As telecom technologies evolved and changed radically, this unleashed new concepts that have managed to push the services toward the best. But every change needs a transit to result in good operability and interoperability between the new and the current working technologies.
Source: https://www.aitnews.com
NEXT GENERATION NETWORK (NGN)
The Next Generation Network (NGN) inferred to the new network capable of provisioning users with all IT and Telecomm needs in one network. One of the most controversial subjects was the change of the transmission carrier from TDM to IP, which provides substantial advantages such as voice, data and video convergence, greater platform densities, rapid service and feature enhancements. However, the change is not that easy.
TDM vs IP
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path using a time-based method by means of synchronized equipment at each end of the transmission line so that each signal appears on the line only a fraction of the time in an alternating pattern.
TDM is a circuit switched technology where the end-to-end (E2E) connection is already made prior to the connection request like in the leased lines. A multiplexing clock used to identify the time-slot in which the payload is available for the individual E2E connection.
Source: https://circuitglobe.com/difference-between-fdm-and-tdm.html
In IP networks, the packet switched technology is used. There is no E2E connection prior the communication start, but the incoming packets/payload is identified based on the identity that each payload has. The Source Address tells where from this payload it is coming and the destination IP tells where the payload is going to.
DIFFICULTIES OF THE TRANSMISSION TRANSITION
TDM was dominating most of telecom operators' networks and could smoothly support voice services by nature.
That is not the only thing about TDM, as it also was preferred by the operators who utilized its definite bandwidth, especially when leasing to customers.
Also, in the Ethernet environment, when using Ethernet as a carrier for smaller Ethernet services, we encounter problems like optimizing the bandwidth for ports and using OAM facilities.
THE SWITCHING TRANSIT STEPS TOWARD The CHANGE
The transit in the Switching passed through using media gateways (MGC), then softswitches and IP Multimedia Subsystems (IMS) afterwards. The emulation for TDM (E1/T1) services depended firstly on separating the control signaling of the traffic, but with Transmission the conversion from TDM to Ethernet should encompass the whole signal transparently, regardless of the switch signaling and traffic.
Using Media Gateways in Switches to enable transition between TDM and Ethernet (Source: https://www.gl.com/mgcp-protocol-simulator-maps.html)
THE TRANSMISSION SOLUTION FOR TECHNOLOGY TRANSITION
One of the most important Transmission solutions for Technology Transition is PWE3 (Pseudo-Wire Emulation Edge to Edge). The ITEF launched PWE in 2001 to enable carrying the Layer 2 services of CEs like ATM, frame relay, Ethernet, low-rate TDM, or SDH on the packet network through tunnels (such as MPLS LSPs, TE tunnels, or GRE tunnels).
PWE3 offered to support the Emulation of TDM services over Ethernet also.
PWE3 used to send Ethernet to leverage the use of Ethernet services, like sending Data Units (PDUs), optimizing L2VPN with extra features and strong OAM not available in the pure Ethernet environment.
PWE3 carring Ethernet/ATM/TDM
PWE3 CREATING METHODS
Static PW
In a static PW, no signaling protocol is used to negotiate the parameters; instead, the relevant information is manually specified through commands.
Dynamic PW
The dynamic PW refers to the PW that is set up through a signaling protocol. PWE3 takes LDP as the signaling protocol.
THE BASIC TRANSPORT COMPONENTS OF PWE3
PWE3 takes LDP as the signaling protocol (when using dynamic creation) and transmits the Layer 2 packets of CEs through tunnels (such as MPLS LSPs, TE tunnels, or GRE tunnels). The basic transport components of the PWE3 network are as follows:
AC - an AC is a physical or virtual circuit attaching a CE to a PE. An AC can be, for example, an Ethernet port, a VLAN, or a TDM link;
PW - a PW is a mechanism that carries the emulated services from one PE to another PE over a PSN. By means of PWE3, point-to-point channels are created, separated from each other. Users' Layer 2 packets are transparently transmitted on a PW;
forwarder - a forwarder selects the PW for the service payloads received on an AC. The mapping relationships can be specified in the service configuration, or implemented through certain types of dynamically configured information;
tunnels - a tunnel provides a mechanism that transparently transmits information over a network. In a tunnel, one or more PWs can be carried. A tunnel connects a local PE and a remote PE for transparently transmitting data;
PW signaling protocol - PWE3 takes LDP as the signaling protocol.
PWE3 Transport Component
PWE3 ENCAPSULATION FORMAT
The specific PWE3 encapsulation format varies slightly according to the type of emulated service, but a generic encapsulation format is also available.
A PWE3 packet contains the MPLS label, control word and payload.
PWE3 Encapsulation Format
MPLS Label
The MPLS labels include tunnel labels and PW labels, which are used to identify tunnels and PWs respectively. The format of the tunnel label is the same as that of the PW label.
Control word
The 4-byte control word is a header used to carry packet information over an MPLS PSN.
The control word is used to check the packet sequence, to fragment packets and to restructure packets. The specific format of the control word is determined by the service type carried by PWE3 and the encapsulation mode adopted.
The encapsulation format is classified into:
TDM PWE3 (SAToP / CESoPSN);
ATM PWE3 (ATM N-to-one / ATM one-to-one);
ETH PWE3 (Ethernet encapsulation).
In here, the focus will be to the TDM encapsulation methods.
ENCAPSULATION METHODS OF TDM IN PWE3
The TDM gets encapsulated in the PWE3 using 2 ways:
Structure-Agnostic Time Division Multiplexing Over Packet (SAToP) - SAToP segments and encapsulates TDM services as serial bit streams and then transmits the bit streams in the PW tunnels. Although it disregards the TDM frame structure, it supports the transmission of synchronous information;
Circuit Emulation Service Over Packet Switched Network (CESoPSN) - CESoPSN does not transmit idle timeslot channels; instead, CESoPSN extracts only the usable timeslots from the service flow and then encapsulates these timeslots as PW packets for transmission. The CESoPSN needs to be manually specified.
I hope this article is useful to you!
