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What�s Behind the Ethernet Jack � Data 101

By Tony Rybczynski


�Can you give me a high-level description of how a data network works?� This question (and it�s not all that uncommon in this ever evolving industry) came to me during a side discussion with an end user at a recent Internet Telephony Expo. Let�s start with how you connect to the network, and then look at what happens in the network once you are connected.

Getting connected
The connection between you and the network is a high speed on-ramp based on one technology family, called Ethernet. Ethernet is referred to as a Layer 2 technology, while IP routing is Layer 3 (Layer 1 is the physical wired or wireless connection).

Plugging your PC into any Ethernet jack (slightly bigger than a phone jack) at your desk or in a conference room provides a dedicated link to the wiring closet, an aggregation point on your floor typically handling 100 users. This is most likely over copper with a reach of 100 meters. The network automatically connects you at 10 or 100 Mbps and, more recently, at 1 Gbps, the actual speed depending on hardware used and transmission characteristics. It�s all plug and play. Traditionally, you would have had a separate telephone and data jack at each desk. If you have an IP phone, plug the PC into the phone and the phone into the Ethernet jack and you�re in business. DC power over Ethernet avoids the need for an AC plug on each phone � but is not enough to power your laptop.

You can also plug in wirelessly, but you�ll need wireless Ethernet (WLAN) capability on your laptop (e.g., a wireless LAN adaptor card or a built-in capability) and, of course, you�ll need to be near a WLAN Access Point (AP). An AP covers roughly a 50 meter radius in an office environment � the range is dependent upon walls and other obstacles as well as materials. The throughput you achieve also is dependent on a number of factors (e.g., technology used supporting 11 or 54Mbps peak, number of users, distance to the AP). The AP, which can look like a smoke detector with internal antenna, is Ethernet connected and powered back to the wiring closet. In a large site, you might have hundreds of APs, particularly if they are needed to provide coverage for roaming voice users.

You can also get connected remotely. You might be working from home or a hotel with some form of high-speed access, or from a coffee shop wireless hot spot. Your laptop could have security software (called a Virtual Private Network or VPN client) that you would use to set up an encrypted �tunnel� across the Internet back into your enterprise. Across this tunnel, you could access all the data applications that you would have access to at your desktop. You could even set up an IP Telephony or multimedia call and use a headset plugged into your laptop and, optionally, a video camera. In this way, you can take your office anywhere and stay connected. The public Internet doesn�t give voice packets priority, but you probably won�t notice (most of the time).

Behind the Connection
Early Ethernet LAN technology was developed to allow users to share local printers and servers across a LAN segment, using only an Ethernet MAC (Media Access Control) address. The MAC address of every device is assigned in the factory. Using MAC addresses does not scale to allow you to connect to any of hundreds of millions of devices across the Internet.

Enter IP. Both Ethernet and IP are based on packet switching, whereby user information is broken into packets up to 1500 bytes, each identified by an address. This address is used to either switch traffic across the LAN using a flat MAC addressing space, or route traffic across the network using a hierarchical IP addressing space. Each time you connect at the Ethernet level, you are dynamically assigned an IP network address.

The function of IP routing is to dynamically adapt to network topology changes and route packets most effectively among LAN segments across the network. An IP router or switch keeps a routing table and, after examining the IP address contained in the voice or data packet, forwards the packet on the �best� link towards the ultimate destination. Data packets also carry a Transmission Control Protocol (TCP) header, which allows lost packets to be retransmitted; not so for voice packets, since there�s no time. So, your packets flow from your wiring closet Ethernet switch to either a very high capacity campus routing switch or to a branch router. After a number of routing hops, it reaches the LAN, which connects to the destination device (perhaps a server, a PC, or IP phone).

Added Complexities
If only networking life was so simple.

At the next level of detail, there are added complexities and challenges associated with IP networking, some of which are identified here.

Converged networking: Voice has some specific requirements: end-to-end delays below 150 msec, as much as 80 Kbps per voice call, depending on coding scheme; the need to handle 50 packets per second for every voice call without packet loss, which requires end-to-end QoS and comprehensive traffic management capabilities; and sub-second failure recovery in various parts of the network.

Routing, traffic management, and timely recovery: The objectives are clear: optimally use the network resource and keep networks running while meeting application/user needs. These systems have been evolving over decades.

Multicast and streaming: To efficiently handle audio and video streaming, these streams can be optimally replicated close to the listeners (rather than at source). This has resulted in development of numerous �multicast� mechanisms.

Layered security: This includes endpoint, perimeter, communications, and core network security functionality. Today, security and availability are inextricably linked.

Added application intelligence in the network: While IP networks were originally intended to be fairly simple (with intelligence left to the endpoints), enterprise core networks have evolved to provide value-added capabilities by looking deep in the packet to make applications work better and to offload servers.

Wire-speed operation: High-end routers and switches need to support the above functionality over multiple 10Gbps Ethernet links without dropping a packet.

The Data Networking Paradox
So there you have it. For users, it�s a plug and work utility that allows connectivity across the enterprise or around the world. For IT, it�s a strategic asset that needs to be planned, designed, and operated. Today, that often requires specialized networking, operational, and security skills. The good news is that vendors are moving towards real-time secure multimedia networks that dynamically adjust to changing traffic, topology, and threat conditions to optimize network performance and user quality of experience. IT

Tony Rybczynski is Director of Strategic Enterprise Technologies at Nortel (quote - news - alerts). He has over 30 years experience in the application of packet network technology. For more information, please visit

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