1. Basic concepts of digital data network
With the development of data communication services, relatively fixed users have a relatively large business volume, and requires stable delay and high real-time performance. Driven by market demand, a semi-permanent connection method between permanent connection and switched connection has emerged.
Digital data network is a transmission network that uses digital channels to transmit data. Its transmission media include optical cables, digital microwaves, and satellite channels. The user side can use ordinary cables and twisted pair . Compared with traditional analog channels, the data signal is transmitted using digital channels. It has a series of advantages such as high transmission quality, fast speed and high bandwidth utilization.
DDN provides users with semi-permanent digital connections, and does not perform complex software processing along the way, so the delay is small, avoiding the disadvantage of large and unfixed transmission delay in packet switching networks; DDN adopts a digital cross-connection device (its basic concept will be described below), and can connect the required bandwidth lines within the agreed time according to user needs. The allocation and connection of channel capacity is carried out under computer control, which has greater flexibility.
According to the above definition, DDN has the following advantages:
- DDN is a synchronous data transmission network, does not have switching functions, and can provide users with fixed or semi-permanent channels through digital cross-connection devices, and provide access at multiple rates;
- transmission rate is high and the network delay is small. Currently, NX64kbit/s~2Mbitml6/s data services are provided;
- DDN is a fully transparent network that can be supported and not restricted by any protocol, so as to meet the needs of various services such as data, images, and sound.
2. The concept of 64kbit/s digital channel multiplexing and digital cross-connection
1.64kbit/s digital channel multiplexing
In DDN, the rate is less than 64kbit/s is called the sub-rate. The circuits of each sub-rate are multiplexed to the digital channel of 64kbit/s. Generally, according to the original CCITTX.50, X.51, X.58 recommendations, the synchronized user data stream is multiplexed into a 64kbit/s set rate signal, which is called D0MUX. The multiplexing of
subrates is in the Envelope format. my country adopts the X.50 standard, and its frame structure is an 8-bit encapsulation format with (6+2). Interleaved multiplexed framing in 8-bit encapsulation. The encapsulation structure is shown in Figure 5.15, where F is a fixed frame bit, S is a state bit, and 2~7 bits are user information bits (D). F bits form a frame synchronization sequence when multiplexing, S bits are used to represent the contents of 6 Dbits, S=1 means D is data information; S=0 means D is control information.
Figure 8-bit encapsulation structure in X.50
contains only one character in one encapsulation, and the other 2 bits are encapsulated by the next encapsulation, so 4 (6+2) encapsulations are first encapsulated before forming a frame. Includes a complete 3 characters in an encapsulation group, as shown in Figure 5.16.
Figure In the encapsulation group format
Figure, P, Q.R are 3 8-bit characters; SA, SB, and SC represent the status bits associated with P.Q.R respectively; S. Used to provide synchronization of 4 8bit encapsulation groups.
's frame structure consisting of 20 encapsulations is shown in Figure 5.17. This is a frame structure of 5-way 9.6kbit/s information multiplexing.
Figure 20 encapsulated frame structures
The above talks about the principle of sub-rate multiplexing. The multiplexing of rates above 64kbit/s is consistent with the standards of my country's digital transmission network. Therefore, DDN is just a sub-network developed according to user needs and is attached to the telecom digital transmission network.
2. Digital cross-connection definition
Traditionally, the intelligence of the network is concentrated on the switching device. The current development direction is to put some intelligence into the transmission system, so that the transmission system has certain switching functions. CCITT defines digital cross-connection as: it is a device that has one or more digital ports that comply with the G.702 (quasi-synchronous) or G.707 (synchronous) standards, and can controllable connection and reconnection of at least one port signal (or sub-rate signal) and other port signals (or sub-rate signal). In layman's terms, a digital cross-connect device is a multiplexer controlled by a microcomputer and an distribution frame . It is a static switch that is not controlled by signaling , and is controlled by a program to form a semi-permanent connection.
3. DDN network service category and user access method
1.DDN network service category
DDN network mainly provides users with dedicated circuits, including point-to-point or point-to-multipoint digital dedicated circuits at specified rates and dedicated circuits with specific requirements, as well as frame relay services and compressed voice/G3 fax services. frame relay service provides permanent virtual circuit (PVC) service through the introduction of frame relay service module (FRM); and provides compressed voice/G3 fax service through the introduction of voice service module (VSM).
2. User access method
(1) User terminal device access method
- When the user is relatively far from the access point of DDN, the modem is used to connect to DDN;
- is connected to DDN through the remote data terminal device provided by DDN, and there is no need to add a separate modem. Its characteristic is that the DDN network management center can perform remote system configuration, parameter modification and daily maintenance management of the data terminal devices placed by the user;
- is connected to the 2Mbit/s port of DDN through the user centralized device. The user concentrator can be a zero-subgroup multiplexing device or a small multiplexer provided by DDN;
- is connected to DDN through the analog circuit and directly connect to the DDN audio interface.
(2) User network interconnects through DDN
- LAN interconnects through routers using DDN;
- packet switches interconnects through DDN;
- dedicated DDN interconnects through public DDN;
- user switches interconnects through DDN.
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