and Frame Relay WANs inside their network......

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Regards - replace xyz with ntl


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Thanks for that

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