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July 1999

The Role Of Layer 3 Switching In Telephony Grade Campus/ LAN Infrastructures


Internet Protocol (IP) telephony provides a way to consolidate traffic in the WAN, resulting in significant savings. More importantly, it opens up the door for new types of clients leveraging Internet, intranet, and LAN connectivity, and enables extensions to existing applications and a new range of IP-enabled applications. Whether Internet telephony is addressed as an evolution of the existing voice communications environment or as an application added on top of an IP network, the IP networking infrastructure has to provide the reliability and quality of service (QoS) expected by telephony users. In the campus environment, Layer 3 routing switches supporting high levels of reliability, multi-gigabit optical networking, and policy-enabled networking are the fundamental infrastructure building blocks required to support Internet telephony.

The key requirements placed on the campus networking infrastructure to support IP telephony applications fall into the following categories:

  • Scalable platforms with competitive price/performance to accommodate voice and data traffic demands, both at the workgroup and campus backbone levels.
  • Optional switch redundancy at the workgroup and campus backbone levels with networking options that provide fast recovery from failures.
  • Differentiated application networking to allow networks to meet application and user QoS and security needs across the campus and into the WAN.
  • Simplified network management that supports the objectives for a high-quality infrastructure and operational practices compatible with business critical operation.

Today’s installed base of campus network infrastructures is seen as a bottleneck in meeting the infrastructure needs of Internet telephony, by not being able to differentiate between voice and data applications (without a significant performance hit), and not being reliable enough without costly equipment duplication. Campus networks vary in size from a couple of workgroups to hundreds of workgroups handled by multiple campus switches. Campus networks today consist of a complex multi-tiered network, e.g.:

  • The access or workgroup tier;
  • The campus distribution tier;
  • The campus core tiers; and
  • The server aggregation tier.

This mixture of shared media LAN hubs and switches, multilayer switches, and/or routers impedes the scalability of the solution. Often the network deployment has been driven by minimizing the price per user, with little consideration for the reliability implications of adding business-critical applications, such as telephony. This complexity and the lack of affordable switch and network redundancy make outages common, resulting in a network that is orders of magnitude less reliable than required for telephony. While bandwidth is relatively inexpensive in the in-building environment, the unpredictable nature of congestion conditions makes the best effort handling of packets unacceptable for high-quality telephony. The networking design principles that work very well for TCP/IP data applications are inadequate for Internet telephony. Finally, in many enterprises, network management systems are optimized for traditional data traffic rather than for multimedia and reliability-critical operations.

The core of the vision for next generation campus networks is a platform, delivering on the infrastructure requirements of Internet telephony and business critical data applications, without compromising high performance even when providing extensive classification and security functionality. This platform is a routing switch that can also be configured as a Layer 2 switch having Layer 3 and up application-awareness. It can be deployed in a considerably simplified and more resilient two-tier campus network topology consisting of the access/workgroup tier and a core switch tier. This vision of next generation campus platforms eliminates the need for a distribution layer between the access and core layers, as well as the server aggregation layer between the core layer and server farms. The overall benefits include lower cost, lower latency, and higher affordable availability. Platform consolidation can be extended to incorporate switch server functionality, integrated IP telephony call server capabilities, and interworking with wireless LAN systems.

This consolidated platform scales to hundreds of Gbps and millions of packets per second, with trunking at speeds of tens of Gbps. The transmission infrastructure is based on twisted pair to the desktop (complemented by wireless technologies), and single and multimode fiber in the riser — on backbone links and to high-capacity servers. Current fiber distributed data interface (FDDI) and 100 Mbps campus and metropolitan area network (MAN) links running over dedicated fiber can be upgraded to gigabit Ethernet over distances as large as 50 km. Gigabit Ethernet running multilink trunking (MLT) effectively increases trunk capacity to N Gbps (N up to 16). Ten-Gbps Ethernet is also going to be available, for example, based on OC192c components from SONET (much the same way gigabit Ethernet ‘borrowed’ technology from fiber channel). The next plateau is running IP on SONET and on Dense Wavelength Division Multiplexing (DWDM) over this fiber.

Switch and Network Resilience
This platform cost effectively supports power, interface, control and switching fabric redundancy, and hot swappability as required of an infrastructure supporting Internet telephony. Resilience is provided at Layer 1 through SONET and DWDM features in the extended campus or MAN. At Layer 2, mechanisms such as MLT provide instantaneous recovery from failures. At Layer 3, resilience is provided through dynamic routing protocols such as OSPF, complemented by Equal Cost MultiPath (ECMP) routing and Virtual Router Redundancy Protocol (VRRP). Multigigabit optical networking not only provides the bandwidth required by enterprises but also, through close integration with switching and routing technologies, is the basis of new levels of resiliency required by enterprise networking. A key capability is the support of redundant MLT at all trunking levels across the campus. Redundant MLT spreads MLT trunks across multiple switch interface cards, enhancing reliability — and very importantly, allowing quick recovery from trunk and card failures compatible with the needs of telephony applications.

Differentiated application networking
A comprehensive set of IP QoS capabilities are provided across the campus, operating under a policy management framework. In an ideal world, all applications would indicate their requirements at Layer 3 and there would be one QoS standard. Unfortunately, there are a number of competing standards. For example, some applications will use the Type of Service (TOS) bits in the Layer 3 IP header as specified in the Differentiated Services (DiffServ) architecture. Others will use Resource reSerVation Protocol (RSVP) Layer 3 signaling as specified in the Integrated Services (IntServ) architecture. Yet others may indicate their requirements in the MAC header using IEEE 802.1p, these being mapped mostly to DiffServ as soon as they leave the local LAN. However, most current applications can’t do this. Application awareness is being built into intelligent workgroup and campus core levels. Technically, this is also referred to as deep packet filtering, whereby the Layer 2 or 3 switch examines the received packet header fields beyond those associated with Layer 2 or 3 respectively, to ascertain which preferential treatment is called for in the network. For example, an application-aware switch can examine TCP port numbers as discussed above, or fields of a Layer 5 Real-Time Protocol (RTP) header to detect the start of an Internet telephony call and ensure appropriate treatment across the network. Adding Layer 3 intelligence and application awareness at the edge of your network — without incurring a price/performance penalty — is very important to achieving the key objective of being able to offer preferential treatment for certain applications (such as IP telephony), and end users who can’t signal their needs to the network at Layer 3.

Unified management
Unified management consists of three key components: Network management, policy management, and service level management. Network management includes performance and fault management capabilities that can significantly enhance network reliability as perceived by telephony users. Configuration management, including integrity checks, can avoid configuration errors that result in loss of logical connectivity that would impact telephony and data users alike. Providing preferential treatment for certain applications and users is a key requirement for IP telephony, and is provided in next generation campus infrastructures, through the addition of switch and network level QoS and security capabilities under policy management.

Policy management provides a structure of network-wide control mechanisms that ensure that the ‘right’ applications and end users have access to network resources. Policy management is an implementation of a set of rules or policies which dictate the access and use of resources on a per user, application, or company basis to meet established business objectives. It is essentially focused on providing end-to-end QoS (bandwidth, latency, priority) and security (authentication, authorization, auditing). Policy-enabled networking ensures that applications such as voice, e-commerce, supply chain management and Web access are given the appropriate treatment. It also ensures that the highest availability (even under failure conditions) is provided to business critical applications, simplifies operations by providing a unified directory environment, and generally lowers the total cost of ownership by making the best use of available bandwidth.

Finally, service level management is a set of client and management capabilities that allow the IT manager to proactively track the performance of the network from the end user application perspective.

IT managers need to develop infrastructures that can support a broad range of applications. With the increased focus on e-business applications (including e-commerce, enterprise resource planning, and e-customer care), all of which are business critical, there is a growing emphasis on the same level of infrastructure reliability that is required for IP telephony. For example, in a comprehensive supply chain management application environment, a customer query with a response time requirement of three seconds may necessitate a large number of back-office network transactions (e.g., to the factory, to accounting, to inventory databases). Due to the cumulative nature of delays, individual transactions may have latency requirements below 100 ms (in the same order of magnitude as voice). Enterprises need solid networking and management infrastructures that cost-effectively support this breadth of needs and allow switch and network redundancy to be deployed where it is needed. Scalable price/performance, optional switch and networking redundancy, application-optimized networking, and simplified management are the key requirements of business-grade networking infrastructures for business-critical applications such as IP telephony and e-business.

In the campus environment, IT managers need to invest in routing switches with scalable performance even when classification and security features are turned on. These switches should incorporate resiliency at both the switch and trunking level. They need to support comprehensive IP QoS and traffic management capabilities under network, policy, and service level management. Next generation campus infrastructures based on routing switches deliver three key benefits for enterprise users:

  • Business-critical availability.
  • Operational simplicity and application optimized performance.
  • Lower total cost of ownership.

Partnering with vendors who are industry leaders in both telephony communications and applications, and in routing switches is a key strategy in meeting the challenge of building a telephony-grade networking infrastructure. c

Tony Rybczynski is director of strategic marketing and technologies for Nortel Networks’ Enterprise Solutions. This business unit offers a full range of enterprise terminal, workgroup, campus, and wide-area unified networks and applications, through direct and indirect channels. For more information, visit the company’s Web site at www.nortelnetworks.com. E-mail questions or comments to the author at [email protected].

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