Information Technology Reference
In-Depth Information
The 802.3ab Task Force has been assigned the task of defining the 1000BASE-T physi-
cal layer standard for gigabit Ethernet over four pairs of Cat-5 UTP cable, for cable dis-
tances of up to 100 m, or networks with a diameter of 200 m. As it can be used with ex-
isting cabling, it allows easy upgrades. Unfortunately, it requires new technology and
new coding schemes in order to meet the potentially difficult and demanding parameters
set by the previous Ethernet and fast Ethernet standards.
26.13.2 Fibre Channel components
The IEEE 802.3 committee based much of the physical layer technology on the ANSI-
backed X3.230 Fibre channel project. This allowed many manufacturers to re-use physical-
layer Fibre channel components for new gigabit Ethernet designs, and has allowed a faster
development time than is normal, and increased the volume production of the components.
These include optical components and high-speed 8B/10B encoders.
The 1000BASE-T standard uses enhanced DSP (digital signal processing) and enhanced
silicon technology to enable gigabit Ethernet over UTP cabling. As Figure 26.23 shows, it
does not use the 8B/10B encoding.
26.13.3 Buffered distributors
Along with repeaters, bridges and switches, a new device called a buffered distributor (or
full-duplex repeater) has been developed for gigabit Ethernet. It is a full-duplex, multiport,
hub-like device that connects two or more gigabit Ethernet segments. Unlike a bridge, and
like a repeater, it forwards all the Ethernet frames from one segment to the others, but unlike
a standard repeater, a buffered distributor buffers one, or more, incoming frames on each link
before forwarding them. This reduces collisions on connected segments. The maximum
bandwidth for a buffered distributor will still only be 1 Gbps, as opposed to gigabit switches
which allow multi-gigabit bandwidths.
26.13.4 Quality of service
Many, real-time, networked applications require a given quality of server (QoS), which
might relate to bandwidth requirements, latency (network delays) and jitter. Unfortunately,
there is nothing built into Ethernet that allows for a QoS, thus new techniques have been de-
veloped to overcome this. These include:
•
RSVP - allows nodes to request and guarantee a QoS, and works at a higher level to
Ethernet. For this, each network component in the chain must support RSVP and com-
municate appropriately. Unfortunately, this may require an extensive investment to to-
tally support RSVP, thus many vendors have responded in implementing proprietary
schemes, which may make parts of the network vendor-specific.
•
IEEE 802.1p and IEEE 802.1Q - allows a QoS over Ethernet by tagging packets with an
indication of the priority or class of service desired for the frames. These tags allow ap-
plications to communicate the priority of frames to internetworking devices. RSVP sup-
port can be achieved by mapping RSVP sessions into 802.1p service classes.
•
Routing - implemented at a higher layer.
26.13.5 Gigabit Ethernet migration
The greatest advantage of gigabit Ethernet is that it is easy to upgrade existing Ethernet-
based networks to higher bit rates. Typical migration might be:
•
Switch-to-switch links - involves upgrading the connections between switches to
1 Gbps. As 1000BASE switches support both 100BASE and 1000BASE then not all the
