Serial Communication Standards and DTE DCE

Serial Communication Standards and DTE DCE

In this chapter you will learn about Serial Communication and its Standards, parallel communication and DTE DCE. One of the most common types of WAN connections, especially in long-distance communications, is point-to-point connections , which are also called “ serial connections ” or “ leased lines ”. Because, in general, these connections are provided by a service provider, such as a telephone company, the boundaries between what the provider manages and what the customer manages must be clearly established.

SERIAL AND PARALLEL PORTS

As shown in Image, point-to-point connections are used to connect LAN networks to WAN networks of a service provider, as well as to connect LAN segments within a business network.

A point-to-point connection from LAN to WAN is also called a “leased line connection”. This is because the lines are leased from a service provider (usually a telephone company) and are dedicated for use by the company that leases the lines. Companies pay a continuous connection between two remote sites, and the line is continuously active and available.
It is important to understand how point-to-point serial communication works through a leased line to have a general concept of how WANs work.
Communications through a serial connection are a method of data transmission in which the bits are transmitted sequentially over a single channel. Serial ports are bidirectional and are often referred to as "bidirectional ports" or "communications ports."

PARALLEL CONNECTION

This is different from parallel communications, in which the bits can be transmitted simultaneously over several cables. In following image a parallel connection transfers data eight times faster than a serial connection.

According to this theory, a parallel connection sends 1 byte (8 bits) at the time when a serial connection sends a single bit. However, parallel communications have problems with crosstalk through cables, especially as their length increases.
The clock skew is a problem with parallel communications. Clock bias occurs when the data does not arrive at the same time through the different cables, which creates synchronization problems.
On most computers, parallel ports and RS-232 serial ports were replaced by higher-speed universal serial bus (USB) interfaces. However, for long-distance communications, many WANs still use serial transmission .

SERIAL COMMUNICATION

In following picture, a simple representation of a serial communication through a WAN is shown.

The communications protocol used by the sending router encapsulates the data. The encapsulated frame is sent to the WAN by a physical means. There are several ways to cross the WAN, but the receiving router uses the same communications protocol to uncapsulate the frame when it arrives.
There are different serial communication standards, and each uses a different signaling method.

• RS-232 : Most serial ports on personal computers comply with the RS-232C standard or the latest RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mouses and printers.
• V.35 : This ITU standard for synchronous high-speed data exchange combines the bandwidth of several telephone circuits and is generally used for modem to multiplexer communication. In the USA In the US, the V.35 interface standard is the one used by most routers and DSUs that connect to T1 carriers.
• High Speed ​​Serial Interface (HSSI) : An HSSI supports transmission speeds of up to 52 Mb / s. Engineers use HSSI to connect routers in LANs to WANs through high-speed lines, such as T3 lines. In addition, engineers use HSSI to provide high-speed connectivity between LAN networks using Token Ring or Ethernet.

POINT-TO-POINT COMMUNICATION LINKS

When permanent dedicated connections are required, a point-to-point link is used to provide a single pre-established WAN communications route from the customer's facilities to a remote destination through the provider's network, as shown in Image.

A point-to-point link can connect two geographically distant sites , such as a corporate office in New York and a regional office in London. For a point-to-point line, the carrier dedicates specific resources to a line leased by the customer (leased line). In general, point-to-point links are more expensive than shared services. The cost of leased line solutions can be considerable when used to connect several sites over increasing distances.

MULTIPLEXING AND TYPES

With a leased line, despite the fact that customers pay for dedicated services and that dedicated bandwidth is provided to the customer, the carrier continues to use multiplexing technologies within the network.
Multiplexing refers to a scheme that allows several logical signals to share a single physical channel. Two common types of multiplexing are time division multiplexing (TDM) and statistical time division multiplexing (STDM).

TIME-DIVISION MULTIPLEXING: TIME DIVISION MULTIPLEXING (TDM)

In the beginning, Bell Laboratories invented TDM to maximize the amount of voice traffic that was transported by means. Before multiplexing, each phone call required its own physical link. This was an expensive and impossible to scale solution.
TDM divides the bandwidth of a single link into separate time intervals. TDM transmits two or more channels (data stream) through the same link by allocating a different time interval for the transmission of each channel. In effect, the channels take turns using the link.
TDM is a physical layer concept. It does not take into account the nature of the information that is multiplexed in the output channel. In addition, it is independent of the layer 2 protocol used by the input channels.
TDM can be explained by an analogy with motorway traffic:

• To transport four-way traffic to another city, all traffic can be sent on a lane if roads are served equally and traffic is synchronized.
• If each of the four roads places a car on the main highway every four seconds, it receives a car with a frequency of one per second.
• While the speed of all cars is synchronized, there are no collisions. At the destination, the opposite is true, and cars are taken off the highway and placed on local roads using the same synchronous mechanism.

This is the principle that is used in synchronous TDM when sending data through a link. TDM increases the capacity of the transmission link by dividing the transmission time into shorter equal intervals, so that the link transports the bits of several input sources.

MULTIPLEXER (MUX)

In Image, a multiplexer (MUX) in the transmitter accepts three different signals. The MUX divides each signal into segments. The MUX places each segment in a single channel by inserting each segment in a time interval.

A MUX at the receiving end reassembles the TDM transmission in three different data streams solely on the basis of the arrival time of each bit. A technique called " bit interleaving " tracks the number and sequence of bits of each specific transmission so that they can be reassembled quickly and efficiently in the original form of reception.
Byte interleaving performs the same functions, but because there are 8 bits in each byte, the process needs a larger or longer time interval.

EXAMPLES OF TDM: SONET AND SDH

On a larger scale, the telecommunications sector uses the synchronous optical network standard (SONET) or synchronous digital hierarchy (SDH) for optical transport of TDM data.
SONET, used in North America, and SDH, used in the rest of the world, are two closely linked standards that specify interface parameters, speeds, frame formats, multiplexing methods and synchronous fiber optic TDM management.

SONET / SDH takes n streams of bits, multiplexes them and modulates the signals optically. It then sends the signals through a light-emitting device via fiber optic with an equal bit rate anx (incoming bit rate).
In this way, the traffic that reaches the SONET multiplexer from four locations at 2.5 Gb / s comes out as a single flow at 4 x 2.5 Gb / s or 10 Gb / s. This principle is explained in the illustration, which shows an increase in bit rate by a factor of 4 in the time interval T.

STATISTICAL MULTIPLEXING BY TIME DIVISION (STDM)

In another analogy, TDM is compared to a train with 32 cars.

• Each car belongs to a different cargo company, and every day the train leaves with the 32 connected cars.
• If one of the companies has cargo to ship, the car is loaded. If the company has nothing to send, the car is empty, but remains on the train.
• Shipping empty containers is not very effective.
• TDM shares this inefficiency when traffic is intermittent, since the time interval is still assigned even when the channel has no data to transmit.

STDM was developed to overcome this inefficiency. STDM uses a variable time interval , which allows channels to compete for any free interval space. It uses a buffer memory that temporarily stores data during periods of increased traffic.

DEMARCATION POINT

Prior to deregulation in North America and other countries, telephone companies owned the local loop, including wiring and equipment at customer facilities.
The local loop refers to the line from the facilities of a telephone subscriber to the central office of the telephone company.
Deregulation forced telephone companies to disarm their local loop infrastructure to allow other providers to provide equipment and services.
This led to the need to delineate the part of the network that belonged to the telephone company and the part that belonged to the customer. This delineation point is the demarcation point .
The demarcation point marks the point at which the network communicates with a network that belongs to another organization. In telephony terminology, this is the interface between the client's local computer (CPE) and the network service provider's equipment. The demarcation point is the one where the responsibility of the service provider ends.
A router serial port is required for each leased line connection. If the underlying network is based on carrier technologies T or E, the leased line is connected to the carrier network through a CSU / DSU. The purpose of the CSU / DSU is to provide a clock signal to the client equipment interface from the DSU and terminate the carrier's channeled means of transport in the CSU.
Serial DTE DCE
From the point of view of the connection to the WAN, a serial connection has a DTE device at one end and a DCE device at the other.

The connection between the two DCE devices is the transmission network of the WAN service provider, as shown in Image 9. In this example:

• The CPE , which is usually a router, is the DTE. The DTE could also be a terminal, a computer, a printer or a fax machine if they connect directly to the service provider's network.
• The DCE , usually a modem or a CSU / DSU, is the device used to convert the DTE user data to an acceptable format for the transmission link of the WAN service provider. This signal is received at the remote DCE, which decodes the signal again in a sequence of bits. Next, the remote DCE indicates this sequence to the remote DTE.

SERIAL CABLES

Originally, the concept of DCE and DTE was based on two types of equipment: the terminal equipment that generated or received data, and the communication equipment that only retransmitted data.
There are two different types of cables: one to connect a DTE to a DCE, and another to connect two DTEs directly to each other.
The DTE / DCE interface for a given standard defines the following specifications:

• Mechanical and physical : number of pins and type of connector.
• Electric : defines the voltage levels for 0 and 1.
• Functional : specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface.
• Procedure : specifies the sequence of events to transmit data.

The original RS-232 standard only defined the connection of the DTEs with the DCEs, which were modems. However, to connect two DTEs, such as two computers or two routers in a laboratory, a special cable called a " null modem cable " eliminates the need for a DCE. That is, the two devices can be connected without a modem.
A null modem is a communication method to directly connect two DTEs using an RS-232 serial cable. With a null modem connection, the transmission (Tx) and reception (Rx) lines intersect, as shown in Image.

The cable for connecting DTE to DCE is a shielded serial transition cable. The router end of the shielded serial transition cable can be a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card. The other end of the serial transition cable is available with the corresponding connector for the standard to be used.

CLOCK SIGNAL

When using a null modem, synchronous connections require a clock signal. An external device or one of the DTEs can generate the clock signal. By default, when a DTE and a DCE are connected, the serial port on a router is the DTE end of the connection, and generally a CSU / DSU or similar DCE device provides the clock signal.

However, when a null modem cable is used in a router-to-router connection, one of the serial interfaces must be configured as the DCE end to provide the clock signal for the connection.

SERIAL BANDWIDTH

Bandwidth refers to the speed at which data is transferred through the communication link .
The underlying carrier technology depends on the available bandwidth. There is a difference in the bandwidth points between the North American specification (carrier T) and the European system (carrier E). Optical networks also use another hierarchy of bandwidth, which also differs between North America and Europe.
In North America, bandwidth is usually expressed as a digital signal level number (DS0, DS1, etc.), which refers to the speed and format of the signal.

• The most elementary online speed is 64 kb / s, or DS-0, which is the bandwidth required for an uncompressed digitized phone call.
• The bandwidths of the serial connections may increase more and more to meet the need for faster transmission.
• There are leased lines available with different capacities and, generally, the price is based on the bandwidth required and the distance between the two connected points.

OC transmission speeds are a set of standardized specifications for the transmission of digital signals that are transported through SONET fiber optic networks. The designation uses OC followed by an integer representing the basic transmission rate of 51.84 Mb / s.
For example, OC-1 has a transmission capacity of 51.84 Mb / s, while OC-3 would be 51.84 Mb / s by three, or 155.52 Mb / s.