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NE40E V800R010C00 Feature Description - WAN Access 01

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Basic Concepts of TDM

Basic Concepts of TDM

TDM

Time Division Multiplex (TDM) is implemented by dividing a channel by time, sampling voice signals, and enabling sampled voice signals to occupy a fixed interval that is called timeslot according to time sequence. In this way, multiple ways of signals, through TDM, can be combined into one way of high-rate complex digital signal (group signal) in a certain structure. Each way of signal is transmitted independently.

Figure 7-1  Multiplexing and demultiplexing for TDM

Traditional Transmission Mode

‎After processed by Pulse Code Modulation (PCM), voice signals, together with other digital signals, are transmitted through Plesiochronous Digital Hierarchy (PDH) or Synchronous Digital Hierarchy (SDH) connections by using the TDM technology. Generally speaking, PDH/SDH services are called TDM services.

Service System

TDM services are classified by transmission mode as follows:

  • In the PDH system, E1, T1, are usually used.
  • In the SDH system, the STM-1, STM-4, and STM-16 are usually used.

Clock Synchronization

TDM services require clock synchronization. One of the two parties in communication takes the clock of the other as the source, that is, the device functioning as the Data Circuit-terminal Equipment (DCE) outputs clocks signals to the device functioning as the Data Terminal Equipment (DTE). If the clock mode is incorrect or the clock is faulty, error code is generated or synchronization fails.

The synchronization clock signals for TDM services are extracted from the physical layer. The 2.048 MHz synchronization clock signals for E1 are extracted from the line code. The transmission adopts HDB3 or AMI coding that carries timing information. Therefore, devices can extract clock signals from these two types of codes.

TDMoPSN

Based on TDM circuits on a PSN, TDM Circuits over Packet Switching Networks (TDMoPSN) is a kind of PWE3 service emulation. TDMoPSN emulates TDM services over a PSN such as an MPLS or Ethernet network; therefore, transparently transmitting TDM services over a PSN. TDMoPSN is mainly implemented by means of two protocols: Structure-Agnostic TDM over Packet (SAToP) and Structure-Aware TDM Circuit Emulation Service over Packet Switched Network (CESoPSN).

  • CESoPSN

    The Structure-aware TDM Circuit Emulation Service over Packet Switched Network (CESoPSN) function simulates PDH circuit services of low rate on E1/T1 interfaces. Different from SAToP, CESoPSN provides structured simulation and transmission of TDM services. That is, with a framed structure, it can identify and transmit signaling in the TDM frame.

    Features of the structured transmission mode are as follows:

    • When services are carried on the PSN, the TDM structure needs to be protected explicitly.
    • The transmission with a sensitive structure can be applied to the PSN with poor network performance. In this manner, the transmission is more reliable.

    The structure of CESoPSN packets is shown in Figure 7-2.

    Figure 7-2  CESoPSN

    • MPLS Lable

      The specified PSN header includes data required for forwarding packets from the PSN border gateway to the TDM border gateway.

      PWs are distinguished by PW tags that are carried on the specified layer of the PSN. Since TDM is bidirectional, two PWs in reverse directions should be correlated.

    • PW Control Word

      The structure of the CESoPSN control word is shown in Figure 7-3.

      Figure 7-3  PW Control Word

      The padding structure of the PW control word on the NE40E is as follows:

      • Bit 0 to bit 3: padded with 0 fixedly.
      • L bit (1 bit), R bit (1 bit), and M bit (2 bits): Used for transparent transmission of alarms and identifying the detection of severe alarms by an upstream PE on the CE or AC side.
      • FRG (2 bits): padded with 0 fixedly.
      • Length (6 bits): length of a TDMoPSN packet (control word and payload) when the padding bit is used to meet the requirements on the minimum transmission unit on the PSN. When the length of the TDMoPSN packet is longer than 64 bytes, padding bit field is padded with all 0s.
      • Sequence number (16 bits): It is used for PW sequencing and enabling the detection of discarded and disordered packets. The length of the sequence number is 16 bits and has unsigned circular space. The initial value of the sequence number is random.
    • Optional RTP

      An RTP header can carry timestamp information to a remote device to support packet recovery clock such as DCR. The packet recovery clock is not discussed in this document. In addition, packets transmitted on some devices must include the RTP header. To save bandwidth, no RTP header is recommended under other situations.

      The RTP header is not configured by default. You can add it to packets. Configurations of PEs on both sides must be the same; otherwise, two PEs cannot communicate with each other.

      Figure 7-4  RTP header

      The padding method for the RTP header on the NE40E is to keep the sequence number (16 bits) consistent with the PW control word and pad other bits with 0s.

    • TDM Payload

      The length of TDM payload is the number of encapsulated frames multiplied by the number of timeslots bound to PW (bytes). When the length of the whole PW packet is shorter than 64 bytes, fixed bit fields are padded to meet requirements of Ethernet transmission.

  • SAToP

    The Structure-Agnostic TDM over Packet (SAToP) function emulates PDH circuit services of low rate.

    SAToP is used to carry E1/T1 services in unframed mode (non-structured). It divides and encapsulates serial data streams of TDM services, and then transmits encapsulated packets in a PW. SAToP is the most simple method to handle transparent transmission of PDH low-rate services in TDM circuit simulation schemes.

    Features of non-structured transmission mode are as follows:

    • The mode does not need to protect the integrity of the structure; it does not need to explain or operate the channels.
    • It is suitable for the PSN of higher transmission performance.
    • It needs to neither distinguish channels nor interrupt TDM signaling.

    The structure of SAToP is show in Figure 7-5

    Figure 7-5  SAToP

    • MPLS Lable

      The MPLS label for SAToP is the same as the MPLS label for CESoPSN.

    • PW Control Word

      The structure of the CESoPSN control word is shown in Figure 7-6.

      Figure 7-6  PW Control Word

      The padding structure of the PW control word on the NE40E is as follows:

      • Bit 0 to bit 3: padded with 0 fixedly.
      • L bit (1 bit) and R bit (1 bit): Used for transparent transmission of alarms and identifying the detection of severe alarms by an upstream PE on the CE or AC side.
      • RSV (2 bits) and FRG (2 bits): padded with 0 fixedly.
      • Length (6 bits): length of a TDMoPSN packet (control word and payload) when the padding bit is used to meet the requirements on the minimum transmission unit on the PSN. When the length of the TDMoPSN packet is longer than 64 bytes, the padding bits are padded with all 0s.
      • Sequence number (16 bits): It is used for PW sequencing and enabling the detection of discarded and disordered packets. The length of the sequence number is 16 bits and has unsigned circular space. The initial value is the sequence number is random.
    • Optional RTP

      The optional RTP for SAToP is the same as the optional RTP for CESoPSN.

    • TDM Payload

      The length of TDM payload is the number of encapsulated frames multiplied by 32 (bytes). When the length of the whole PW packet is shorter than 64 bytes, the fixed bits are padded to meet requirements of Ethernet transmission.

IP RAN

IP RAN, mobile carrier, is a technology used to carry wireless services over the IP network. IP RAN scenarios are complex because different base stations (BSs), interface technologies, access and convergence scenarios are involved.

  • 2G/2.5G/3G/LTE, traditional BSs/IP BSs, GSM/CDMA, TDM/ATM/IP (interface technologies) are involved.
  • Varying with the BS type, distribution model, network environment, and evolution process, the convergence modes include microwave, MSTP, DSL, PON, and Fiber. You can converge services on BSs directly to the MAN UPE or through convergence gateways (with functions of BS convergence, compression optimization, packet gateway, and offload).
  • Reliability, security, QoS and operation and maintenance (OM) are considered in IP RAN scenarios. In some IP RAN scenarios, transmission efficiency is concerned.

Other Key Technologies

  • Jitter Buffer

    After traversing the MPLS network, PW packets may reach the egress PE at different intervals or packet disorder may occur Therefore, the TDM service flow must be reconstructed on the egress PE according to the interval at which PW packets smoothed with the jitter buffer technology.

    The jitter buffer of a larger capacity can tolerate a greater jitter in the transmission interval of packets on the network, but it causes a longer delay in the reconstruction of TDM service data flows. A jitter buffer can be configured based on delay and jitter conditions.

  • Analysis on Delay of Data Packets

    Most TDM services are voice services and therefore require short delay. ITU-T G.111 (A.4.4.1 Note3) points out that when the delay reaches 24 ms, a human ear can feel the echo in the voice service.

    Generally, the TDMoPSN processing delay is calculated as follows:

    TDMoPSN service processing delay = Hardware processing delay + Jitter buffer depth + Packet encapsulation time + Network delay

    Where:

    • The hardware processing delay is fixed and inevitable.
    • The jitter buffer depth is configurable.
    • The packet encapsulation time equals 0.125 ms multiplied by the number of frames encapsulated into a packet.
    • The network delay refers to the transmission delay between two PEs.
  • Clock synchronization

    TDMoPSN service packets are transmitted at a constant rate. The local and remote devices must have synchronized clocks before exchanging TDMoPSN service packets. Traditional TDM services can synchronize clocks through a physical link but TDMoPSN services are carried on a PSN. TDM services lose synchronization clock signals when reaching a downstream PE.

    A downstream PE uses either of the following methods to synchronize clocks:

    • Obtains clock signals from an external BITS clock.
    • Recovers clock signals from packets.

      Downstream PEs, by following an algorithm, can extract clock signals from received PWE3 packets. Clock recovery is further classified as adaptive clock recovery (ACR) and differential clock recovery (DCR) according to implementation.

  • QoS processing

    TDM services require low delay and jitter and fixed bandwidth. A high QoS priority must be specified for TDM services.

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Updated: 2018-07-04

Document ID: EDOC1100027168

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