The basic components of a data network

Basic Data Networks and Protocols

Modern data communications networks are built up in a ‘modular’ fashion — using standardised network components, interfaces and protocols based on digital line transmission, packet switching and layered communications protocols. In this chap-ter we present the basic components of a data network, and explain in detail the

‘networking’ or lower-layer protocols (protocol layers 1 – 3) which make them work.

We shall explain physical and electrical interfaces and connectors, as well as physi-cal, datalink, network, transport and higher-layer protocols: everything that goes to ensure efficient propagation across a network. Complex though it may seem, these are only the network basics!

3.1 The basic components of a data network

The basic components of a data network are illustrated in Figure 3.1. A personal computer (PC) and a mainframe computer — both examples of DTE (data terminal equipment) — are connected to the network (shown as a ‘cloud’) by means of DCE (data circuit-terminating equipment). The DCE is a device which provides the ‘starting point of the long-distance network’ and is a device installed either near to the DTE, or sometimes is incorporated into it.

The network of Figure 3.1 (providing the connection between the DCEs) comprises a meshed network of 4nodes.¹

The devices (DTEs, DCEs and nodes) in Figure 3.1 are interconnected by means of stan-dardised communications interfaces, and every possible physical, electrical and protocol aspect of the interconnection is precisely defined.

There are different classes of general interfaces describing different types of connections, as we learned in Chapter 1.User-network interfaces (UNIs) are used to connect end devices (DTEs) to the network and are generally ‘asymmetric’ — the balance of control of the interface being with the network.Network-node interfaces (NNIs), meanwhile, are more ‘symmetric’

arrangements intended to be used to interconnect nodes of equal importance in the main backbonepart of the network.

Both UNI and NNI specifications define in precise detail the physical and electrical aspects of the interconnection, as well as the lower-layer or networking protocols (layers 1 – 3 of

1Anodeis any kind of switch-type device, upon which numerous communications links and other connections converge. A node might technically be aswitch, arouter, aLAN hub, amultiplexor or some other kind of exchange. In this example, as often, the exact technical nature of the node does not concern us.

Data Networks, IP and the Internet: Protocols, Design and Operation Martin P. Clark

2003 John Wiley & Sons, Ltd ISBN: 0-470-84856-1

Figure 3.1 The components and interfaces making up a simple data network.

Figure 3.2 The protocol layers of the OSI (Open Systems Interconnection) model.

Figure 3.2) which must be used at the interfaces. It is the detailed functions of these layers (layer 1 —physical layer; layer 2 —datalink layer and layer 3 —network layer) which will be the main subject of discussion in this chapter. We start with an overview of the various layer roles, before we go on later to review the details.

Physical layer (layer 1)

The physical layer (layer 1) interface and protocol specification define how the physical medium (e.g. the copper cable, optical fibre or radio channel) is to be used to transport the bitstream which comprises a digital signal: the precise shape and electrical or optical characteristics of the signal, the bit rate and clocking, as well as the physical connectors to be used.

Different physical layer specifications correspond to different physicalmediaand modula-tion orline coding schemes. Thus one physical layer specification might cover the use of a twisted-pair copper cable to carry a signal of a given bit rate, while another physical layer specification may define the use of light pulses of a given laser light wavelength — to be sent over an optical fibre medium.

The basic components of a data network 69 While one physical layer (i.e., the medium and its related modulation, control and line coding — theprotocol) may be used as an alternative for another, media and protocols within the physical layer are not interchangeable. Thus, for example, it may be possible to use either a twisted-pair line or a fibre-optic-based communications system to carry a given signal, but it is ludicrous to consider using an optical transmitter and detector (intended for use with a fibre cable) in conjunction with a copper cable!

The physical layer interface usually provides for a simple point-to-point connection or link between two end-points. Alternatively, the physical layer of a shared medium (e.g., radio) provides for the ‘broadcast’ of a signal from any of a number of devices sharing the communications medium to all of the others. In the case of a shared medium, all but the intended recipient(s) will ignore the transmission. The most common shared medium systems used for data communications are LANs (local area networks) and wireless data networks.

Physical layer interfaces specifically designed for long-distance lines or NNIs (network-node interfaces) are usually designed for long-distance, permanently established point-to-point links. Suchline interfacesuse a minimum number of cable leads for the connection.

Each link of an end-to-end connection across a network may use a different physical medium and thus a different physical layer. Thus, for example, the path across the network from DTE-to-DTE in Figure 3.1 (a minimum of five links) might traverse different fibre, copper and radio communications media along the way. Each node will perform the necessary physical layer adaptations and signal conversions to achieve this.

In the case of physical layer interfaces intended for use as UNIs (user-network interfaces), the role of a special ‘network terminating’ or ‘adaption’ device is defined: the DCE — data-circuit terminating equipment. As we shall learn, the DCE undertakes a physical layer con-version of the short-range communications port capabilities of the DTE into aline interface format suitable for long-distance communication. The role of the DCE is to do the following:

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

• provide for network termination, 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;

• clockandbit-synchronise²the 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).

Datalink layer (layer 2)

Thedatalink layeranddatalink protocolare concerned with the formatting of the ‘raw’ stream of bits carried across a link by the physical layer into a byte-synchronised and structured form recognisable asblocks orframesof data. Apart from this, thedatalink layer is also concerned withdata flow control. The sending and receiving devices at either end of the physical link need to be coordinated to make sure that they ‘listen’ when ‘spoken to’ and only ‘speak’ when it is ‘their turn’. A frame check sequence (FCS)may be applied by thedatalink protocol in

2See Chapter 2.

the case where the physical medium is likely to be prone to a high level of bit errors. We discussed all this in Chapter 2.

Together, a datalink layer protocol, a physical medium and a physical layer protocol are all that is needed for point-to-point transport ofblocks or frames of data. A network layer (layer 3) protocol only becomes necessary when a network (i.e., a meshed topology of numerous links) needs to be traversed.

While various different physical layer media and protocols may be used with any given datalink layer, not all datalink protocols are suitable for all physical media and physical protocols. MORAL: you can mix and match to some degree — but not all combinations of physical layer (layer 1) and datalink layer (layer 2) protocols are viable!

Network layer (layer 3)

The network layer (layer 3) protocol is used to combine a number of separate individual datalinks into a ‘chain’ — thereby forming a complete path across a network comprising multiple nodes (as, for example, in Figure 3.1).

Thenetwork protocol is concerned withpackets andpacket switching. As we learned in chapters 1 and 2,packets of data are analogous to the parcels and letters of a postal service.

The frames of data carried by the datalink layer (layer 2), meanwhile, are analogous to the postal delivery vans which run from one postal depot or sorting office to the next. In just the same way in which postal delivery vans are loaded before a ‘run’ from one depot to the next, and then completely emptied and ‘re-sorted’ before packing into another van for the next

‘leg’ of the journey, so the frames of the datalink layer are filled and emptied of their packet contents before and after each datalink of the path through the network. The frames of the datalink protocol only carry the data for a single leg of its journey. The packets of the network protocol, on the other hand, are transferred from one end of the complete path to the other.

Network layer protocols have a number of responsibilities to fulfil:

• identification of destinationnetwork address;

• routing or switching of packets through the individual network nodes and links to the destination;

• statistical multiplexingof the data supplied by different users for carriage across the network;

• end-to-end data flow control: the flow control conducted by layer 2 protocols only ensure that the receiving data buffers on each individual link can receive more data. The layer 3 protocol has the more onerous task of trying to ensure a smooth flow of data across all the datalinks or subnetworks taken together. Uneven ‘hold-ups’ along the route need to be avoided;

• correct re-sequencing of packets (should they get out of order having travelled different routes on their way to the destination);

• error correction or transmission re-request on a network end-to-end basis.

Now, let’s get down to details.

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