VoIP and E911 1
Running head: VoIP and E911
Modern problems: VoIP and E911
Cory M Langston
University of Central Florida
VoIP and E911 2
This paper discusses some of the issues facing VoIP as it continues to mature. After a brief
introduction to the technology, it focuses on Quality of Service and compliance with the FCC’s
VoIP and E911 3
Modern problems: VoIP and E911
For years, technologists have spoken about digital convergence. The goal, too integrate
voice service with our existing packet switched data networks. The ability to use one network to
carry voice, data, and video lowers infrastructure costs and give us more benefits than
maintaining two separate networks for voice and data. Voice over the Internet Protocol (VoIP) is
one of the technologies making this goal a reality.
Essentially, VoIP lets us digitize a voice stream and then we chop that data into little
slices called packets and then we send them over the internet (Hardy & NetLibrary Inc., 2003).
Since the dawn of the broadband era, internet service providers have increased the bandwidth
available to consumers so even residential customers have the spare bandwidth to consider
migrating their voice service from the public switched telephone network (PSTN) to VoIP.
Service providers like Vonage are already boasting more than 2 million subscribers and they off
a slew of features like free long distance, caller ID, and much more.
However, VoIP service providers still have several issues to overcome before they can
match the PSTN. One issue is maintaining an acceptable level of Quality of Service, availability,
and reliability (Khasnabish & NetLibrary Inc., 2003). Another issue is integrating with the
Emergency Public Safety Answering Point (PSAP) so users can call 911 directly and trust that
their location will automatically be conveyed to the operator.
Quality of Service
A major issue facing VoIP is Quality of Service (QoS). To improve the quality of service
of VoIP sessions the IETF has defined Differentiated Services (a.k.a. DiffServ) which helps
provide priority based handling for packets that need real time service. DiffServ helps
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administrators prioritize voice packets on their network (Nichols, 1998). Prioritization is
necessary to help prevent packet loss and excess delay.
Packet switch networks were originally designed for transmitting data. Data
communications are not as disrupted by packet loss as voice communication. If someone sends
an email and the network gets congested then routers will start dropping the extra packets they
can’t process. If one or two packets of your email gets dropped, the packet is simply resent after
a watchdog timer expires on the transmitter. However, VoIP expects all of its packets to arrive
within a certain period.
If a VoIP call misses a few packets, the packets are not resent and the recipient does not
get the whole message. This can lead to a choppy phone conversation as the speakers voice cuts
in and out. Unfortunately, packet loss usually tends to occur in bursts during brief periods of
congestion at network peering points, which can completely ruin a conversation.
A priority based handling lets routers differentiate between packets that need real-time
priority. Differentiated service lets applications use the type of service (TOS) field in the IP
header to indicate when a packet needs increased priority. If routers are configured to use this
service, they will give them preferential treatment when they get congested they will drop excess
packets that do not need real-time service first (Nichols, 1998). One of the issues with
differentiated service is it must be enabled by each network to maintain a quality connection
between networks. Your ISP may use it for their network, however, there is no guarantee the
intermediary networks handling your call support differentiated service.
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Latency is another issue that must be overcome by VoIP services. Since packet
switching networks dynamically route information VoIP packets can traverse any path. The
service can not depend upon a virtual circuit with a fixed amount of latency. Instead, it has to be
prepared to handle a stream of packets with a variable amount of latency. The difference in the
amount of latency experienced is called jitter. When latency exceeds 150 milliseconds in one
direction the natural flow of conversation is strained[…] (Phil & Cary, 2004).
To compensate VoIP applications utilize a jitter buffer, which essentially introduces a
fixed amount of delay prior to playback in order to give packets enough time to arrive. To help
reduce latency VoIP uses the connectionless user datagram protocol (UDP) to send information.
The real-time protocol (RTP), which rides on top of UDP, is used to handle packet sequence
information and timestamps so endpoints can manage jitter. RTP helps the service dynamically
adjust the jitter buffer based on changing network conditions (Phil & Cary, 2004).
One of the biggest challenges facing VoIP is availability. VoIP service may be lost in the
event of a power outage (FCC, 2006). In this regard, VoIP service is not as reliable as the PSTN.
The PSTN provides central power. In the event of a power outage, the PSTN is still able to
provide dial tone. Local exchange carriers have invested in generators to power the central
office and battery backups in their SLECs to provide power in the event of a power failure so
your service will always be available.
Major VoIP service providers use the Internet to route calls. The Internet is a distributed
network that depends on edge equipment and access units to provide their own power. In order
for your VoIP service to work properly, your broadband modem must have power as well as
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your network service provider. Currently, some modem manufacturers provide a built-in lithium
ion battery to provide power in the event of a power failure (WikiPedia, 2006). Businesses that
implement a VoIP infrastructure with an IP enabled PBX have the option of providing central
power to their VoIP phones by using Power over Ethernet (PoE). Power over Ethernet lets a
switch use one of the unused pairs in a CAT5 cable to provide DC to power a phone.
Central power is a critical component for availability. A system is only as robust as its
weakest component. Since power cables are not always buried underground, the power grid is
vulnerable to failure. Consider what happens to the power grid in the event of some common
Florida emergencies. A car crashes into a power pole outside of your business resulting in
injuries and a local power outage. Central power lets you call for help. A squirrel running
across a power line shorts a circuit, is electrocuted, and trips the inline breakers on the power
pole. Central power lets you call the power company for help. A cell phone could also work in
Due to availability issues, companies like Vonage recommend keeping a landline if you
have an alarm system that needs to be able to dial out in an emergency. VoIP providers offer a
network availability feature to address incoming calls during a disruption. When enabled,
network availability lets a person designate a phone number to forward their calls to during
The FCC’s E911 mandate
In June 2005, the FCC mandated that interconnected VoIP services provide access to
Enhanced 911 (E911) for every customer. E911 is a mechanism that automatically transmits
your location and phone number to an emergency operator when you dial 911. The 911 calls
from VoIP system may work differently than calls from a landline.
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Currently, local exchange carriers provide access to the local public safety answering
point (PSAP). When you dial 911 from a landline, your phone number is transmitted to the local
PSAP operator via an automatic number identification (ANI) message. The incumbent local
exchange carrier maintains an automatic location identification (ALI) database that matches each
phone number with a location. The operator uses the phone number lookup the location of the
caller in the ALI database (WikiPedia, 2006).
Under the FCC mandate, interconnected VoIP services must; deliver all 911 calls to the
local emergency call center; deliver the customer’s call back number and location information
where the emergency call center is capable of receiving it; and inform their customers of the
capabilities and limitations of their VoIP 911 service (FCC, 2006).
Today, when you dial 911 from a VoIP connection, it may not go directly to the PSAP.
Instead you may be routed to the VoIP’s emergency call center so they can verify your location
information and route you to the appropriate local PSAP (FCC, 2006). Alternatively you may be
routed to the wrong PSAP or your call may not transmit your location or call back number
correctly. Implementing these features is a challenge that VoIP providers are trying to address.
AT&T has come up with a solution to address their “nomadic” users. Nomadic users are
those who are mobile and travel from one area to the next. When a nomadic user powers down
their phone, CallVantage will disconnect their service. The next time they power up they will be
asked if their location has changed. If it has not then service is restored. If it has changed then
they’re directed to update their location information by calling an 800 number or visiting a web
page (Cauley, 2005). The drawback of this method of maintaining location information is that it
completely depends upon the user to enter address updates accurately and on demand whenever
they change. An automatic solution would make it much more reliable and user friendly.
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VoIP may promise to offer more features with less cost, but it has several issues to solve
as it matures. Although VoIP can provide free long distance for those wishing to place
international calls, it is struggling to deal with new regulations that require service providers to
integrate with local public safety access points. Will VoIP continue to be affordable as the
government imposes new regulations? The Quality of service (QoS) of VoIP calls is threatened
by jitter and congestion. Peer networks that carry VoIP without properly implementing
differentiated service also threaten QoS. The distributed nature of the Internet makes it difficult
to control quality. The lack of central power means service is lost during a power outage. VoIP
has a long way to go, if it is going to offer the same reliability and quality as circuit switched