LEONI High Speed Cables
Technical document
May 19, 2004
LEONIZELL® - Propagation Delay and Delay Skew considerations
Velocity of propagation (vp) is the speed at which a signal travels over transmission medium. This value is expressed as a
percentage of the speed of light (300 million m/sec.), which is used as the reference velocity. The value of vp can be calculated
, where k is the dielectric constant of the insulating material. This
from the following empirical equation:
equation shows that vp increases as dielectric constant decreases. Therefore, the speed of transmission along a conductor is a
function of the insulation’s electrical properties. It’s important to note that the dielectric constant of some materials changes
with frequency. This means that vp will also vary slightly.
The dielectric constant and hence the vp of melt extrusion foamed fluorocarbon dielectrics do change appreciably as in the
frequency range of approximately 1,5 GHz and beyond, Same undesired effect is experience in foamed Polyolefin materials
where accelerating chemicals and nucleating agents are mixed in the melt to create cell structures within the dielectric.
Transmission speed is a critical property of single and even more so in multiple-parallel pair cables because it can affect the
design and reliability of high speed networks.
Since vp can vary significantly with frequency due to contamination of dielectric in the case of chemical polyolefin foam, and or
fluorocarbon materials , application of refined legacy dielectric technologies as well as new materials and processes are need to
respond effectively to the new challenges that new high bandwidth interconnect solutions require
However, nvp is only a part of the story. For network engineers and interconnect system designers, the actual time required for
a signal to travel from sender to receiver is far more important. The time needed to transmit a signal over a specific distance of
parallel pair cable is expressed as propagation delay. Propagation delay skew is the difference in delay between the fastest and
slowest pairs within the same cable sheath.
Important factors other than control of absolute dielectric constant and the hereby resulting skew performance are application
of pair shields around the parallel pair as well as uniformity of drain wires applied within the differential pair.
Any compression and or crush of the individual differential pair will cause abnormal behavior and change of dielectric constant
within the pair resulting in increased skew over the acceptable limits for performance of the network.
Importance of Delay Skew in Network Standards and Reliability
Steady increase in demand for faster networking speeds has led to the development of next-generation high-speed I-O
interconnects operating at 2500 Mbps and beyond. To deliver the required bandwidth, new standards IEEE 802.ak3 for Gigabit
interconnect is being rapidly developed, as well as newly developed standards such as Infiniband, that rely on simultaneous
transmission over more than two pairs within the cable.
If the difference in propagation delay between pairs, or delay skew, exceeds specified limits, network performance can be
compromised. Transmission of parallel data streams over multiple pairs must remain closely synchronized. Data bits must arrive
LEONI High Speed
Cables GmbH
Naverland 2
DK-2600 Glostrup
Denmark
USA toll-free: 1-800-903-7153
Phone +45 434-747-25
Telefax +45 434-747-45
e-mail: info@lhsc.leoni.com
www.leoni-high-speed-cables-com
Page 1 of 5
LEONI High Speed Cables
Technical document
May 19, 2004
at the receiver within a specified time limit if they are to be successfully recombined.
Exceeding skew limits results in unacceptable levels of transmission errors and collisions that reduce effective bandwidths and
system reliability.
To assure good performance, the above referenced IEEE and Infiniband standards define a maximum limit on delay skews.
Compared to legacy standards for high speed such as i.e. the TIA/EIA standards, propagation delay and delay skew
specifications require that delay skew shall not exceed 50 nanoseconds (ns) at any frequency between 2 and 25 MHz, and shall
not vary by more than ± 10 ns within this frequency range. The new standards initiatives are requiring astonishing maximum
of 10 Pico seconds per meter at frequencies up to 2,5 Gigabit per second. A thousands fraction on acceptable skew at 1000
times the frequency.
Length anomalies
Propagation delay skew between pairs also affects the basic link or channel length. Hence the geometric lay-up of cables with
multiple pairs is crucial to the overall performance of the cable.
Testing used is a method known as Time Domain Reflectometry (TDR) to calculate length. A pulse is transmitted down the
length of cable and reflected back to the testing equipment. The tester measures the time delay between transmission and
reception of the pulse. Once delay is known, the tester computes the pair length using the nvp (nominal velocity of
propagation) measured.
Since the mechanical length of the pairs within the cable are different due to the circumference of the pitch diameter of the
pair ,Some length variation within a given link or channel is inherent. Technologies of matching propagation delay of individual
pairs much be considered before making the differential single parallel pairs into the final cable bundle, and example of the
matching is 24 Pairs within the same cable using 3 layers of pairs.
Different rates of cabling twist mean longer or shorter linear footage among pairs. Pair length is always greater than linear
cable length. Depending on cabling technologies there can be skew variations from pair to pair exceeding 50 Picoseconds per
meter.
Performance choices
Developments of precisely manufactured parallel pair cable technologies has resulted in pushing the envelope of the need for
fiber optics, recent studies suggest that for short haul cabling within racks, and or between racks with transmission speeds of
up to 11,2 Gb/sec. in SerDes Applications.
LEONI High Speed
Cables GmbH
Naverland 2
DK-2600 Glostrup
Denmark
USA toll-free: 1-800-903-7153
Phone +45 434-747-25
Telefax +45 434-747-45
e-mail: info@lhsc.leoni.com
www.leoni-high-speed-cables-com
Page 2 of 5
LEONI High Speed Cables
Technical document
May 19, 2004
For those applications were legacy speeds of category LAN cables and or ATM bandwidth a foamed dielectric using chemicals
to create the foam and co-polymers is sufficient in order to meet or exceed the limits specified in the standards, However as
bandwidth increases to comprise Infiniband and or 10 Gigabit Ethernet the application of these legacy dielectric technologies
become more and more difficult due to the unpredictable skew performance and uniformity of cell dispersion throughout the
length of the entire.
Premium performance differential parallel pair cable dielectric technologies are available with new dielectric technologies with
polyolefin based dielectric where the cell structure is created with inert gas injected in to the melt during extrusion.
The LEONI High Speed Cables GmbH unique newly developed LEONIZELL®
technology and associated manufacturing process is developed to provide
answers and performance unseen in high speed cables.
The LEONIZELL® technology comprises a precise process control extrusion
process where a inner skin and an outer skin is simultaneously being extruded
with the cell expansion created between the inner skin and the outer skin.
Extrusion head of LEONIZELL®
By applying the inner skin ensures that no avoids are created close to the
centre conductor with potential performance degradation as a result,
The outer skin ensures mechanical strength and durability protecting the foam
for mechanical impacts during caused by compression and crushing of the
cable, making the ParaLink ® cable immune to mechanical stress such as cable
ties and tight bend radius.
The new developed Computer integrated
LEONIZELL® Extrusion line
The ParaLink ® cable utilizing the LEONIZELL® skin foam skin dielectrics offer high value and future proofing by providing
maximum headroom with absolute world class minimal delay skew. In addition, since all pairs have the same dielectric
properties, transmission characteristics are uniform resulting in world class cable balance.
LEONI High Speed
Cables GmbH
Naverland 2
DK-2600 Glostrup
Denmark
USA toll-free: 1-800-903-7153
Phone +45 434-747-25
Telefax +45 434-747-45
e-mail: info@lhsc.leoni.com
www.leoni-high-speed-cables-com
Page 3 of 5
LEONI High Speed Cables
Technical document
May 19, 2004
Below are tables of various within pair skew performance:
ParaLink ® Skew within
pair ps/6 meter
Form Factor
Industry Skew within
pair ps/ 6meter
Improvement
Form factor 1X (2 Pair)
18
100
5x
Form factor 4X (8 Pair)
25
100
4x
Form factor 12X (24 Pair)
30
100
3x
Within pair time delay skew effects on insertion loss
Within pair skew adds to absolute insertion loss in certain frequencies as well as creating an insertion loss suck.
Depending on cable quality the effects of additional insertion loss added by within pair skew can have significant negative
effects on the overall performance of the system, potential creating a system down scenario.
Below are graph shows the effects of additional insertion loss as well as the impact of time delay skew on insertion loss suckouts:
Infiniband Skew performance requirements
50 Pico Seconds (10 m)
0
10
a /dB
20
30
40
50
60
0
2
4
6
8
10
f /GHz
No significant impact on the insertion loss at the fundamental frequency of 1,25 GHz caused by within pair skew
LEONI High Speed
Cables GmbH
Naverland 2
DK-2600 Glostrup
Denmark
USA toll-free: 1-800-903-7153
Phone +45 434-747-25
Telefax +45 434-747-45
e-mail: info@lhsc.leoni.com
www.leoni-high-speed-cables-com
Page 4 of 5
LEONI High Speed Cables
Technical document
May 19, 2004
General industry performance 166 Pico sec. (10 m)
0
10
a /dB
20
30
40
50
60
0
2
4
6
8
10
f /GHz
A suck-out occurs at app. 3 GHz as a result of within pair time delay skew, as well a significant increase in absolute insertion loss
at the fundamental frequency 1,25Ghz
ParaLink ® Performance 30 Pico Sec. (10m)
0
10
a /dB
20
30
40
50
60
0
2
4
6
8
10
f /GHz
ParaLink ® cable performance exhibits no suck-out as well as no additional insertion loss at the fundamental frequency at 1,25
GHz.
ParaLink ® is the best choice for high performance cable for high bandwidth application, ensuring optimal signal integrity and
secures the long term investment. For further information please contact you local sales representative.
LEONI High Speed
Cables GmbH
Naverland 2
DK-2600 Glostrup
Denmark
USA toll-free: 1-800-903-7153
Phone +45 434-747-25
Telefax +45 434-747-45
e-mail: info@lhsc.leoni.com
www.leoni-high-speed-cables-com
Page 5 of 5