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7 DTE ( data terminal equipment), DCE ( data circuit- terminating equipment), line interfaces and protocols
The Internet, email, ebusiness and the worldwide web (www)
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 and wide 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
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 a transmit (Tx) data path and a receive (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 all DTE/DCE interfaces 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-called synchronisation) of the communication
taking place between DTE and DCE. It does this in accordance with the constraints of
Though 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
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
• 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 a DTE or a DCE.
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 a serial interface or referred to by one of the specifications
which define it: ITU-T recommendation V .24 or EIA RS-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.
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 and line interfaces 2
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?
See Chapter 3.
The Internet, email, ebusiness and the worldwide web (www)
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 for network 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 and network 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 existing data network (at a DCE — data circuit-terminating equipment) using a protocol or suite
of protocols. The combination of a DTE, DCE and relevant protocol specification describes
a type of interface sometimes called a user-network interface (UNI). For some types of networks (e.g., X.25, frame relay and ATM — asynchronous transfer mode), a single document
(the UNI specification) replaces separate specifications of DTE, DCE and protocol. A usernetwork interface (UNI) is illustrated in Figure 1.5. Despite the fact that the term UNI 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 also NNIs (network-network interfaces
or network-node interfaces) and INIs (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 equipment and the associated proprietary techniques of data transport and network management
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)
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 (intercarrier interface). This is the type of interface used between networks under different ownership, 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 reconfiguration 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.
1.9 Open systems interconnection (OSI)
In the early days of computing, the different computer manufacturers developed widely diverse
hardware, operating systems and application software. The different strengths and weaknesses
of individual computer types made them more suited to some applications (i.e., uses) than
others. As a result, enterprise customers began to ‘collect’ different manufacturers’ hardware
for different departmental functions (e.g., for bookkeeping, personnel records, order-taking,
The business efficiency benefits of each departmental computer system quickly justified the
individual investments and brought quick economic payback. But the demands on computers
and computer manufacturers quickly moved on, as company IT (information technology)
departments sought to interconnect their various systems rather than have to manually re-type
output from one computer to become input for another. As a result, there was pressure to