Year: 2021

Legacy Transmission & Line Codes

Before Fibre, there was copper!       

It’s almost difficult to believe that not so very long ago (ok, going back maybe more than 50 years) there were no optical fibre or digital transmission paths of any flavour of technology providing our communications infrastructure.

Analogue FDM

From early to mid 20th Century, an extensive core copper cable network was rolled out, based on analogue FDM (frequency Division Multiplexing) over coaxial pairs, with the valve-based technologies occupying a lot of space and consuming much power. 

Digital PCM

The late 1960s saw the introduction of digital PCM (Pulse Code Modulation) sampling at 8kHz. The ITU-T (International Telecommunication Union – then known as CCITT) standardised 30-channels at 64kbit/s in a 2.048Mbit/s multiplexing system, using 8-bit A-law algorithm (the USA adopted 24-channel 1.544Mbit/s with μ-law algorithm).

Problems with high bit-rates

The higher bit rates gave rise to crosstalk interference problems on many existing cables. Also, data signals transmitted as voltage levels in unipolar NRZ (Non-Return to Zero) format are not self clocking and have a significant DC component, wasting power. Bipolar RZ (Return-to-Zero) type AMI (Alternate Mark Inversion) coding prevents the build up of the DC-component for longer distance and addresses the issue of data containing multiple ones. However, long sequences of zeros still present problems with a lack of transitions causing difficulties maintaining synchronisation.

Introduction of Line Codes

Line Coding of the format mB-nB was introduced to overcome these issues. Initially 4B3T (four Binary, three Ternary) was used. This encodes each 4-bit input group into a 3 symbol output using the three states of positive, negative and no pulses.

e.g. ‘0000’ is coded as ‘+0-‘

This improved efficiency in terms of bit per symbol over AMI, which itself is an example of a 1B1T code. Improvements in transverse screened cables were also made. However, transmission problems with high-speed digital data were still encountered due to unsuitable copper cabling which needed to be addressed.

PDH Higher Order Multiplexing

By the late 1970s, the UK had adopted the ITU-T recommended PDH (Plesiochronous Digital Hierarchy) of E-carrier higher-order multiplexing at 8Mbit/s, 34Mbit/s (in the US, T-carrier at 6Mbit/s, 45Mbit/s) and 140Mbit/s. 

The lower rates of the E-carrier system adopted HDB3 coding, which replaces 4 ‘0’s with ‘000V’ or ‘B00V’ (or in the US for T1, B8ZS coding which replaces 8 ‘0’s with ‘000VB0VB’).

CMI (Coded Mark Inversion) was included in the ITU-T standards for higher-order PDH at 140Mbit/s PCM (as well as SDH at 155Mbit/s electrical STM-1). This is a 1B2B type of NRZ coding where a ‘0’ is represented by ’01’ and a ‘1’ as an alternatively ’00’ and ’11’, with +V and -V representing the coding levels.  

The advantage of the coding is it makes clock recovery by the receiver simple and for maintaining synchronisation alignment with a long sequence of ‘0’s or ‘1’s.     

Line Coding examples

Optical fibre systems

From the beginning of the 1980s, early optical-fibre multi-mode systems operating at 850nm were deployed, and later single mode at 1300nm, using the PDH multiplexing capacities. 

Typical of long-haul PDH optical-fibre systems, the 2 Mbit/s, 8 Mbit/s and 34 Mbit/s ‘Dynanet’ products from Nokia have ITU-T G.703 compliant digital interfaces using the HDB3 code, but using an optical transmission Line Code of 5B6B. This is another type of mB-nB code, where in this case 5 bit data words are coded using 6-digit code words

e.g. ‘00000’ being represented as ‘100111’. 

As well as its use on electrical systems, CMI Line Coding has also been popular for use on short-haul optical-fibre transmission such as ’tactical’ fibre optical systems operating at 2 Mbit/s.

SDH / SONET – A different approach

For optical SDH systems, STM-1 and above, scrambling is employed instead of line codes to ensure the incoming bit stream contains sufficient transitions for maintaining synchronisation. This works by combining the data signal with a pseudo-random bit sequence generated by a scrambler polynomial generator.

i.e. with a sequence of length of 127, the generating polynomial is 1+x6+x7 , leading to input data ‘00000000001111111111’ being scrambled as ‘11111110000001000001’.

Optical PDH still serving

In most cases higher-order optical PDH has been decommissioned, but optical transmission at 2Mbit/s is still in operation for many low-data rate applications, where costly replacement with SDH, WDM or carrier Ethernet would bring no advantage. An example product is the Nokia DF2-8 which continues to offer reliable access services, particularly in the Utilities and Transportation industries.

DF2-8 – TA 21518

Copper systems still in operation 

Though core copper electrical transmission systems have now been discontinued, much of ‘last mile’ telephony and related broadband connections are still copper access. For extended data transmission applications, copper systems are still deployed and maintained. Such products include the Nokia DSL2i copper line equipment (including power feeding repeaters) using SHDSL (Single-pair High-speed Digital Subscriber Line). This uses TC-PAM (Trellis-Coded Pulse-Amplitude Modulation) which is a 4B1H Line Coding, since translates 4 binary digits into 1 Hexadecimal (16) levels. It improves range, especially when used with regenerative repeaters, and improved ADSL (Asymmetric Digital Subscriber Line) compatibility. 

ACL2i PF GEN – T65580

Feedback and assistance

This has been a necessarily very brief run-through of legacy transmission and some of the Line Codes employed. @YellowsBestLtd would be keen to hear your experiences and knowledge of transmission systems and performance of Line Codes. If we can be of any assistance with your solution requirements, including both new and legacy technologies, then please get in touch. 

Lessons from the ‘Lockdowns’ for Business

Seeking the ‘new normal’

Most workplaces have seen some considerable disruption over the last year due to the restrictions necessary to deal with the global Covid-19 virus pandemic. Hopefully things are going to get easier over the coming months. But before we race to ‘get back to normal’ (if that’s indeed possible), let’s consider some unexpected benefits we might want to hang onto.

Work is what you do, not where you do it

Commuting has always been a drag. The time wasted driving, not to mention the cost, in order to reach an office in which documents are written, emails are read and replied to, and phones calls are made. Or instead, various ‘productivity’ applications are used. All of which could be done from home. What is needed is a ‘mind-shift’ to recognise that “I’m off to work” can mean engaging in an activity rather than physically travelling somewhere. 

What’s the point of an office?

The broad acceptance that an office is where ‘work happens’ is due to the familiarly of their existence over a number of years. Once upon a time there were good reasons why work had to be so: people needed the facilities they provided, including main-frame computers, desk telephones, fax machines, printers, typing-pools (yes, really – people once didn’t type their own documents!) And memos – remember those ‘internal mail’ envelopes? But now, with laptops and mobile phones and broadband internet, it’s no longer critical to all share the same space.

People ‘like’ keeping in touch

The reality of the office is that it’s no longer a critically functional resource hub, but there are some social benefits over working remotely. It’s a place to meet and greet, share ideas and stories, help each other and generally contribute to high morale. People enjoy discussing last night’s TV or the football. Lasting bonds and relationships are formed, sometimes even being introduced to future partners. Not sure all employers would see this to be their ‘role’; the social side can of course be achieved in other ways. Anyway, flexible remote working offers the opportunity for better work-life balance.

Meanwhile, bosses like collecting their workers in one place as then it’s easier to ‘manage by walking about’. There’s a trust element: how can the staff be really hard at work if they’re not visible, aka ‘chained to the desk’. But following McGregors’s ‘Theory X (authoritarian) and Theory Y’ (participative) style of management, you either micromanage them because they’re not motivated, or trust people to take pride in their work and get the job done. So forcing people into an office isn’t the answer to productivity. Rather, pick the right people, train and support them, give them ownership of their tasks. Let them work where and when they need to. Use performance reviews as a tool (not a chore) to keep on track and set rewarding goals.   

Quantity or Quality

The crazy thing about the 9-5 office culture is people vary between not having enough time to get a job done, and piloting a desk ‘looking busy’, because they’re supposed to be ‘in’. Flexible working on the other hand recognises that people have lives with things that need scheduling from time to time, around varying business demands and commitments. Allowing people the discretion to manage their work-life balance means better motivated and focussed staff who will put the extra effort in when needed. Or else, managers need to take strong decisions on appropriate resources and team composition. Working ‘smarter not harder’ certainly doesn’t mean forcing everyone into an office and making them work all hours.

Meetings expand to fill the time available

It seems like ‘work’ to spend hours in meetings showing each other an endless supply of presentation slides. Discussions often arise involving only a few participants while others wait passively. The reality is very little is accomplished that couldn’t have been better reviewed remotely, in one-to-one conversations or communicated more broadly via team or company-wide bulletins.

Keep your germs to yourself!

Due to the emphasis on ‘attendance’ (perhaps ingrained in people from their school years), there’s often a culture of ‘bravely struggling in’ when ill with a cold, thus almost guaranteeing the sharing amongst all colleagues. Above all else, the pandemic has shown the sense in keeping people separated to reduce the spread of illness. 

Better for you, better for the environment

Not everyone can work from home, and certain tasks can’t be done remotely. But it’s time for a re-evaluation of what journeys are ‘necessary’ and what are the most productive work patterns, both in terms of getting the job done (without sitting in traffic jams for hours) and maintaining a flexible, motivated workforce. Not least because of the unsustainable effect on our planet’s finite resources and impact of climate change due to limitless business activities and excessive travel.

Are you ready for the ‘paradigm shift’?

@YellowsBestLtd we’d be interested to hear your thoughts and feelings about the changes brought about by Covid-19, and how you see habits changing for the future. Will you be rushing back to the office, or reaping new flexibility from remote working? Please get in touch, and let us know how we can help with your continuing business requirements. We look forward to hearing from you.

International Trading (Post Brexit)

New rules for Business

It’s been an uncertain period leading up to the United Kingdom (UK) leaving the European Union (EU). Now that that ‘Brexit’ has happened, as of 1st January 2021, businesses are having to work out the practical changes having an impact on international trading.

The reality of the situation is that it will take time to fully understand the new rules and what effect they have on imports and exports. Due to this uncertainty, some companies are choosing to curtail international activities, which is having an effect on availability of supply.

So this is a subject that will need to be revisited as experience is gained. What seems likely is that there will be more administration and higher costs.

@YellowsBestLtd has a 5-year history of trading not just within the EU, but globally, and it’s this experience which is helpful in understanding the adjustments needed. Put simply, all import and export business within and beyond the EU is now ‘similar’ in principle in terms of customs and tax, with or without government ‘trade deals’ being agreed.

VAT charging and reclaim in the UK

It’s helpful to first understand how VAT works on a national level. In the UK, it’s charged on the sale of many goods where ‘value has been added’ at a rate of 20% of the selling price. To consumers, that’s just a tax they pay within the total purchase price. For businesses, that element of the sale is ‘collected’ on behalf of the government, totaled and paid quarterly via their tax return. However, VAT-registered businesses are able to ‘claim back’ the taxed paid on their own purchases. The logic being, that when you’re ‘trading’ i.e. buying and selling, the amount of tax due on goods is that relating to the ‘value add’ – so the difference between the tax charged on the cost price and that charged on the selling price. Which of course should be more, if you’re making any kind of profit.     

How VAT previously applied for UK businesses trading within the EU

To avoid the complications of how to ‘settle’ the collection and claiming back of taxes on purchases made between EU member states, an arrangement is in place such that each country is responsible for  their own VAT affairs.  Here’s how charging for VAT works for businesses when trading within the EU:

  1. The goods are zero-rated for VAT, from suppliers that are VAT-registered. This means that the export invoice, stating “intra-community supply”, charges VAT at 0%.
  2. The VAT-registered buyer (importing company) then declares the ‘acquisition’ on their VAT return (in “Box 2”)
  3. The buyer simultaneously also ‘reclaims’ the VAT (included in “Box 4”) on the same VAT return
  4. The logic behind these transactions is that the purchaser acts as both the “seller and the buyer”, for VAT purposes, Hence the transaction is accounted for entirely within one member state, and no funds need to be transferred to or from the tax authorities.  

When these arrangements applied to UK businesses selling goods to buyers in EU member states, a ‘VAT EC Sales List” also needed to be completed and sent to the HMRC.      

VAT and trading globally

Now the UK is ‘outside’ the EU, the ‘intra-community’ arrangement for VAT no longer applies from 1st January 2021. Instead, all international trading follows the same process, for all countries both within and beyond the EU.

UK businesses making global purchases receive an invoice from their international supplier which has no VAT added. But the UK government will separately charge VAT at 20%; this amount needs sending directly to HMRC. Typically, this amount will be collected by the courier physically transporting the goods, and normally an administration fee will be additionally charged for this transaction. If an agreement hasn’t been made with the seller, it is often the case the buyers find they need to pay these additional costs before the imported goods will be released from customs and delivered. Reclaiming of import VAT will then be made via the VAT return as an input tax, following the normal rules for UK-paid VAT. 

Alternatively, import VAT can be accounted for using “postponed VAT accounting” on the VAT-return, which essentially is a similar scheme to the EU “intra-community” arrangement. This requires ensuring that the courier is advised how import VAT will be accounted, so they can complete the customs declaration appropriately.

Similarly, UK businesses selling goods internationally don’t add VAT to their invoices, but are likely to have import taxes added by the authorities in the receiving country. If the buyer has not agreed to include these additional costs within their purchase order, this can mean that the seller needs to settle these charges as part of their cost of supply, typically being charged by the courier used to deliver the goods. The difficulty is knowing in advance what is likely to be charged.

Customs Duty and Commodity Codes

In addition to VAT, businesses trading internationally need to take into account Customs Duty (and for certain products, Excise Duty as well) that may be charged on imports.

For UK businesses, this means checking with the HMRC and specifying the correct ‘Commodity Code’ for the goods, which will determine how much is charged. Unfortunately, there are thousands of such codes, covering all manner of products, so identifying the right code can take some searching.

@YellowsBestLtd typically imports a range of telecommunications spare parts, and can therefore advise that for similar requirements the following is applicable: Commodity Code: 8517620000

Commodity Code: 8517620000

According to the HMRC website, sales of these goods should have no Duty to pay on imports. It’s less clear what may need to be paid on exports since this will vary by country. The ongoing concern is trying to anticipate these costs in advance, but by making an excessive allowance for them can mean an uncompetitive offer, causing a loss of business.    

Your Experiences and Questions

We’d like to hear about your concerns or practical experiences of international trading, both before and after Brexit, within and beyond the EU. Please get in touch, and let us know how we can help with your continuing business requirements. We look forward to hearing from you.

New Year, New Stock! ‘Dynanet’ Spares

Nokia ‘Dynanet’ PDH Transmission products

For many years, the ‘Dynanet’ family of PDH Transmission telecoms products have well served Public Operator and Private Network Customers across the Telecoms, Utilities, Transport and Public Safety markets with high availability mission critical infrastructure, and indeed some networks are continuing to provide good operational service.

They were first introduced by Nokia over 20 years ago, and were continued in recent years by DNWP. Production of the majority of the product range was ceased in 2019. 

Spare parts for continued operational service

@YellowsBestLtd satisfies world-wide customer product sourcing requirements for current and ‘legacy’ equipment technologies from a wide range of Original Equipment Manufacturers (O.E.M.s).

For the ‘Dynanet’ range, we have recently obtained of a number of additional  refurbished and surplus equipment items. Hence, for those customers continuing to maintain their networks, there now exists the opportunity to increase stocks of spare parts to take advantage of the current availability.

Stocklist of items for immediate supply

Here is a list of the main items currently in stock, though there may be a few additional parts that can be supplied.  Hence, please check and if you do have any requirements, please let us know. We look forward to hearing from you.

Part NumberDescription
CC 24002DB2 2×2 Mb/s Branching Unit (B2), 75ohm
B2
DB2 B2 – CC 24002
Part NumberDescription
CC 24011DB2 2 Mb/s Switching Unit (X2), 75ohm
X2
DB2 X2 – CC 24011
Part NumberDescription
CC 24101DN2 2×2 Mb/s Interface Unit (IU2), 75ohm
IU2
DN2 IU2 – CC 24101
Part NumberDescription
CC 24111DN2 Control Unit (CU), 75 ohm
CG 24170DN2 Bus Power Unit (BPU)
CG 24171DN2 Extended Bus Power Unit (EBPU)
CU 24013Data Interface Unit (DIU) 2M, nx64k: G.703/704, 75ohm
nx64k
DIU nx64k – CU 24013
Part NumberDescription
D-21470Euro Connector, 3×7
3x7
Connector 3×7 – D-21470
Part NumberDescription
D-24204Optical Teleprotection Interface Unit, C37.94
C37.94
C37.94 – D-21470
Part NumberDescription
T31094.01DCN Adapter C4.0
DCNA
DCN Adapter C4.0 – T31094.01
Part NumberDescription
T37870.01NDM 19in 17-slot Subrack
Subrack
19in Subrack – T37870.01
Part NumberDescription
T37871.01NDM DN2 19in 17-Slot Subrack
T37882.02NDM DC Unit (NDUe)
NDUe
NDUe – T37882.02
Part NumberDescription
T37885.01NDM Ring Generator
Ring Generator – T37885.01
Part NumberDescription
T37885.02NDM Ring Generator + DC/DC converter
Ring Generator – T37885.02
Part NumberDescription
T37889.01NDM Backup Unit (NBU)
NDM Backup Unit – T37889.01
Part NumberDescription
T65580.01ACL2i PF GEN Line Terminal Card
ACL2i PF GEN – T65580
Part NumberDescription
TA 21513Optical Line Terminal Repeater (DF2-8), 1300 nm LED MM/SM
TA 21516Optical Line Terminal Repeater (DF2-8), 1300 nm LASER SM
TA 21518Optical Line Terminal Repeater (DF2-8), 1300 nm LASER LP
DF2-8 – TA 21518
Part NumberDescription
TC 21101DM2 Multiplexing Unit, 75ohm
DM2
DM2 – TC 21101
Part NumberDescription
TC 21301DM8 Multiplex Equipment, 75ohm
TC 21705Supervisory Substation
TC 21710.01TMS Adapter
TU 21122.5Data Interface Unit (DIU) 48..64k, V.11, 10ch
TU 21124Data Interface Unit (DIU) nx64k, V.11/V.35/X.21, 2ch
TU 21124.05Data Interface Unit (DIU) nx64k, V.11/V.35/X.21, sync
TU 21125Data Interface Unit (DIU) 48..64k with sync, X.21
DIU X.21 – TU 21125
Part NumberDescription
TU 21205Channel Unit SUB/SUB
TU 21206Channel Unit SUB/SUB
Channel Unit Subscriber / Subscriber – TU 21206
Part NumberDescription
TU 21215Channel Unit SUB/EXCH
TU 21216Channel Unit SUB/EXCH
Channel Unit Subscriber / Exchange – TU 21216
Part NumberDescription
TU 21234.20Channel Unit E&M/VF-P, 10 ch, 20 E&M ch
TU 21236.10Channel Unit 8ch E&M/uP: 1xUKe&m/VF
VF E&M
VF E&M – TU 21236.10
Part NumberDescription
TU 21255.01Channel Switch, 4 port