Local area networks

In the mid-1980s, several protocol suites emerged to fill the gap between the data link
and applications layer of the OSI reference model. These were Appletalk, IPX and
NetBIOS with the dominant protocol suite during the early 90s being IPX due to its
popularity with MS-DOS users. TCP/IP existed at this point but was typically only used
by large government and research facilities.[54] However as the Internet grew in
popularity and a larger percentage of local area network traffic became Internet-related,
LANs gradually moved towards TCP/IP and today networks mostly dedicated to TCP/IP
traffic are common. The move to TCP/IP was helped by technologies such as DHCP
introduced in RFC 2131 that allowed TCP/IP clients to discover their own network
address — a functionality that came standard with the AppleTalk/IPX/NetBIOS protocol
suites.


However it is at the data link layer that modern local area networks diverge from the
Internet. Where as Asynchronous Transfer Mode (ATM) or Multiprotocol Label
Switching (MPLS) are typical data link protocols for larger networks, Ethernet and
Token Ring are typical data link protocols for local area networks. The latter LAN
protocols differ from the former protocols in that they are simpler (e.g. they omit features
such as Quality of Service guarantees) and offer collision prevention. Both of these
differences allow for more economic set-ups. For example, omitting Quality of Service
guarantees simplifies routers and the guarantees are not really necessary for local area
networks because they tend not to carry real time communication (such as voice
communication).

Including collision prevention allows multiple clients (as opposed to
just two) to share the same cable again reducing costs. Though both Ethernet and Token
Ring have different frame formats, it is in terms of collision prevention that the two
present the greatest difference. With Token Ring a token circulates the network and
clients only transmit when they have the token. The token must be managed to ensure it
is not lost or duplicated. With Ethernet any client can transmit if it thinks the medium is
idle, but clients listen for collisions and if one is detected suspend communication for a
random amount of time.


Despite Token Ring’s modest popularity in the 80’s and 90’s, with the advent of the
twenty-first century, the majority of local area networks have now settled on Ethernet. At
the physical layer most Ethernet implementations use copper twisted-pair cables
(including the common 10BASE-T networks). Some early implementations used coaxial
cables. And some implementations use optical fibres. Optical fibres are also likely to feature prominently in the forthcoming 10-gigabit Ethernet implementations.Where optical fibre is used, the distinction must be made between multi-mode fibre and single-mode fibre. Multi-mode fibre can be thought of as thicker optical fibre that is cheaper to manufacture but that suffers from less usable bandwidth and greater attenuation.