Gigabit Ethernet


Gigabit Ethernet.  Gigabit Ethernet is a 1000 Mbps enhancement of the Ethernet standard largely to be used for backbones.  This 802.3z standard is an augmentation of the IEEE 802.3 specifications for Ethernet.  Gigabit Ethernet has been designed to use the standard Ethernet frame format.  This establishes compatibility with previous installed 10 Mbps and 100Mbps Ethernets and requires no frame translation.  The push for Gigabit Ethernet seems to be largely due to its inherent compatibility with Ethernet access LANs.  Gigabit Ethernet is the main competition for ATM.  It can also scale to WANs.

Gigabit Ethernet can be looked at as having two major categories.

  • 1000BaseT - Like its slower cousins, it makes use of UTP cabling for distances up to 100m.
  • 1000BaseX - this is usually divided into three subcategories
    • 1000BaseSX - This relies on multimode fiber optics for standard use giving short haul runs up to 200m.
      • based on short wave length laser which suggests the S in the name
    • 1000BaseLX - This is most often based on single mode fiber optics for long runs up to 10km.
      • based on long wavelength laser which suggests the L in the name
      • apparently there are implementations that work with multimode fiber if it is mode conditioned
    • 1000BaseCX - This relies on a shielded copper medium designed for short patches between devices with distances limited to 25m.
      • it is based on a new type of shielded cable
      • doesn't make use of standard UTP or IBM Type I or II
      • conceived for data center interconnections or intra/inter-rack connections

1000BaseSX is the most common of the fiber optic solutions and the least expensive.  As you'd expect the 1000BaseCX requires a fair number of special features to implement.

Due to the high speed of Gigabit Ethernet, in order to avoid collisions, the frame size must be less than 512 bytes!  This also means that every frame carries an unusual amount of overhead relative to the actual data being communicated.  To alleviate this the Gigabit standards specify a burst mode to be specified if desired.  This burst mode allows small frames to chain together by sending carrier extensions in the interframe gap.  Other stations wanting to transmit see the interframe gap but still detect carrier and avoid transmission.  This burst mode standard allows for up to 64Kb of burst mode traffic to pass before sending a standard interframe gap.

Gigabit Ethernet Capabilities.  Gigabit Ethernet is ATM's main competition for backbones.  ATM was generally replacing FDDI, but Gigabit Ethernet has been giving it some serious competition.  There seem to be several major reasons for this competitiveness.

  • speed capabilities
  • administrator's/designer's/implementer's familiarity with Ethernet
  • lack of need for LANE adaptations/transformations required to interface ATM with Ethernet
  • potential for 10 Gigabit Ethernet
    • requires either multimode or single mode fiber
    • still expensive
      • Catalyst 10GBase-EX4 Metro 10 Gigabit Ethernet Module still costs around $80,000!

But Gigabit Ethernet doesn't enjoy the inherent QoS - Quality of Service characteristics that ATM does mostly due to its variable frame sizes of 64 bytes to over 1400 bytes.