Layer 1 — physical layer interface: DTE/DCE, line interfaces and protocols

The physical layer interface defines the manner in which the particularmedium (e.g., twisted pair cable, coaxial cable, fibre line or radio link) should be coded to carry the basic digital

Layer 1 — physical layer interface: DTE/DCE, line interfaces and protocols 71 information. At a minimum, the physical interface specification needs to define the precise nature of the medium (e.g., wire grade, impedance etc); the exact electrical (or equivalent) signals which are to be used on the line and the details of theline codewhich shall be used for bit synchronisation as discussed in Chapter 2. Many modern physical interface specifications also stipulate the precise mechanical connectors (i.e., plugs and sockets) which should be used for the interface, but this is not always defined. As well as the basic electrical (radio or optical) interface, the physical layer specification defines control procedures (thephysical layer protocol) which allows one or both of the devices at either end of the line (i.e., the physicalmedium) to control the line itself.

There are three main types of physical interface to be distinguished from one another:

• DTE-to-DCE interfaces. These are the asymmetric point-to-pointuser-network interfaces (UNIs)used typically to connect end devices (e.g., computers) tomodems, line terminating units (LTU), channel service units (CSU), data service units (DSUs), network terminating units (NTorNTU). All the latter are examples ofdata circuit terminating equipment (DCE);

• Line interfaces or trunk interfaces (symmetrical point-to-point line interfaces) — most NNIs and some UNIs are of this type;

• Shared medium interfaces (point-to-multipoint interfaces) — usually used as UNIs. [The most commonly used shared media are local area networks (LANs). LANS are widely used to connect end-users PCs to internal office data networks].

In general terms, the UNI (user-network interface) always employs a DTE-to-DCE type interface, while trunk interfaces tend to use symmetrical, higher bit rate NNI (network-node interface).

Figure 3.3 shows a network in which three routers and two DTEs are interconnected. Both DTEs are connected to the network by means of DTE/DCE interfaces. Routers C and B are also connected to the line which interconnects them by means of DTE/DCE (e.g., V.24 or RS-232) interfaces. Routers A & B and A & C, meanwhile, are interconnected by means of direct trunk interfaces (DCE/DCE³). In these cases, the DCE function is included within the router itself.

Figure 3.3 DTE/DCE Interface and Trunk or line interface.

3The ‘DCE/DCE interface’: tempting as it may be, it is not correct to call the long-distance connection between DCEs a ‘DCE/DCE interface’. Instead, one should refer to theline interface ornetwork interface.The term

‘DCE/DCE interface’ is reserved for the case in which the UNI interface (normally used to connect a DTE to a DCE) is used to connect a second DCE instead. A cross-cable is used for this purpose, as we discover in Figure 3.8b.

DTE-to-DCE interfaces

DCEs (data circuit-terminating equipment)form the ‘end-point’ of a network or long-distance telecommunications line, allowing point-to-point communication between remote computers and terminals (which are generically calledDTE ordata terminal equipment).

The first DCEs were modems and were designed to be used at either end of a datalink created by means of a dial-up telephone connection. The modem (like all other types of DCE) had to provide an interface for connecting to theserial data communications port of the computer, but also be capable ofsetting up, receiving, clearing and controlling telephone connections.

As we have already discussed, the basic functions of all types of DCE are to:

• convert the physical interface emanating from the DTE (data terminal equipment) into aline interface format suitable for long-distance transmission, and provide for digital/analogue signal conversion if necessary;

• provide for networkterminationof the long-distance line, being a source of power for the line and network as necessary;

• forward data received from the DTE to the network;

• deliver data received from the network to the DTE;

• clock and bit-synchronise⁴the data transmission of the DTE during thedata transfer phase;

• set up the physical connection forming the medium and clear it as required and/or requested by the DTE. This may be necessary where the physicallinkis actually a dial-up connection across a telephone network, or a temporary radio path.

Some DCEs are controlled by the associated DTE, others act autonomously on behalf of the DTE. The user’s data and the control signals (functions) are conveyed from DTE to DCE or DCE to DTE by means of dedicated control leads defined as part of the DTE – DCE interface.

Detailedproceduresdefine how the functions are used and how the states of the DTE and DCE can be changed fromidle, throughready to thedata transfer phase and afterwards arrange forclearing of the connection.

Typical physical layer interface specifications for a DTE-to-DCE interface comprise up to four different components, including a definition of:

• the physical connector;

• the electrical interface;

• the controls and functions for establishing the link: changing from one state (e.g., idle) to another (e.g., ready or data transfer) or vice versa;

• the procedures (i.e., sequences of commands) which define the use of the controls. (The controls and procedures form together form the physical layer protocol.)

The most commonly used DTE/DCE interfaces are illustrated in Figure 3.4. There are two main categories of DTE/DCE interfaces: X.21-type interfaces (for digital lines) and X.21_bis-type interfaces (used inmodems for analogue lines). The other V-series and X-series recommen-dations listed in Figure 3.4 define individual aspects of specific interfaces.

4See Chapter 2.

Layer 1 — physical layer interface: DTE/DCE, line interfaces and protocols 73

Note: You may be wondering why all the specifications and standards defining DTE/DCE interfaces have such different designations. This is because different naming standards are used by the different standards-publishing organisations. ISO (International Organization for Standardization) uses a simple numbering scheme (but I haven’t yet worked out the logic behind the individual numbers). I T U - T (International Telecommunications Union — Standardization sector) issues recommendations in various series. The X-series defines ‘interfaces and procedures intended for use in general data communications’. The V-series defines‘ data communications over the telephone network’ (e.g.

modems). Sometimes the same recommendation is issued with a recommendation number in both series (e.g.V.10/X.26).The nomenclature RS, mean while, stands for recommended standard. This designation is used by the United States EIA/TIA (Electronic Industries Alliance or Association/

Telecommunications Industries Association).

Figure 3.4 Standards, specifications and ITU-T recommendations defining DTE/DCE interfaces.

DCEs intended for use between DTEs and analogue wide area network (WAN)lines are calledmodems. Such modems conform at their DTE/DCE interface with ITU-T recommen-dations X.21bis and V.24/V.28. X.21bis sets out the entire framework of the DTE-to-DCE interface used by the modem. V.24 (and RS-232 as well) set out the signals and circuits (together with their names and numbers) used at the interface. But which signal is sent on exactly which wire and via which pin of the connector is defined by either V.28 (RS-232), V.35 or V.36 accordingly.

There are five DTE-to-DCE interfaces in common usage. These (as shown in Figure 3.4) are:

• V.24/V.28 (25-pin DB-25 plug) is the most common interface used between computers and analogue modems. In Europe this interface is simply referred to as ‘V.24’ and in North America as RS-232.

• V.36 (37-pin plug), usually referred to as RS-449) is the most commonly used interface in North America, the UK and France for high bit rate DCEs and those used to connect digital lines. Very confusingly, many people refer to ‘V.35’ even though they really mean

‘V.36’.

• V.35 is a similar interface to V.36 but with a different connector. Its usage is restricted to certain types of IBM computers and networking equipment.

• X.21 (V.11) is the main interface used in Germany, Austria, Switzerland for interfacing digital line DCEs and high bit rate lines to DTEs. It is often referred to simply as ‘X.21’

without specifying V.11.

• X.21 (V.10) is the version of the X.21 interface designed for use in conjunction with coaxial cables. It is also simply referred to as ‘X.21’ without specifying V.10.

Unfortunately, newcomers to DTE/DCE standards can easily be confused by the various spec-ifications, since lazy experts often do not define in full the interfaces they wish to refer to. It is commonplace, for example, to refer only to ‘V.24’ when in fact the complete V.24/V.28 interface is meant. The V.24/V.28 (or RS-232) interface uses the 25-pin DB-25 connector (ISO 2110) commonly seen on older modems. Similarly, when referring to DCEs used on digital line circuits, people often speak of ‘X.21’ without saying whether the interface is V.10 (forunbalanced circuits, i.e., coaxialcables) or V.11 (forbalanced circuits, i.e.,twisted pair cable).

Network synchronisation

One of the major functions of the DCE (data circuit terminating equipment) is to ensure that the DTE (data terminal equipment) transmits its data in a manner bit-synchronised with the network (i.e., at precisely the right bit rate and at the correct interval in time).

In a public digital transmission network, the network operator uses an extremely accurate master clock (typically a caesium clock or the extremely accurate clock signal of the satellite global positioning system) to synchronise his or her entire network. This ensures that slip, jitter, wander and other undesirable effects (as discussed in chapter 2) do not affect signals as they move from one node to the next through the network.

The clock signal is sent to all devices within the network, by means of a hierarchical syn-chronisation plan (Figure 3.5). Each node in the network is configured to receive aprimary clock signal and asecondary (orback-up)clock signal. When theprimary source fails, the node reverts to the secondary. The clock is propagated as far as the DCEs, and from each DCE is passed on to the corresponding DTE, thus ensuring that all the DTEs maintain an accurate and network-compatible transmitting bit rate.

Figure 3.5 Hierarchical network synchronisation plan with nominated primary and secondary sources DCEs and DTE/DCE Interfaces intended for use with analogue lines (Modems).

Layer 1 — physical layer interface: DTE/DCE, line interfaces and protocols 75 In the case of analogue transmission lines, the DCE cannot rely on the network to provide accurate clocking information, since the bit rate of the signal received from the remote end is not accurately regulated by thePTO’s (public telecommunications organisation or operator) analogue network. For this reason, an internal clock is needed within the DCE in order to maintain an accurate transmitting bit rate.

DTE/DCE control signals

DTE/DCE interfaces were originally multi-lead interfaces, with separate wires or circuits dedicated for each control action. The control signals defined by ITU-T recommendation V.24 and EIA RS-232 are the most widely used. These are listed in Table 3.1.

DTE/DCE electrical interface

As an example of a standardised physical layer electrical interface, Figure 3.6 illustrates the

‘negative logic’ voltages defined by RS-232 and V.28 for use on each of the DTE/DCE circuits defined in Table 3.1. For a mark (binary value ‘1’), the transmitter should output a voltage between−5 V and−15 V. The receiver, meanwhile, interprets any received voltage between

−3 V and −15 V as a mark. Similarly, when transmitting a space (binary value ‘0’), the

Table 3.1 DTE/DCE control signals defined by EIA RS-232/ITU-T recommendation V.24 Signal

CTS Clear-to-send 5 8 106 CB From DCE to DTE.. ‘I am

ready when you are’

DSR data set ready 6 (not always used)

GND Ground 7 5 102 AB Signal ground — voltage

reference value RC Receiver clock 17 (little used) — 115 DD Receive clock signal

(from DCE to DTE)

RI Ring indicator 22 9 125 CE From DCE to DTE.. ‘I

have an incoming call for you’

RTS Request-to-send 4 7 105 CA From DTE to DCE..

‘please send my data’

RxD Receive data 3 2 104 BB DTE receives data from

DCE on this pin TC Transmitter

clock

15 (little used) — 114 DB Transmit clock signal (From DCE to DTE)

TxD Transmit data 2 3 103 BA DTE transmits data to

DCE on this pin