Asynchronous Transfer Mode (ATM) vs VoIP

Asynchronous Transfer Mode (ATM) and Voice over Internet Protocol (VoIP) are two popular technologies used for transmitting real-time data like voice and video over networks.

ATM uses fixed-size data packets called cells and dedicated virtual circuits to send data. It provides a predictable quality of service by reserving network bandwidth.

ATM networks are complex and expensive to build. ATM rose to prominence in the 1990s as high-speed backbone networks were being built by telecom carriers.

VoIP sends voice traffic as IP packets over packet-switched networks like the Internet. It is a cost-effective way to transmit voice calls and integrates voice and data networks. Quality of service is not inherent in IP networks and needs to be engineered.

VoIP has become widespread since the 2000s as high-speed broadband internet has become common.

This article provides a detailed comparison between the two technologies across various parameters like architecture, QoS, bandwidth utilization, costs, etc. It also discusses the pros and cons of ATM and VoIP networks and when each technology is better suited.

1. Architecture

ATM Architecture

ATM uses virtual circuits to send data over networks. A fixed-size cell of 53 bytes is used regardless of the application.

For each call, a virtual circuit needs to be established between the source and destination through the network. This provides a dedicated path for the call’s data.

The ATM cell has a 5-byte header and a 48-byte payload. The header contains identifiers used by switches to route the cell along the virtual circuit. The payload contains the actual user data.

ATM uses out-of-band signaling protocols like Q.2931 to set up and manage virtual circuits. Routing of cells is done using virtual path (VP) and virtual channel (VC) identifiers in the cell header. This allows faster switching in hardware.

ATM uses layered architecture with the ATM adaptation layer (AAL) responsible for segmenting upper-layer data into fixed-size cells. The ATM layer adds the 5-byte cell header and sends cells over the physical medium.

VoIP Architecture

VoIP uses existing IP network infrastructure to transmit voice calls. No dedicated circuits are established.

Voice data is first digitized using codecs and then broken down into data packets. These voice packets are transmitted over the IP network along with other types of data.

The voice codec used impacts call quality and network bandwidth requirements. Common codecs include G.711, G.729, and Opus which have varying levels of compression and voice quality.

For IP transport, the Real-time Transport Protocol (RTP) is used on top of UDP for the transmission of voice packets.

SIP or H.323 signaling protocols are used for call establishment and management. QoS mechanisms like traffic shaping, compression, and prioritization are used to improve call quality.

Various standards bodies like the ITU and IETF have defined protocols at different layers of the VoIP architecture. This has enabled interoperability between equipment from different vendors.

2. Quality of Service


ATM provides inherent Quality of Service guarantees as part of the network architecture. By reserving dedicated bandwidth in the form of virtual circuits, ATMs can provide fixed latency and jitter levels required by real-time traffic like voice and video.

ATM uses traffic contracts between the user and network while establishing each VC.

Parameters like peak cell rate, sustained cell rate, burst tolerance, etc are specified depending on application requirements. The network guarantees QoS as per these contracts.

Complex buffering and traffic shaping techniques like leaky buckets and early packet discard are used in ATM switches to manage congestion and maintain QoS.

Traffic can be prioritized by using separate VCs or by adjusting VC parameters.

Overall, ATM provides robust, predictable QoS but at the cost of increased network complexity. The dedicated circuits are inefficient for burst data traffic.


IP networks are inherently best-effort with no inherent QoS capabilities.

VoIP traffic competes with other data on the network leading to issues like latency, jitter, and packet loss. This degrades call quality.

Various QoS mechanisms have been developed to improve VoIP performance over IP networks:

  • Traffic shaping using queues to avoid congestion and bound delays
  • Packet prioritization using DiffServ or MPLS to tag voice packets
  • Call admission control limits the number of calls to prevent oversubscription
  • Jitter buffers at the receiver to minimize delay variation
  • Echo cancellation deals with voice echo issues

Additional link bandwidth provisioning also helps improve VoIP QoS. Despite these mechanisms, QoS is still a challenge in VoIP.

The public internet does not provide any QoS guarantees due to its decentralized nature.

3. Network Bandwidth Utilization

ATM provides dedicated circuits so bandwidth utilization is highly efficient. The entire VC bandwidth is available only to that call’s traffic.

VoIP shares bandwidth with other IP traffic so utilization is less efficient. Voice packets compete with data packets for buffer space in routers.

VOIP codecs can use silence suppression and compression to improve bandwidth efficiency.

4. Hardware Complexity

The fixed cell format and virtual circuit architecture make ATM networks complex to design and build.

High-speed ATM switches need to be used to route cells using hardware lookups. Buffering and traffic policing capabilities add more hardware complexity.

VoIP does not need specialized hardware since it runs over standard IP networks. Existing routers and switches used for data networks can be used.

QoS features do add some hardware complexity. Overall VoIP hardware is simpler and cheaper than ATM.

5. Network Scalability

The signaling protocols used in ATMs to establish VCs limit their scalability for large networks.

Each switch maintains the complete path which adds state information. Routing complexities also increase.

VoIP scales well since it uses distributed, hierarchical IP routing protocols like BGP which can handle large networks.

The stateless nature of IP makes it very scalable. VoIP networks can potentially scale to a global size.

6. Resilience and Reliability

ATM’s virtual circuits provide predictable reliability during the call.

But loss of any link breaks the circuit resulting in call failure which requires re-establishing the path. Resilience mechanisms like protected switching exist but add cost and complexity.

VoIP’s use of IP networks makes it robust to link failures due to the inherent redundancy of mesh-type networks.

Dynamic routing protocols like OSPF allow finding new paths in case of failures. So VoIP can potentially recover faster from network issues.

7. Cost Comparison

ATM requires significant capital expenditure to build dedicated VC networks with specialized hardware like ATM switches.

It is optimized for voice and video traffic. Data networks require separate equipment.

VoIP rides on top of existing IP networks built for data and can utilize the same infrastructure for voice.

No major new investment is needed and the incremental cost per voice call is very low. This makes VoIP networks very cost-effective to deploy.

8. Use Cases

ATM rose to prominence in the 90s when phone companies needed high-speed backbones to carry growing voice and video traffic.

The dedicated circuits allowed guaranteed QoS and efficient transport. ATM dominated for a decade before being superseded by IP.

VoIP is ideal for any application where cost savings and integration with data services are important.

VoIP is ubiquitous in enterprise networks where voice can leverage the installed LAN/WAN infrastructure. VoIP-based cloud telephony platforms are also gaining traction nowadays.

Despite higher costs, ATM networks are still useful where very high reliability and low jitter are critical. For example, ATM is still used in some military networks or 911 infrastructures where QoS is paramount. But most telecom networks have shifted to IP.


ATM and VoIP represent two different network architectures suited for real-time traffic like voice and video calls.

ATM provides robust QoS but requires expensive custom hardware. VoIP leverages standard IP networks and is more cost-effective. But QoS is a concern over the public internet.

ATM made sense in the 90s when phone companies were building dedicated voice backbone networks.

However, with the growth of data services and the public internet, VoIP has emerged as the preferred technology for integrated voice-data networks. Improvements in VoIP quality and widespread broadband availability have fueled its adoption.

While VoIP is ubiquitous today, ATM still retains some niche use cases where guaranteed quality is a requirement regardless of cost.

Most telecom networks have evolved to all-IP but retain some legacy ATM links at the core. For greenfield deployments, VoIP has become the de facto standard for voice networking needs.

Frequently Asked Questions

Ques 1. What are the main differences between ATM and VoIP architectures?

Ans. ATM uses dedicated virtual circuits with fixed bandwidth allocation while VoIP sends voice packets over shared IP networks.

ATM provides inherent QoS guarantees while VoIP needs additional mechanisms for QoS.

Ques 2. Why is QoS better with ATM compared to VoIP?

Ans. ATM’s virtual circuits reserve dedicated bandwidth end-to-end for each call ensuring low latency and jitter.

VoIP competes with other traffic on the IP network leading to variable QoS.

Ques 3. What makes VoIP more cost-effective than ATM?

Ans. VoIP leverages existing IP network infrastructure while ATM requires building separate networks with expensive hardware like ATM switches.

VoIP has a very low incremental cost per call.

Ques 4. When is ATM still used compared to the more popular VoIP?

Ans. ATM remains useful for niche applications where guaranteed QoS is critical regardless of cost, like military networks or 911 infrastructure. Most telecom networks have migrated to VoIP.

Ques 5. How does network scalability compare between ATM and VoIP?

Ans. VoIP scales very well using IP routing protocols like BGP. ATM signaling protocols limit scalability as each switch maintains a complete path state. VoIP networks can potentially scale to a global size.

Evelyn Brown
Evelyn Brown

Evelyn Brown is a knowledgeable and dedicated reviewer of business communication softwares. When she's not testing the latest platforms or providing in-depth analyses for his readers, you can find her playing guitar and hiking local trails.