Do X.25/Frame Relay/ATM/MPLS have their own isolated physical infrastructure? Or are they technologies invented to utilize the then existing infrastructure of the PSTN?

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    Dec 17, 2020 at 15:47

3 Answers 3


Short answer: No, and no.

Long answer: At the time X.25, Frame Relay, and ATM were in widespread use, voice networks were still largely "circuit switched," meaning, a dedicated data channel was likely to exist between the PSTN switches terminating each end of a phone call. The PSTN inter-exchange switches themselves handled the circuit-switching.

The technologies you mentioned are all "packet switching" (or cell switching) technologies allowing for over-subscription. This is important when serving data terminals (such as early cash dispensers or finance applications.)

Those packet-switching technologies used some of the same underlying link-layer technologies as voice networks. For example, a DS1/T1 might've carried 24 voice calls or 1.5Mb/s of packet data.

  • What other technologies are analogous to DS1/T1?
    – Noob_Guy
    Mar 7, 2020 at 10:41


The PSTN used to have an analogue core with various analogue signalling methods. This was replaced by digital T/E carriers with CAS signalling to support the analogue signalling methods like loop start, by giving indication of on-hook status in the robbed bits, until the core signalling was replaced with digital SS7 CCS signalling. These carriers were part of the PDH hierarchy, which were eventually bundled into SDH carriers over WDM in the core network. The PSTN local loop is still analogue which is now referred to as POTS and uses loop start signalling.

POTS lines used to connect to an MDF in the exchange and then connect to a class 5 telephone SSP switch line card that performs the analogue BORSCHT functions and convert to SS7 signalling to be used between the SSP and STP from there to connect the call between tandem class 4 SSP switches.

In DSL, the DSL modem would connect to a DSLAM line card in the exchange that would filter off the voice back to the MDF and then it would connect to the class 5 switch line card. The modem to DSLAM connection was PPP over ATM over DSL, and the DSLAM to BRAS connection was PPP over ATM over SDH over Fibre/WDM. This was replaced with an Ethernet backhaul between the DSLAM and BRAS and the DSLAM was moved to a street cabinet, and eventually EFM (Ethernet in the first mile) replaced ATM over DSL. For FTTC it is now PPP/IP over ethernet over VDSL PTM mode.

The voice is filtered off at the DSLAM cabinet line card and goes over the old analogue voice pair to the exchange. In the exchange it connects to an MSAN line card, which performs POTS BORSCHT functions, but this is now an AGW (access gateway) that converts the analogue signalling to a H.248 VoIP session with a wholesale AGC, which acts as an MGC for residential POTS gateways (i.e. it implements the AGCF function which is slightly different to the MGCF function, which is for interacting with T/E carriers and SS7). The AGC is a P-CSCF that connects to an ISP S-CSCF call server via SIP. The AGC controls the RTP session between the MSAN MGW and the destination MSAN MGW.

The PSTN essentially ends at the MSAN now and has been reduced to just the POTS network in many areas. Soon POTS will be replaced with VoIP from the router, and the PSTN will disappear altogether.

The MSAN also offers ADSL2 broadband, and the connection to it is PPPoEoA/IPoEoA I think. The PPPoE connection is carried via fibre ethernet to the BRAS. The CMSANs are aggregated using AMSANs and then FMSANs and then the FMSAN connects to the BRAS. Similarly, the DLSAMs in the street cabinets have their own aggregation, they connect to L2Ss and then EESs before connecting to the BRAS. The link to the BRAS is C/DWDM but the links in the aggregation network are probably non-WDM fibre.


The GMSC is just another SSP that connects to the MSC SSP. The SSP converts between ISUP SS7 signalling to BSSAP SS7 signalling and then the BSC converts it to BTSM LAPD SS7, and the BTS converts it to the GSM air protocols. This was upgraded in like 3GPP release 4 to a VoIP core. It's now an RTP session between CS-MGWs controlled by MSCs acting as MGCs via H.248, and the connection between MSCs is BICC or SIP. The MSC converts this to BSSAP SS7 signalling with the BSC or uses a signalling gateway. This was upgraded in release 5 to VoIP to the mobile, with the GGSN connecting to a P-CSCF in the IMS subsystem via a PDF. So essentially, the introduction of MGCs/MGWs was like the residential upgrade from class 5 switches to AGCs/MGWs, and the 3G VoIP upgrade is like VoIP to the router, which will mean the phasing out of the AGCs/MGWs. The IMS system can interface with the SS7 system via the MGCF<->IMSMGW<->SSP and MGCF<->IMSSGW<->STP. The MGCF acts as the P-CSCF. This is the function of the MGCF – connecting VoIP and SS7 via H.248. AGCF is for connecting POTS and VoIP via H.248.


MPLS is now used in the core to provide a VPRN or VPLS VPN between the BRAS and ISP LNSs. For PPPoE I have seen PPP L2TPv3 tunnels over VPRN to the ISP LNS, with tunnels set up using PADI authentication NAS-port-ID verified on the BRAS RADIUS server, with IPs being assigned by the ISP LNS RADIUS server IP pools via IPCP after the tunnel gets set up. I think with DHCP/IPoE, the ISP uses a DHCP server to assign IPs based on option 82 or 61 etc., receiving a DHCPNAK if the information is incorrect, and then IP packets from the client are routed from the wholesale provider from a dot1q tunnel to a H-VPLS VPN to the ISP by assigning the ISP a certain outer VLAN. One advantage of VPRN/VPLS is that ISPs can use CG-NAT, and the network provider can route to those ISPs despite any private IP clashes because each ISP has a separate VPN. There are various other advantages of using MPLS instead of plain IP.


The first commercial WAN protocol over analogue lines, had lots of built in error correction and retransmission. This was due to the high error rate of the analogue modem standards. A digital signal was modulated with analogue carriers and the noise in this analogue signal would accumulate over analogue trunks and FDM switching in the exchange. When the core was upgraded to E1/T1 PCM standard, the PCM encoded analogue signal was transmitted across trunks and recovered perfectly and the noise added would not remain in the signal. It was then placed on a new E1/T1 line without the noise carrying over.

Frame Relay

Originally an ISDN service, based on X.25 but without the error correction and retransmission. If the packet has a error, it is dropped. Retransmission must be requested by higher layer protocols. In the ISDN stack it is LAPD.

Before DSL, in the UK BT had ISDN, Frame Relay, SMDS, X.25, POTS dialup, leased line services. For dialup I assume the BRAS/NAS used to be connected to an SSP and was dialed up by a modem, and the broadband connection was a VPDN. You could not make a call at the same time, so ISDN replaced analog signalling in some areas to allow for broadband and a telephone connection simultaneously.


A cell relay WAN technology developed for B-ISDN, an extension of ISDN, aimed at creating a global packet switched broadband over an end to end circuit switched network, resembling the nature of the voice services. ATM over SDH/SONET used to be used in the core network before being overtaken by the concept of the Internet and IP over MPLS VPNs. It is still used in the access network in some places. The small cell sizes allow for time sensitive information like voice to be packet switched without delay or long processing/queueing. GR-303 DLE protocols were then used to connect the VoATM voice to the class 5 switch (but of course with DSL, voice is just sent in the POTS band and not over ATM). As data rates increased, the need for small cells was no longer important, and as hardware improved, IP routers were able to keep up with high data rates.

X.25, Frame Relay, SMDS, ATM and MPLS all use the virtual circuit concept.


When the telephone networks were digitalized in the 1980s and 90s using ATM and SDH, protocols such as PSTN, X.25, Frame Relay or MPLS became applications running on top of those networks. Dedicated networks were possible but uncommon for wide-range communciation.

  • wait.. PSTN is a protocol??
    – Noob_Guy
    Mar 6, 2020 at 14:19
  • Not really (more a stack), but it depends on how you look at it. In any case, telephony runs as an application on top of the digital networks, circuit-switched or packet-switched.
    – Zac67
    Mar 6, 2020 at 14:33
  • X.25/Frame Relay/MPLS run on top of ATM?? Aren't they all link layer protocols?
    – Noob_Guy
    Mar 7, 2020 at 2:52

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