The ever-increasing demand for various multimedia applications and services as well as the need for bandwidth expansions and faster data rates are becoming a challenge for every data center everywhere.

1. Trends in Optical Networking
1. Executive Summary of the development of the25G Standard
The ever-increasing demand of diverse multimedia applications and services as well as
the re- quirement for bandwidth expansions and faster data rates becomes a challenge
for every data center everywhere. Emerging technologies like cloud computing also
acted as catalyst in driving the industry to develop a new approach to adopt in these
fast-charging trends.
Reliance on networking permeates every aspect of our world, and data center
bandwidth re- quirements are expanding at double-digit rates-along with an equally
urgent push to contain costs. New technologies necessitate that data centers remain
flexible and scalable enough to adapt to changing requirements. The rise of cloud
providers changed the data center Ethernet landscape, creating a viable market for
high-speed, reasonably-priced connectivity.
Leading cloud and telco providers are clamoring for even more network performance in
order to meet the needs of their web-scale data centers and cloud-based services,
without compromis- ing the cost-to-performance ratio. To help address network
performance needs, leading man- ufacturers have joined forces to define and drive the
25 Gigabit Ethernet (25GbE) technology.
To suffice the increasing demands of collaborative multimedia services and
applications, to an- swer to the fast-changing traffic patterns, and to improve
accommodations of users’ bandwidth requirement for communication. This is one of
the reasons why an industry consortium was formed to create a new Ethernet
connectivity standard in data centers. The consortium’s goal is to enable the
transmission of Ethernet frames at 25 or 50Gb per second (Gbps) and to promote the
standardization and improvement of the interfaces for applicable products. Last July
2014, the IEEE agreed to support the development of this 25GbE standards for
servers and switching due to the increasing demand for a much faster network
performance while maintaining Ether- net economics. This standard was called 25
Gigabit Ethernet or 25Base-T, developed by the IEEE 802-3 task force P802.3by. This
standard was derived from 100GbE, since its operation works with four 25Gbps that
are running on four fibers in each direction. The IEEE 802.3by 25GbE standard is
technically complete and ratified on June of 2016.
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2. 2. The Push for higher Bandwidth for Mediaand
Transceiver Modules
The requirements in the market for Ethernet are constantly changing for different
applications and the coherent need of speed, further distance and lower costs. Various
speeds are also needed for specific applications like Wireless Access Points that use
2.5GbE and 5GbE; servers which need up to 25GbE and lastly core networks which
operate with up to 400GbE.
Due to this exponentially growing, global bandwidth requirements, Ethernet speeds are
also in constant development at a high rate to stay ahead of these demands. However,
innovation is occurring within varied application spaces at lower speeds. Most of
todays’ servers are still using GbE, and some users do not care for a foreseeable
future about higher speeds like Terabit Ethernet (TbE) and 400GbE.
There is diversification of efforts to coupe up these requirements, but a common goal
will un- dermine the diversities, which is a global requirement for a market-driven
standard, fostering innovation and enabling multi-vendor interoperability across
whatever application area the world’s growing cast of Ethernet users seeks to enable.
Ethernet has always been and will always be about connectivity and how far this
technology can go, but the Ethernet community embraces that the need for speed is
relative to the given application.
In years, since 1995 until 2010, the Ethernet’s evolution was somewhat slow in pace
and in- novation was mostly simple. Ethernet speed increased linearly - approximately
an order to an extent every few years like 10 Mbit/s to Fast-Ethernet 100 Mbit/s and
from 1G to 10G. Conse- quently, around 2010, the first 100G Ethernet version
100GBASE-SR10 was introduced. Below are some of the trends for this development.
25GBASE-SR: Over the development of 100GBASE-SR4 in 2015, it became deem
compulsory to develop the intermediate speed 25GBASE-SR to comply on standards
as IEEE 802.3 by 2016. This transceiver utilizes the popular SFP+ form factor with
LC-Duplex connector interface but with 25G speed. Meanwhile known as SFP28 and
known to have collective four 25G server ports at one 100G switch port.
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3. 50GBASE-SR: Designed to combine four 50G server ports to a 200G switch port.
Available in the market since 2018. The transceiver has the SFP+ form factor with LC-
Duplex connector inter- face, but in this case it runs with 50G speed and is called
SFP56.
100GBASE-SR2: The objective of this standardization project is to aggregate two 50G
server ports at one 100G switch port. This Ethernet version will be available along with
the 50GBASE- SR. The transceiver will have the QSFP form factor with LC Duplex
connector interface.
200GBASE-SR4: It will become available to the market together with 50GBASE-SR4
and 100GBASE-SR2. The configuration is expected to conform with the earlier series
of Ethernet SR4 devices.
Based on the released road map of the Ethernet Alliance in 2015, which outlines the
response of Ethernet to the ever-increasing desire for higher bandwidth in data
centers, Ethernet speed is shown to have unprecedented level of activity in the low
end of the market, this roadmap also shows Ethernet speed in the future.
After the ratification of IEEE 802.3by 25GbE in June 2016, we can now see that a new
Ether- net speed becoming the new common standard. 25GbE which is designed to
replace 10 GbE because of its cost effective and power efficiency to various
application like ToR switching for cloud providers. This new trend will also help the
demand for the huge data volume as well as the speed needed for the internet of
Things. Furthermore, this development focuses primarily to the fiber-based SFP28 and
QFSP28 market for the purpose of backbone or longer-haul con- nections.
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4. 3. TheAdvantages of aQSFP28TransceiverSolution
The QSFP28 transceiver is projected as the prospective interface by the Ethernet
Alliance. This transceiver enables network bandwidth to be cost effective and resource
saving. It is designed for 100GbE speeds using the 4x25GbE wiring specification,
hence the “Q” which stands for quad. The QSFP28 form factor retains the usual
density of 48 ports in a 1U tall switch which is very advantageous to existing systems
without the need to migrate the network to a new stan- dard. Moreover, QSFP28
increases the density but minimizes the power consumption. QSFP is slowly
becoming the universal form factor for various reasons.
First, it increases front panel density by maintaining the form factor and the maximum
number of ports but increases the lane speed from 10Gbps to 25Gbps. Secondly, it
supports both cables and transceivers. Using QSFP28, a one-rack unit switch can
accommodate up to 36 QSFP ports. Lastly, QFSP28 can use either VCSELs for short
distances or Silicon photonics for longer distanc- es to support data center to reach for
more than 2 kilometers of interconnection.
With all these benefits coming to a single form factor, the next versions of high-
bandwidth switches, routers and adapters will feature QSFP28 ports to ensure
100GbE data center inter- connection.
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4. 25GbESFP28, today’s new TransceiverStandard
25GbE is already an emerging standard for Ethernet connectivity that will be beneficial
to cloud and enterprise data center environments. Due to the Consortium that was
established in June 2014 an Ethernet Task Force was created to encourage 25GbE
technology and successively, to further develop the standard - the IEEE P802.3by.
Moreover, the IEEE P802.3bq 40GBASE-T Task Force adopted objectives to likewise
improve BASE-T support for 25GbE.
There are various market drivers why 25GbE standard emerged. The main reason is, it
provides a server connection speed faster than 10GbE that is optimized for cost,
throughput and efficien- cy. Moreover, it maximizes efficiency of server connections to
access switches in data centers. Lastly, this leverages four 25Gbps lanes (IEEE
802.3bj) running on four-fiber or copper pairs in- dividually transmitting at 25Gbps. This
sums up to a backplane of 100GbE per form factor. Every lane needs a Serializer /
Deserializer (SerDes) chipset. This twisted-pair concept originates from the 40GbE
standard development. The below table shows a synopsis of fundamental upcoming
IEEE standard interfaces that specify 25GbE.

5. IEEE 802.3 Standard Interfaces that specify
25GbE
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Physical Layer Name ErrorCorrection
MMF Optics 25GBASE-SR RS-FEC
Direct Attach Copper 25GBASE-CR BASE-RFECor RS-FEC
Direct Attach Copper 25GBASE-CR-S BASE-RFECor disabled
Electrical Backplane 25GBASE-KR BASE-RFECor RS-FEC
Electrical Backplane 25GBASE-KR-S BASE-RFECor disabled
Twisted Pair 25GBASE-T N/A
There are various fundamental benefits of 25GbE, it allows network bandwidth to be
cost-efficiently scaled in support of succeeding generation server and storage solutions
existing in cloud and web-scale data center settings. Below are its most noticeable
benefits.
1. Reduced CAPEX
- Lower Costcompared to 40GbE
- Fewer ToRswitches and cables
2. Maximum switchInput / Output performance andfabriccapability
- Four times the switch port density versus 40GbE(one-lane versusfour-lane)
- Higher performance than the existing10GbE
-Asingle lane per physical port maximizesthe number of connected servers
or uplinks perswitch
3. Fastmaturation byleveraging the current IEEE100GbE standard
4. ReducedOPEX
- Lesserpower, cooling and smaller footprint requirements
Some members of the 25 Gigabit Ethernet Consortium who are prominent suppliers of
Ethernet switching solutions are now offering 25GbE-capable Ethernet switch
platforms. Most Ethernet switching solutions including 10GbE, 25GbE to 100GbE
support multiple Ethernet rates, which means consumers have absolute cable choice
for network connectivity. Some of the notable vendors who use multispeed
10/25/40/50/100GbE switch platforms that offer these emerging standards are Cisco,
Arista, Broadcom and Mellanox.

6. In addition, Network Interface Cards (NIC) for the 25GbE standard are also rapidly
released. As a result, cables are also adopting this emerging standard. It is essential
that 25GbE and 50GbE channels compel on all the “channel characteristics” described
in IEEE standards under 802.3bj, section “Physical Medium Dependent (PMD)
sublayer and baseband medium, type 100BASE- CR4,” section 92.9., this is according
to the 25Gb Ethernet Consortium. A number of converter and cable combinations can
meet the features.
Specifics of 25GbE and 50GbE PMDs are both low-cost and with twin-axial copper
cable avail- ability. 25Gbps operation however, requires only two twin-axial cable pairs
whereas 50Gbps needs only four-twin-axial cable pairs.
ToR switches usually connect to servers via a link based on copper twin-axial cables as
well as the intra-rack connections between switches and/or routers.
Moreover, cables that connect to higher speeds and “fan out” (to multiple lower speed
links) are able to connect to 10/25/40/50Gbps speed which is now possible by the use
of multi-mode or Single-mode fibers, copper cables, matching reach-range to the
specific application need.
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Typeof Technology Media Standards Power ReachRange
100GBASE-SR4 Multi-mode 100GbE/25GbE 3.5 w 100meters
100GBASE-LR4 Single-mode 100GbE 4.5w 2km or 10km
PSM4 Parallel Single-mode 100GbE/25GbE 3.5w 500meters
CWDM4 CWDMsingle-mode 100GbE 3.5w 2 meters to 2km
100GBASE-CR4 PassiveCopper 100GbE/25GbE 3.5w 5 meters

7. ATechnical Overview of the 25GbESFP28
CLOCK-RATE
Inside Ethernet NICs or switches, a serial component called SerDes connects all of the
high- speed components which take data to transmit and then serialize it. Then, a
Deserializer on the recipient side reconstructs the serial stream of bits into data for the
final receiver. Over the years, the SerDes technology advanced to the newest 25Ghz
rate.
The mechanisms that consist of 10GbE switches run at 12.5Ghz SerDes with a clock-
rate of 10.3125GHz. Nevertheless, the present 40GbE NICs and switches utilize four
parallel SerDes links having a clock rate of 10.3125GHz each. In the contrary, the
components encompassing the 25GbE NICs and switches use a single lane SerDes
with a clock rate of 25.78125GHz. Below is a table that shows the clock rates, lanes
and performance 25GbE SFP28.
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Ethernet Clock Rate Lanes DataRate
1 GbE 1.25GHz 1 1 Gbps
10 GbE 10.31GHz 1 10 Gbps
25 GbE 25.78GHz 1 25 Gbps
40 GbE 10.31GHz 4 40 Gbps
100GbE 25.78GHz 4 100Gbps
Number of transmission connections withSerDes
The actual Ethernet ports in a usual 10GbE/40GbE Ethernet switch are SerDes
connections coming from the switching chip pins, which were used to connect directly
to the SFP+ and QSFP optics cages or other Ethernet or fabric chips (for blade
servers). Communication amid an SFP+/ QSFP in the front of the switch and the
switching chip runs on top of one of these SerDes con- nections. With this, the term
“Lanes” is used to call the number of SerDes connections required to drive a switch
port.
Currently switches use components that are all run by SerDes with a clock rate of
about 10Ghz, delivering a 10Gb transfer rate among every component, permitting for
the encoding overhead. SerDes technology improved in the past several years that it
reached 15Ghz SerDes, it turned out to be economically doable and all of the different
physics linked challenges in signal integ- rity found dependable solutions.

8. Four parallel SerDes links between the Ethernet chip and the QSFP pluggable module
is the composition structure of the so-called 40GbE interface. It remains indispensable
to have four parallel 10Gb streams in extending QSFP onto fiber to transport this to
the receiving QSFP (i.e. parallel optics). Short reach QSFP interfaces utilize four pairs
for the transmission. Long-reach QSFP interfaces use an internal Coarse Wave
Division Multiplexing (CWDM) to transport the four 10Gb streams over a single pair of
fiber. The requisite of four lanes substantially decreases switch port density per
switching chip and escalates the cost of cabling and optics.
The 25GbE standard leverages the availability of a 25Ghz SerDes and requires only a
single SerDes lane, while delivering 2.5 times more throughput compared to 40GbE
solutions and significant CAPEX savings compared to 40GbE solutions.
Moreover, existing several blade server chassis solutions have limits of only two
SerDes lanes for their LAN on Motherboard (LOM) networking ports, hence, they can’t
implement a four- lane 40Gbps interface.
IEEEerror correction code for 25 GigabitEthernet
Starting with 10GbE and 10Gb Fibre Channel Inter-Switch Links (ISLs), the “64b/66b”
encoding scheme is used to develop data transfer efficiency.
The 64b/66b encoding outcomes a 3% overhead (66-64) /66 on the raw bit rate. To pay
off, Clause 74 (Fire code) FEC was presented to deliver extra error protection.
25GbE specification both supported Clause 74 FEC and Clause 91. Auto-negotiation
can deter- mine whether Clause 74 FEC, Clause 91 FEC, or no FEC is employed on
the link.
Auto-Negotiation for 25 GigabitEthernet
Specifics of auto-negotiation capabilities aren’t fully established or implemented. The
25GbE and 50GbE solutions are nevertheless backward and forward compatible with
10GbE, 40GbE, 100GbE and 400GbE products since they use the same IEEE 802.3
frame format. However, switch port capability to automatically select a different speed
is still under development.
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9. Different Form Factors for 100G and25G
Shown here below, the 25GbE physical interface description supports various form
factors:
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Currently some available Switches don’t support direct 25GbE connections by means of
a SFP28 Direct Attach Copper (DAC) cable. The usage of a breakout cable that permits
4x 25GbE ports to connect to a 100GbE QFSFP28 switch port is one of the suggested
solutions. Lengths of Direct Attach Copper (DAC) cable are limited to five meters for
25GbE. To support longer lengths, Ac- tive Optic Cable (AOC) solutions can be utilized
as well.
PCIExpress (PCIe) Interfaces
The PCIe 3.0 interface is present-everywhere transversely in shipping server
platforms. Due to cost, the preference in cloud and web-scale server deployments is
headed for single-port Eth- ernet connectivity. These volume servers usually have
PCIe 3.0 x4 slot(s).
The table below shows why 25GbE is an easier move to upgrade from 10GbE lanes for
it entails half the number of PCIe lanes and fits into the existing model against 40GbE
lanes, resulting to better PCIe bandwidth usage and lower power impact.
PCIe 3.0 Lanes required for Ethernet Generations
Form Factor Lanesand Speed
QSFP28 4 x25Gbps
SFP28 1 x25 Gbps
Ethernet SinglePort Dual Port
10GbE 2 4
25GbE 4 8
40GbE 8 16
100GbE 16 32

10. Conclusion: 25GbEreplaces 40GbE
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The consortium held in July 2014 was a call for interest of members who
unan- imously agreed to support the development of a new standard for
servers and switching. They created the 25Gbps Ethernet Task and
developed the 25Gbps standard. There is a fast progress of this standard
due to the high leverage of the existing standard for 100GbE as the base
standard. The new 25GbE standards maximizes server efficiency to
access switch interconnects and provides an op- portunity for optimum
cost/performance benefits. It will provide up to 2.5 times faster
performance than existing 10GbE connections while maximizing the Eth-
ernet controller bandwidths/pin and switch fabric capability. More than
50% of the rack interconnect cost per unit of bandwidth can be saved
and significantly improve an operator’s bottom line. Furthermore, it
increases network scale and accommodate higher server density within
a rack than what is currently achiev- able with 40GbE ToR links. In short
order, deploying 25GbE solutions, something that solutions built on the
more complex 40GbE standard can never achieve.

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