1. The hSo Guide to
SIP Trunking
The key facts you need to know about SIP
hSo:you can do what you need to do

2. Copyright notice
©Copyright 2008 HighSpeed Office Limited, trading as hSo.
No content from this guide, including trademarks, may be copied without approval.
Neither this guide nor materials from it may be publicly displayed or distributed for any
public or commercial purpose.
Legal Disclaimer
This free guide is provided with the understanding that HighSpeed Office Ltd (aka
‘hSo’) and its employees are not providing professional advice or services to you.
This document is a guide to SIP Trunking. It is not a comprehensive guide to the
subject.
Before taking any action or making any business decisions on the basis of this
document, you should take specialist professional advice from a competent
professional.
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3. Table of contents
SIP Trunking for Beginners .....................................................................................................4
Is SIP Relevant To Your Business?.........................................................................................5
SIP Trunking Benefits..............................................................................................................6
The Risk-Free Hybrid Solution ................................................................................................8
SIP Trunk Reliability ................................................................................................................9
Things to Consider When Picking a SIP Trunk Provider ......................................................10
hSo SIP Trunking Solutions ..................................................................................................12
hSo Contact Details ..............................................................................................................12
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4. SIP Trunking for Beginners
When you make a phone call from your office, your phone talks to a mini telephone
exchange called a PBX. Your PBX then talks to the public telephone network,
connecting to it via a ‘trunk’.
As the name suggests, trunks are far wider than normal phone lines. In fact they
typically have 2 to 30 times the capacity of a standard phone line.
Most trunks in the UK are ISDN circuits. But there’s a new kid on the block called SIP
that’s gaining a lot of attention.
SIP stands for Session Initiation Protocol. It is designed to set up connections between
devices, such as computers, phones, PBXs, video conferencing units and servers.
SIP can be used to support a wide range of services, including video broadcasts,
instant messaging, multi-player gaming and telephone conferences. But the application
we’re interested in is setting up virtual phone lines between your PBX and the public
telephone network. This lets calls travel over your internet connection, and so reduces
or eliminates the need for expensive ISDN circuits.
Many recently manufactured PBXs support SIP. If your PBX doesn’t, don’t worry. You
can still benefit from SIP, by adding a SIP-ISDN Gateway. This is a box that converts
the ISDN signals from your PBX into SIP Trunk signals, and vice versa.
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5. Is SIP Relevant To Your Business?
SIP is relevant to you if:
• You have ISDN30 or ISDN2 circuits
• Your office gets its Internet Connectivity through a leased-line or a dedicated SDSL
circuit
SIP is not relevant if:
• You don’t have any ISDN30 or ISDN2 circuits
• Your office gets its Internet Connectivity through a contended ADSL service or
through Wi-Fi
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6. SIP Trunking Benefits
SIP trunking offers a lot of benefits relative to ISDN:
Only
Only pay for capacity you need
need
Before we begin, let’s define a ‘channel’: a channel allows an outgoing call to be made,
or an incoming call to be taken.
ISDN2 forces you to buy two channels at a time.
ISDN30 forces you to buy a minimum of 8, and a maximum of 30. If you have 30 and
you want 31, you’ll have to pay for 38 channels (30 + 8).
SIP can provide any number of channels, in increments of one. The only limit is your
bandwidth.
Cheaper connectivity
Over the past few years, Internet connectivity prices have fallen through the floor.
Over the same period, ISDN prices have stayed broadly unchanged.
If you get your connectivity by renting ISDN lines, you’re probably paying more than
you need to. SIP Trunking allows you to take advantage of falling Internet connectivity
costs.
costs
No hardware costs
With ISDN, if you need to add another ISDN circuit, you may need an additional ISDN
line card for your PBX. The cost of these, including additional PBX licenses can set you
back about £1500, plus installation charges.
With SIP, adding new trunks to your PBX doesn’t require additional hardware. You
would typically just pay a software license fee to your PBX manufacturer.
ready
Disaster Recovery ready
SIP Trunking can increase your phone system’s resilience, by eliminating its
dependency on individual circuits. Should a particular circuit go down, your calls can
be routed over alternative paths. We’ll explain this in greater detail, later in this report.
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7. Cheaper call rates
rates
A ‘hidden cost’ of ISDN is that ISDN calls are usually more expensive than SIP calls.
Here’s why. When you make a call via ISDN, it is likely that your company uses
something called Carrier Pre-Selection (CPS), to save money. This re-routes your calls,
for the middle leg of their journey, so that they are carried by an alternative ‘carrier’ to
BT. Typically these alternative carriers offer cheaper call rates, in order to entice phone
companies to use them instead of BT. So CPS may save you some money, but not
that much, as the ‘carrier’ has to pay to transport your calls from your local telephone
exchange into its core network (before routing it to the person you are calling).
SIP calls tend to be cheaper as they involve the carrier having to do a lot less work.
Your SIP Trunk provider uses your Internet connection to transport your calls during
the first part of their journey. The calls are handed over inside a data centre, going
directly into the carrier’s core network. The carrier no longer has to worry about
transporting your call from your local Telephone Exchange to their core network;
because they have less work to do, they charge less, and this results in lower call
rates.
Typically, all outgoing calls made via a particular ISDN circuit are routed to a single
carrier, even if that carrier has uncompetitive rates to some call destinations. SIP
Trunking lets your phone company route your calls to a range of different carriers,
allowing it to ‘cherry-pick’ the best call rates to a particular destination, and putting
pressure on preferred carriers to lower particular rates. Some of the savings from this
are passed on to you.
Increased capacity
Increased capacity by pooling Voice/Data connectivity
connectivity
When companies have 60 phones, they don’t rent 60 ISDN channels. Not everyone will
be on the phone at the same time. For most of the time, they could probably get away
with just 6 channels.
But what if more than six people need to make or receive calls simultaneously? Then
there’s a problem. Some people will find that they can’t dial out, because all the
channels are in use.
To avoid these problems, most companies pay for more ISDN channels than they
need. In the above example, they might rent 10 channels, rather than 6. They base their
decision on the maximum number of simultaneous phone calls expected at peak times,
rather than the number typically in use. For most of the day, a vast portion of their
expensive ISDN channels go unused.
The same goes for corporate Internet connections. Most of the time, far less than
100% of the connection is being used, but there will be sporadic bursts to 100%
utilisation.
By pooling the phone connectivity and the Internet connectivity, and prioritising the
phone calls over the data transfers, most companies could increase the simultaneous
number of phone calls that could be made at peak times, while letting the Internet
connection use any spare capacity left by the typical usage of their phones.
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8. Risk-
The Risk-Free Hybrid Solution
If you’re convinced that SIP offers benefits, but would prefer to ‘dip a toe in the water’
before getting rid of your ISDN lines, there’s a hybrid solution that will appeal to you.
Here’s how it works.
Your SIP Trunk provider would put a box between your PBX and your ISDN lines.
All incoming calls to your office would go over ISDN, just as they do now.
All outgoing calls would be re-routed by the box, to go over your Internet connection,
using SIP trunks. In the event that your Internet connection were to go down, the calls
would go out over your ISDN lines, just as they do now.
The result is you get cheaper call rates (because the calls you make go out via SIP
rather than ISDN).
You can reduce the number of ISDN channels you pay for, if you wish, but still have
some left, just in case they’re needed.
And you also get improved resilience:
Traditional ISDN Solution Outbound SIP Solution, with ISDN Backup
Fault Inbound Calls Outbound Calls Inbound Calls Outbound Calls
OK
ISDN Connection Down LOST LOST LOST
(Unaffected)
OK OK OK OK
Primary Internet (Unaffected) (Unaffected) (Unaffected) (Switches to using
Connection Down ISDN or backup
Internet connection)
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9. SIP Trunk Reliability
SIP has many benefits over ISDN, but ISDN is proven, whereas SIP is relatively new.
Is SIP trunking as reliable as ISDN trunking?
With the right set-up, the answer is YES. In fact, with the right set-up, you’ll get
improved reliability.
Traditional ISDN Solution SIP Only Solution SIP With Backup Solution
(Your current solution) (Without backup path) (With ISDN or Internet backup)
Inbound Outbound Inbound Outbound Inbound Outbound
Fault
Calls Calls Calls Calls Calls Calls
ISDN OK OK OK OK
Connection LOST LOST
Down (Unaffected) (Unaffected) (Unaffected) (Unaffected)
OK OK LOST LOST OK OK
Primary
(Unaffected) (Unaffected) (Switches to (Switches to
Internet
using ISDN using ISDN
Connection
or backup or backup
Down
Internet Internet
connection) connection)
The right set-up to ensure reliability involves:
• VOICE PRIORITISATION – the SIP trunks should take priority over the rest of the
traffic, as phone calls aren’t delay-tolerant or loss-tolerant.
• AUTOMATIC ‘FAIL-OVER’ TO BACKUP CONNECTION – there must be at least one
backup-up connection. This can be a backup internet circuit, set to automatically
take over if the main link goes down. Or it can be a left-over ISDN circuit (with
fewer channels than before).
• DIRECT CONNECTIVITY TO YOUR SIP TRUNK PROVIDER – your SIP trunks and
Internet connection should be from the same company, to ensure your phone call
quality is good. Although we talk of your ‘Internet connection’, in reality you only
want your calls to travel over the dedicated, uncongested bit of the Internet that
you control, so that traffic congestion doesn’t ruin your call-quality.
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10. Things to Consider When Picking a SIP Trunk Provider
provider
Direct connectivity to your SIP trunk provider
Voice over IP (VoIP) phone calls are sensitive to packet loss, latency (delay) and jitter
(variations in delay). If you’ve ever had a mobile phone conversation with someone
whose signal is poor, you’ll know the effect these things can have on call quality.
To avoid such problems, it’s important to have a low packet-loss, low latency, low jitter
connection to your SIP trunk provider. And the easiest way to get that is to get a direct
connection to their network.
That way, you avoid call-quality issues.
A direct connection also improves security, as your phone call traffic is only visible to
your SIP trunk provider, and not to a separate ISP and its IP transit providers.
services
Additional services
If you are going to have a direct connection to your SIP trunk provider, you might as
well benefit from economies of scale and use that link for other services, besides
phone calls, such as Internet Access, connectivity to other offices and connectivity to
staff working from home. If your SIP trunk provider doesn’t provide these services, you
can’t aggregate your connectivity bandwidth to take advantage of economies of scale.
Experience
The adoption of SIP is typically quite smooth, but there can sometimes be minor
glitches. So choose a provider that has experience at implementing SIP and helping
customers implement it. Avoid ones that just hand out SIP trunk logon details and
leave you to configure it on your own, with only an FAQ for help.
power
Buying power
The more phone calls your provider buys from its suppliers each month, the cheaper
rates it gets, and the cheaper rates it can pass on to you. Look for a provider that buys
millions of minutes of talk time from its suppliers each month. That way, you’ll get
better value.
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11. independence
Carrier independence
Ensure your SIP trunk provider uses multiple carriers. This ensures that if there’s a
problem with a particular carrier, calls can be routed to an alternative one.
Using multiple carriers enables your SIP trunk provider to put greater pressure on their
carriers to reduce pricing. They can play the different carriers off against each other,
and route the phone calls to Tier-1 carriers offering the best value.
Longevity
The Telecoms industry is brutal. Every year, many telecoms companies come and go,
leaving customers in the lurch. Ensure you pick a SIP trunk provider that has been
around for years, is profitable, and will be around for many years to come.
infrastructure
Resilient infrastructure
Some SIP trunk providers operate out of a single data centre. If anything were to go
wrong in that location, for example a power cut, a fire, or flooding, their service would
be taken down. It’s essential that your provider has carrier connectivity and soft-
switches in multiple geographically dispersed data centres, to ensure you’ll get a
reliable service, whatever happens.
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12. hSo SIP Trunking Solutions
Our SIP Trunking solutions provide:
• Carrier-independence
• Resilient services, thanks to our operation out of multiple data centres
• Attractive call rates, thanks to our leverage with carriers
We’ve been providing a full range of telecoms services for over 8 years, including SIP
Trunks for 5 years.
We provide a full range of telecoms solutions including:
• SIP Trunks
• Standard phone lines
• ISDN circuits
• IP-PBXs and
• Complete phone systems
We can help you get the most out of your connectivity by enabling you to combine
SIP-Trunking with site-to-site connectivity, internet access and VPNs.
To find out more, call us today on 08700 638 739
hSo Contact Details
hSo, Epworth House, 25 City Road, London, EC1Y 1AA
08700 638 739
http://www.hso.uk.com
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