The Transmission Network Basics
For example, telephone networks, television networks, computer networks, etc. are included in the service network, the support network mainly for the communication network to provide clocks and signaling protocols, the transmission network is the basis for the connectivity of each node.
The Transmission Network Architecture
Transmission network architecture is flexible. Different application scenarios have different network architectures. Telecom operators have transmission networks, power, oil and other industries also have their own specialized transmission networks.
Carrier transmission network can be categorized by geographic location and can be divided into two parts:
Trunk transmission network
Local transmission network
The local transmission network is further divided into:
Backbone layer : responsible for carrying the service scheduling between the aggregation side and the core layer
aggregation layer : responsible for service aggregation and uploading
Access layer : responsible for accessing customer-side services.
Transmission network is divided into limited transmission and wireless transmission, (microwave, satellite, etc.), we mainly introduce wired transmission.
Wired transmission consists of transmission equipment and transmission lines.
Common transmission equipment: PDH, SDH, WDH, PTN, OTN
Common transmission lines: fiber optic cable, cable
The Development of Transmission Network Evolution
3.1 PDH (quasi-synchronous digital series, Plesiochronous Digital Hierarchy)
PDH is an early digital transmission standard, which began to appear in the 1980s and developed rapidly.
PDH Disadvantages:
No globally harmonized standard
Complex structure, cost is too high
Maintenance is difficult
3.2 SDH (Synchronous Digital Hierarchy)
SDH is also a digital transmission standard. It is actually a product of the combination of fiber optic transmission technology and intelligent network technology.
Advantages of SDH:
Network management capability is greatly enhanced
Unified standard, unified specification, convenient interconnection of different manufacturers
Suitable for large-capacity transmission
The new concept of self-healing network is proposed, and the protection ability is enhanced.
Adoption of byte multiplexing technology makes it very simple to signal the upper and lower branches in the network.
3.3 MSTP (Multi-Service Transmission Platform)
MSTP is a multi-service transmission platform (based on SDH).
As the name suggests, the core of MSTP is still SDH, which is improved on the basis of SDH.
MSTP = SDH + Ethernet + ATM
MSTP is the addition of an Ethernet interface or ATM interface on top of SDH to realize an IP-based interface.
3.4 PTN (Packet Transport Network)
With the development of communication, the telecommunication business has changed from phone call-oriented to Internet-oriented, and the proportion of data business has increased dramatically. both MSTP and SDH are based on circuit switching (TDM) as the core, which can't carry the data business (IP) better, so PTN has appeared.
In terms of transmission units, PTN transmits IP messages as the smallest unit, while SDH transmits time slots with the smallest unit being E1. PTN has elasticity in message size, while SDH has a fixed circuit bandwidth. This is the most essential difference between PTN and SDH.
MPLS (Multi-Protocol Label Switching)
Multi-Protocol (Multi-Protocol): Supports a variety of Layer 3 protocols (IP, IPv6, IPX, SNA, etc.).
Label Switching: Labeling messages to replace IP forwarding with label switching.
In traditional IP networks, the routing technology is unmanageable and uncontrollable; IP is forwarded level by level, and every router it passes through has to perform a route query (which may be done multiple times), which is slow, and this kind of forwarding mechanism is not suitable for large-scale networks.
MPLS, on the other hand, establishes a label forwarding channel (LSP) for messages through pre-assigned labels. At each device that the channel passes through, only a fast label exchange is required (one lookup), thus saving time.
WDM (Wavelength Division Multiplexing)
WDM is the multiplexing of optical signals of different wavelengths into the same fiber for transmission.
Sparse wavelength division multiplexing (CWDM): large wavelength spacing, generally 20nm. Dense wavelength division multiplexing (DWDM): the wavelength interval is small, less than or equal to 0.8nm.
Advantages of WDM: large capacity, and it can be transmitted over long distances.
Disadvantages of WDM: only point-to-point connection, can not form a ring, can not be flexible scheduling, can not cope with complex network structure.
SDH can form a ring and has strong management capability. Why not introduce the characteristics of SDH into WDM, so there is OTN.
OTN (Optical Trasnport Network)
OTN is based on WDM and incorporates some of the advantages of SDH, such as rich OAM overhead, flexible service scheduling, and perfect protection methods.
The scheduling of services by OTN is divided into :
Optical layer scheduling (can be understood as the category of division WDM)
Electric layer scheduling (can be understood as the scope of SDH)
