DTE (data terminal equipment), DCE (data circuit-terminating equipment), line interfaces and protocols

A simplewide area (i.e., long distance) data communications link is illustrated in Figure 1.4.

The link connects a PC or computer terminal on the left of the diagram to a mainframe computer on the right. The long-distance network which actually carries the connection is shown as a ‘cloud’ (in line with modern convention in the networking industry). It is not clear exactly clear what is in the ‘cloud’ in terms of either technology, the line types and interfaces, or the topology. This is often the case, and as we shall see, need not always concern us. What is more important are the interfaces used at each end of the network to connect the computing equipment. These interfaces are defined in terms of the DTE (data terminal equipment), the DCE (data circuit-terminating equipment)and theprotocols used.

You will note that the communicating devices (both the PC and the mainframe computer) are DTE (data terminal equipment) in the jargon. The ‘T’ in DTE does not necessarily refer to a computer device (computer terminal) with a screen and a keyboard (though this is one example of a DTE). The DTE could be any piece of computer equipment connected to a data

Figure 1.4 Explanation of the terms DTE, DCE and protocol.

network. In contrast to the DTE, the DCE is the ‘start of the long-distance network’ (the first piece of equipment in the long-distance network to which the DTE is connected — i.e., the DTE’s ‘direct communications partner’).

If only a short cable were to be used to connect the two DTEs in Figure 1.4, then the two devices could be directly connected to one another, without requiring the DCEs or the long-distance network. But whenever the distance between the DTEs is more than a few metres (up to a maximum of 100 m, depending upon the DTE interface used), then a long distance communication method is required. In simple terms, the DCE is an ‘adaption device’

designed to extend the short range (i.e., local) communication capabilities of DTE into a format suitable for long distance (i.e.,wide area) data networking. A number of standardised DTE/DCE interfaces have been developed over the years which allow all sorts of different DCEs andwide area network (WAN)types to be used to interconnect DTE, without the DTE having to be adapted to cope with the particular WAN technology being used to transport its data.

The cable connection and the type of plug and socket used for a particular DTE/DCE connection may be one of many different types (e.g., twisted pair cable, UTP (unshielded twisted pair), STP (shielded twisted pair), category 5 cable (Cat 5), category 7 cable (Cat 7), coaxial cable, optical fibre, wireless, etc.). But all DTE/DCE interfaces have one thing in common — there is always atransmit (Tx)data path and areceive (Rx)data path. At least four wires are used at the interface, one ‘pair’ for the transmit path and one ‘pair’ for the receive path. But in some older DTE/DCE interface designs, multiple cable leads and multi-pin cable connectors are used.

DTE/DCE interface specifications are suitable for short-range connection of a DTE to a DCE¹(typical maximum cabling distance 25 m or 100 m). Such specifications reflect the fact that the DTE is the ‘end user equipment’ and that the DCE has the main role of ‘long distance communication’. The three main elements which characterise allDTE/DCEinterfaces are that:

• The DCE provides for the signal transmission and receipt across the long distance line (wide area network), supplying power to the line as necessary;

• The DCE controls the speed and timing (so-calledsynchronisation) of the communication taking place between DTE and DCE. It does this in accordance with the constraints of

1Though intended for DTE-to-DCE connection, DTE/DCE interfaces may also be used (with slight modification, as we shall see in Chapter 3) to directly interconnect DTEs.

DTE, DCE, line interfaces and protocols 9 the wide area network or long distance connection. The DCE determines how many data characters may be sent per second by the DTE and exactly when the start of each character shall be. This is important for correct interpretation of what is sent. The DTE cannot be allowed to send at a rate faster than that which the network can cope with receiving and transporting!

• The DTE sends data to the network on the path labelled ‘Tx’ and receives on the path labelled ‘Rx’, while the DCE receives on the ‘Tx’ path and transmits on the ‘Rx’ path.

No communication would be possible if both DCE and DTE ‘spoke’ to each other’s

‘mouths’ instead of to their respective ‘ears’!

The terms DTE and DCE represent only a particular function of a piece of computer equipment or data networking equipment. The device itself may not be called either aDTE or aDCE.

Thus, for example, the personal computer in Figure 1.4 is undertaking the function of DTE.

But a PC is not normally called a ‘DTE’. The DTE function is only one function undertaken by the PC.

Like the DTE, the DCE may take a number of different physical forms. Examples of DCEs are modems, network terminating (NT) equipment, CSUs (channel service units) and DSUs (digital service units). The DCE is usually located near the DTE.

The physical and electrical interface between a DTE and a DCE may take a number of different technical forms. As an example, a typical computer (DTE) to modem (DCE) connection uses a ‘serial cable’ interface connecting the male, 25-pin D-socket (ISO 2110) on the DTE (i.e., the computer) to the equivalent female socket on the DCE (modem). This DTE/DCE interface is referred to as aserial interfaceor referred to by one of the specifications which define it: ITU-T recommendationV .24 or EIARS-232. The interface specification sets out which control signals may be sent from DTE to DCE, how the timing and synchronising shall be carried out and which leads (and socket ‘pins’) shall be used for ‘Tx’ and ‘Rx’.

In addition to a standardised physical and electrical interface, a protocol is also necessary to ensure orderly communication between DTE and DCE. The protocol sets out the etiquette and language of conversation. Only speak when asked, don’t speak when you’re being talked to, speak in the right language, etc. Understanding the plethora of different protocols is critical to understanding the Internet, and we shall spend much of our time talking about protocols during the course of this book.

Why are there so many protocols? Because most of them have been designed to undertake a very specific function, something like ‘identify yourself’ or ‘send a report’. If you need to

‘identify yourself’ before ‘sending a report’ two different protocols may need to be used.

Line interfaces

Before we leave Figure 1.4, you may have noticed that our discussion has not concerned itself at all with what you might think is the most important part of the communication — conveying the data through the data network from one DCE to the other. Surprising as it may seem, this may not concern us. The realisation of the network itself has been left to the network operator and/or the data network equipment manufacturer! As long as the network transports our data quickly and error-free between the correct two end-points why should we care about the exact topology and technology inside the network? Maybe the internal protocols andline interfaces² of the network are not standardised! But why should this concern us? If there is a problem in the network what will we do other than report the problem and demand that the network operator sort it out?

2See Chapter 3.

The first data networks comprised a number of different ‘switches’, all of which were supplied by the same manufacturer. There are significant advantages to a single source of supply of switches, commercial buying-power being perhaps the most important. In addition, a single source of supply guarantees that devices will interwork without difficulty, and that advanced ‘proprietary’ techniques may be used for both the transport of data between the different switches and fornetwork management.

Using a specific manufacturer’s proprietary transport techniques can be advantageous, because at any one time the agreed public data networking standards are some way behind the capabilities of the most modern technology. A proprietary technique may offer benefits of cost, efficiency, better performance or afford capabilities not yet possible with standardised techniques. Thus, for example, proprietary versions of IP tag-switching appeared before a standardised version (called MPLS — multiprotocol label switching) became available. MPLS we shall meet in Chapter 7.

The advantage of having network equipment andnetwork management system supplied by a single manufacturer is that it is easy to correlate information and to coordinate configuration changes across the whole network. For example, it is relatively easy to change the physical location of a given data network address or destination from one switch to another and to adjust all the network configuration data appropriately. In addition, any complaints about poor network quality can be investigated relatively easily.

1.8 UNI (user-network interface), NNI (network-network interface) and INI (inter-network interface)

The initial priority of interface standardisation in data networks was to create a means for connecting another manufacturer’s computer (or DTE — data terminal equipment) to an exist-ing data network (at a DCE — data circuit-terminatexist-ing equipment) usexist-ing a protocol or suite of protocols. The combination of a DTE, DCE and relevant protocol specification describes a type of interface sometimes called auser-network interface (UNI). For some types of net-works (e.g., X.25,frame relay andATM — asynchronous transfer mode), a single document (the UNI specification) replaces separate specifications of DTE, DCE and protocol. A user-network interface (UNI)is illustrated in Figure 1.5. Despite the fact that the termUNI is not generally used in Internet protocol suite specifications, it is wise to be familiar with the term, since it is used widely in data networking documentation. We explain them briefly here.

A UNI (user-network interface) is typically an asymmetric interface, by means of which an end-user equipment (or DTE) is connected to a wide area network (WAN). The point of connection to the WAN may go by one of a number of different names (e.g., DCE — data circuit terminating equipment, modem, switch, router, etc.), but all have one thing in common — the network side of the connection (the DCE or equivalent) usually has the upper hand in controlling the interface.

As well as UNIs (user-network interfaces), there are alsoNNIs (network-network interfaces ornetwork-node interfaces) andINIs (inter-network interfaces). An NNI specification defines the interface and protocols to be used between two subnetworks of a given operator’s network.

Within each of the individual subnetworks of a large network, a single manufacturer’s equip-ment and the associated proprietary techniques of data transport and network manageequip-ment may be used. The NNI allows the subnetwork (which may comprise only a single node) to be inter-linked with other subnetworks as shown in Figure 1.5.

Unlike the UNI, the NNI is usually a more ‘symmetrical’ interface. In other words, most of the rights and responsibilities of the subnetworks (or single nodes) on each side of the interface are identical (e.g., management control, monitoring, etc.). Since the basic physical and electrical interface technology used for some NNIs was adapted from technology originally

Open systems interconnection (OSI) 11

Figure 1.5 UNI, NNI and INI type interfaces between end devices and data networks.

designed for UNI interface, it is often the case that one of the networks may be required to act as DCE, while the other acts as DTE. Symmetry is achieved simply by allowing both ends to assume either the DCE or the DTE role — as they see fit for a particular purpose. Some physical NNI interfaces are truly symmetric.

The third main type of interface is called the INI (inter-network interface)or ICI (inter-carrier interface). This is the type of interface used between networks under different own-ership, i.e., those administered by different operators. Most INI interfaces are based upon standard NNI interfaces. The main difference is that an INI is a ‘less trusted’ interface than an NNI so that certain security and other precautions need to be made. An operator is likely to accept signals sent from one subnetwork to another across an NNI for control or reconfig-uration of one his subnetworks, but is less likely to allow third party operators to undertake such control of his network by means of an INI. In a similar way, information received from an INI (e.g., for performance management or accounting) may need to be treated with more suspicion than equivalent information generated within another of the operator’s subnetworks and conveyed by means of an NNI.