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5 Ethernet physical layer—basic functions of the physical layer signalling (PLS)

5 Ethernet physical layer—basic functions of the physical layer signalling (PLS)

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136

Local area networks (LANs)
Table 4.4

Ethernet physical layer signalling parameters and constraints
10baseT
(ethernet)

Bit duration
Slot duration
Minimum frame
size
Max. Round trip
delay
Maximum frame
size
Interframe gap
(IFG)
Maximum number
of stations
Usual physical
medium

100baseX
(fast ethernet)

1000baseX
(gigabit ethernet)

100 ns
512 bits
64 bytes
(72 bytes with all
PCI)
51.2 µs

10 ns
512 bits
64 bytes
(72 bytes with all
PCI)
5.12 µs

1 ns
4096 bits
64 bytes

1500 (MTU only)
(1530 with all PCI)
9.6 µs

1500 (MTU only)
(1530 with all PCI)
0.96 µs

1500 (MTU only)
(1530 with all PCI)
0.96 µs

100

100

100

2-pair Cat 5 copper
cable

Cat 5 copper cable
(2-pair for
100baseTX or
4-pair for
100baseT4) or
multimode fibre
(100baseFX)
100 m (unshielded
twisted pair
copper cable)
400 m (half
duplex fibre)
2 km (full
duplex fibre)
250 m

Multimode fibre or
monomode fibre or
150 ohm twinax or cat 5
cable(1000baseT — IEEE
802.3ab)

Maximum
hub-to-station
or
point-to-point
length

100 m

Maximum
collision
domain size

500 m

512 ns

320 m (half duplex [HDX]
fibre) 3 km (full duplex
[FDX] monomode fibre)
200 m copper cable

25 m (1000baseCX) 200 m
(1000baseT) 320 m (half
duplex [HDX] fibre)

transmitting station’s own signal from the most remote end of the LAN would be interpreted
by that station as a signal different to that which it was transmitting, and the collision back-off
procedure would be commenced. Another effect of too long a round-trip delay is that remote
stations would not start to receive a frame of the minimum size until after the sender had completed sending it. In other words, it would not be possible to detect and avert a collision fast
enough: the sender of the packet would not repeat the transmission in the case of a collision
(since it did not detect the collision).
The maximum transmission unit (MTU) size of 1500 bytes (giving a total frame size including preamble of 1530 bytes — see Figure 4.5) is stipulated to ensure that a given station does
not dominate the use of the LAN; and to assist the network interface card (NIC) designers
determine the appropriate size of data buffers.

4.6 Ethernet hubs (half duplex repeaters)
Originally, ethernet was conceived as a bus topology (Figure 4.1 c and Figure 4.3a), but
with the emergence of office structured cabling systems based on twisted pair cabling (either
unshielded twisted pair, UTP or shielded twisted pair (STP) and often to Category 5 (Cat.

Ethernet hubs (half duplex repeaters)

Figure 4.8

137

Broadcasting function of an ethernet hub (half-duplex repeater, HDR).

5) cable specification), the use of ethernet reverted almost exclusively to a star topology
(Figure 4.3b) or point-to-point usage (for direct connection of two intercommunicating devices). The new star topology was heralded by the appearance of the ethernet hub.
An ethernet hub (also called a half duplex repeater, HDR) is a device designed to be
placed at the ‘star point’ of a star topology. It gives each connected device the impression it
is connected to a shared bus. Connections of each of the stations of the LAN (i.e., the DTEs)
are connected to the hub by means of two twisted pairs of wires (usually pins 1;2 and 3;6 of
an RJ-45 connector). The hub receives data from each station on RJ-45 pins 3 and 6 (Tx) and
transmits data to each station on pins 1 and 2 (Rx).5 The main action of the hub is simply
to ‘add’ all the signals it receives from stations together and broadcast the same signal back
to all stations (on the Rx leads). This is illustrated in Figure 4.8. In this way, each station
receives all the signals transmitted by other stations, as if they had all shared the same bus.
Even the transmitting station receives a copy back from the hub of the original signal it sent.
This allows it to check for collisions.
In addition to signal broadcasting, the hub acts to repeat the signals received. In other
words, it cleans up the line coding (regenerates the signal) and strengthens it as necessary.
The fact that at any one time the same signal appears on at least one of the receive leads
(Tx) and all of the transmit (Rx) leads of the hub ports restricts the operation of a normal
ethernet LAN to half duplex operation. This is the so-called shared ports configuration. On
the other hand, it is possible (using a so-called cross-cable or cross-over cable, which switches
pins 1 and 2 at one end of the cable to pins 3 and 6 at the other and vice-versa) to connect
two ethernet DTEs directly to one another without using a hub. In this case a point-to-point
configuration results, which optionally can be used in a full duplex mode if the collision
detection is disabled (defined by IEEE 802.3x). When used in the full duplex mode, LLC
uses the X-ON X-OFF protocol for data flow control. Receipt of the X-OFF character (ASCII
13H) means that the receive buffer is full and sending should stop until a subsequent X-ON
character (ASCII 11H) is received.
Nowadays, it is becoming more common to use LAN switches rather than LAN hubs at the
‘star point’ of the LAN topology. This enables the use of full duplex transmission. In addition,
it is an important means of reducing the broadcast traffic on a LAN. For as the traffic levels
in a LAN grows (which is inevitable as the number of stations and overall usage grows), the
level of broadcast traffic (i.e., the fact that each message has to be sent to each of the stations)
5

Tx and Rx leads are named relative to the DTE — so they appear to be ‘the wrong way round’ at the hub.

138

Local area networks (LANs)

turns out to be a real limitation on the overall traffic capacity of the LAN. More on LAN
switches later in the chapter.

4.7 Alternative physical layers — ethernet, fast ethernet
and gigabit ethernet
So that we don’t constantly have to keep referring back to it, Figure 4.9 is a repeat of
Figure 4.4. It illustrates the various physical layer sublayers defined by the IEEE 802.3 suite
of standards. Here it shall serve to help us explain the functions of the various sublayers and
also compare the slightly different realisations of ethernet (10baseT), fast ethernet (100baseX)
and Gigabit ethernet (1000baseX).

Ethernet physical layer (IEEE 802.3 10baset:IEEE 802.3i)
The basic IEEE 802.3 standards for the ethernet physical layer used in 10baseT evolved
directly from the original standards intended for coaxial cable-based networks (10base5 for
thicknet and 10base2 for thinnet ethernet). Some of the sublayers therefore reflect a split
of the functionality of the physical layer which is more apparent when considering 10base5
and 10base2 networks than it is in a modern 10baseT ethernet. Let us consider each of the
sublayers in turn, starting at the ‘top’.

Figure 4.9

Ethernet protocols and sublayers.

Alternative physical layers — ethernet, fast ethernet and gigabit ethernet

139

The physical layer signalling (PLS) sublayer controls the carrier sensing and reacts to
collision detection as we described earlier. The AUI (attachment unit interface) passes signals
from the PLS to the PMA (physical medium attachment). In practice, AUI is a cable with
DB-15 plugs and sockets. Such connectors (labelled AUI) are still found on older networking
equipment and ethernet network interface cards (NIC). AUI also defines the coding of the
physical layer signal to be Manchester coding (see Chapter 2–Figure 2.18).
The physical medium attachment (PMA) is achieved using a device known as a medium
attachment unit (MAU). It is the PMA which is responsible for the actual detection of collisions,
notifying them by means of the AUI to the PLS for action. It also regulates when transmissions
may be sent onto the medium, but otherwise merely forwards the already line-coded signal,
adapting it for the actual type of coaxial cable or other medium in use. In the later days of
coaxial cable networks, the MAU was a connection device which could be incorporated into the
cable network itself by means of a BNC socket. Alternatively it was sometimes incorporated
into wall sockets (behind which was the coaxial cabling of the bus). The medium dependent
interface (MDI) in this case is the specification of one of the different allowed 50 coaxial
cable types (thicknet, thinnet) and the associated BNC (bayonet connector) connectors.
In modern 10baseT ethernet, it is normal for the AUI and MAU functionality to be combined
into the network interface card (NIC). This is both cheaper and reduces the possible sources
of failures. The standard interface format from the NIC is nowadays an RJ-45 socket. An
RJ-45 category 5 patch cable is used to connect the NIC to a similar socket on the LAN hub.
Should, however, a modern NIC be required to be connected to an older coaxial cable ethernet
or standard AUI (DB-15 connector) then a transceiver is used to do the conversion. This is a
small device with a single RJ-45 socket on one side, and an AUI interface (DB-15 connector or
BNC connector) on the other — for direct connection to the coaxial cable ethernet backbone.

Fast ethernet physical layer (100baseT, IEEE 802.3u)
The active hubs (switches) used in fast ethernet networks make it possible to combine the
different types of fast ethernet and older 10baseT ethernet devices in a single network. Such
backward-compatibility was given high importance by fast ethernet designers. Naturally therefore, the combined 10/100baseT hubs have to cope with much more than the passive hubs of
simple 10baseT networks. First of all, each of the ports of a 10/100baseT active hub may be
running at different speeds (either 10 Mbit/s or 100 Mbit/s). Second, each port speed may first
have to be either autosensed (the hub adjusts to the speed of the device) or auto-negotiated
between the devices (which discover which particular fast ethernet technology is in use (TX,
T4, etc).
There were new considerations to be taken care of in the specifications of fast ethernet:
• The higher bit rate demanded faster interfaces between the protocol layers and on the
physical medium.
• Backward compatibility was essential, in order that older DTE devices with existing ethernet cards could coexist with newer fast ethernet devices in the same LAN.
• Full duplex operation was defined, since point-to-point connections of fast ethernet were
envisaged as backbone links between different ethernets.
At the time of introduction of fast ethernet, modifications had to be made to the ethernet MAC
and physical layers to take account of the much higher speed of transmission. Fast ethernet
uses technology for high speed physical data transfer which came from FDDI (fibre distributed