Ethernet Switch

An Ethernet Switch is a LAN interconnection device which operates at the data link layer (layer 2) of the OSI refrence model. A switch is fundamentally similar to a bridge, but usually supports a larger number of connected LAN segments and has a richer management capability.

Modern LANs have increasingly replaced the shared media with a switched media, by installing Ethernet switches and bridges in place of hubs and repeaters. These logically partition the traffic to travel only over the network segments on the path between the source and the destination. This reduces the wastage of bandwidth which results from sending the packet to parts of the network which do not need to receive the data. There are also benefits from improved security (users are less able to tap-in into other user's data), better management (the ability to control who receives what information (i.e. Virtual LANs) and to limit the impact of network problems), and the ability to operate some links in full duplex (rather than half duplex required for shared access).

Architecture

At the centre of a Switch is a type of switching element which controls the ports to which each frame is forwarded. Three types of switching element have been used, each has some merits in terms of cost/performance:

• Matrix Switches
• Shared bus
• Shared Memory

In addition, many switches include processing capability beyond that required for forwarding. This may be used to implement additional features.

Switches and Multicast Traffic

Many Ethernet switches handle multicast traffic as if were broadcast traffic. When a multicast packet reaches such a bridge/switch, it forwards the packet to all active interfaces, effectively flooding the network. This ensures all clients receive the multicast data, but has the drawback that every LAN segment carries all the multicast traffic, even when the directly connected clients do not require the data. This mitigates most of the advantages of switching when considering multicast traffic.

Multicast Traffic from F is delivered to all output interfaces (ports)

A bridge/switch may be designed to provides multicast support by implementing filters at the output ports of the device which remove the packets for which the clients attached to the ports have no registered interest. This operation resembles the processing for Virtual LANs (VLANs) and may be performed using the same/similar processing engine. Three approaches are possible when configuring this style of operation:

1. Manual table configuration The network manager determines which clients should receive which multicast packets, in the same manner that VLAN membership is configured. Many multicast applications however select their multicast groups dynamically as the application executes.
2. "Snooping" the multicast "Join" and "Leave"messages sent by clients. Clients use a protocol called the Internet Group Management Protocol (IGMP) to register the address groups in which they have an interest with their local multicast router. Some switches (e.g. FORE, 3COM) are able to monitor/emulate these packets and use the information to configure the switch filters dynamically
3. Down-loading a switch filter table from the local multicast router. Each IP multicast network must have at least one multicast. The router must track the multicast membership of each client, and may utilise this information to inform the switch which clients require which packets. In practice, this may be as simple as down-loading a multicast address filter table to all attached switches. Most CISCO equipment support the CISCO Group Management Protocol (CGMP) to provide this function.

With multicast Filtering. The multicast traffic form F is only forwarded to those interfaces which have equipment connected that wishes to receive the multicast packets. In this case, only E and H.

Some level of multicast filtering is highly desirable within a bridge/switch handling multicast traffic. These procedures allow a the device to selectively forward multicast packets only to hosts which register an interest in the corresponding multicast group address. Without the addition of such procedures, Level 2 bridges/switches are forced to broadcast multicast packets to all connected LANs. While manual configuration may suffice for applications such as multicast file transfer, or multicast distribution to network news/web cache clients. For the majority of multicast applications, one of the two dynamic schemes is recommended.

Some level of multicast filtering is highly desirable within a bridge/switch handling multicast traffic. These procedures allow a the device to selectively forward multicast packets only to hosts which register an interest in the corresponding multicast group address. Without the addition of such procedures, Level 2 bridges/switches are forced to broadcast multicast packets to all connected LANs. While manual configuration may suffice for applications such as multicast file transfer, or multicast distribution to network news/web cache clients. For the majority of multicast applications, one of the two dynamic schemes is recommended..

Network Switching

What is a Switch?

Switches occupy the same place in the network as hubs. Unlike hubs, switches examine each packet and process it accordingly rather than simply repeating the signal to all ports. Switches map the Ethernet addresses of the nodes residing on each network segment and then allow only the necessary traffic to pass through the switch. When a packet is received by the switch, the switch examines the destination and source hardware addresses and compares them to a table of network segments and addresses. If the segments are the same, the packet is dropped ("filtered"); if the segments are different, then the packet is "forwarded" to the proper segment. Additionally, switches prevent bad or misaligned packets from spreading by not forwarding them.

Filtering of packets, and the regeneration of forwarded packets enables switching technology to split a network into separate collision domains. Regeneration of packets allows for greater distances and more nodes to be used in the total network design, and dramatically lowers the overall collision rates. In switched networks, each segment is an independent collision domain. In shared networks all nodes reside in one, big shared collision domain.

Easy to install, most switches are self learning. They determine the Ethernet addresses in use on each segment, building a table as packets are passed through the switch. This "plug and play" element makes switches an attractive alternative to hubs.

Switches can connect different networks types (such as Ethernet and Fast Ethernet) or networks of the same type. Many switches today offer high-speed links, like Fast Ethernet or FDDI, that can be used to link the switches together or to give added bandwidth to important servers that get a lot of traffic. A network composed of a number of switches linked together via these fast uplinks is called a "collapsed backbone" network.

Dedicating ports on switches to individual nodes is another way to speed access for critical computers. Servers and power users can take advantage of a full segment for one node, so some networks connect high traffic nodes to a dedicated switch port.

Full duplex is another method to increase bandwidth to dedicated workstations or servers. To use full duplex, both network interface cards used in the server or workstation, and the switch must support full duplex operation. Full duplex doubles the potential bandwidth on that link, providing 20 Mbps for Ethernet and 200 Mbps for Fast Ethernet.

Network Congestion

As more users are added to a shared network or as applications requiring more data are added, performance deteriorates. This is because all users on a shared network are competitors for the Ethernet bus. A moderately loaded 10 Mbps Ethernet network is able to sustain utilization of 35% and throughput in the neighborhood of 2.5 Mbps after accounting for packet overhead, interpacket gaps and collisions. A moderately loaded Fast Ethernet shares 25 Mbps of real data in the same circumstances. With shared Ethernet and Fast Ethernet, the likelihood of collisions increases as more nodes and/or more traffic is added to the shared collision domain.

Ethernet itself is a shared media, so there are rules for sending packets to avoid conflicts and protect data integrity. Nodes on an Ethernet network send packets when they determine the network is not in use. It is possible that two nodes at different locations could try to send data at the same time. When both PCs are transferring a packet to the network at the same time, a collision will result. Both packets are retransmitted, adding to the traffic problem. Minimizing collisions is a crucial element in the design and operation of networks. Increased collisions are often the result of too many users or too much traffic on the network, which results in a lot of contention for network bandwidth. This can slow the performance of the network from the users point of view. Segmenting, where a network is divided into different pieces joined together logically with switches or routers, reduces congestion in an overcrowded network.

Collision rates measure the percentage of packets that are collisions. Some collisions are inevitable, with less than 10% common in well running networks.

The Factors Affecting Network Efficiency

- Amount of traffic - Number of nodes - Size of packets - Network diameter

Measuring Network Efficiency

- Average to peak load devition - Collision Rate - Utilization Rate

Utilization rate is another widely accessible statistic about the health of a network. This statistic is available in Novell's Console monitor and WindowsNT performance monitor as well as any optional LAN analysis software. Utilization in an average network above 35% indicates potential problems. This 35% utilization is near optimum, but some networks experience higher or lower utilization optimums due to factors such as packet size and peak load deviation.

A switch is said to work at "wire speed" if it has enough processing power to handle full ethernet speed at minimum packet sizes. Most switches on the market are well ahead of network traffic capabilities supporting full "wire speed" of ethernet, 14,480 pps (packets per second).

Routers

Routers work in a manner similar to switches and bridges in that they filter out network traffic. Rather than doing so by packet addresses they filter by specific protocol. Routers were born out of the necessity for dividing networks logically instead of physically. An IP router can divide a network into various subnets so that only traffic destined for particular IP addresses can pass between segments. Routers recalculate the checksum, and rewrite the MAC header of every packet. The price paid for this type of intelligent forwarding and filtering is usually calculated in terms of latency, or the delay that a packet experiences inside the router. Such filtering takes more time than that exercised in a switch or bridge which only looks at the Ethernet address but in more complex networks network efficiency is improved. An additional benefit of routers is their automatic filtering of broadcasts, but overall they are complicated to setup.

Switch Benefits

- Isolates traffic, relieving congestion - Separates collision domains, reducing collisions - Segments, restarting distance and repeater rules

Switch Costs

- Price: currently 3 to 5 times the price of a hub - Packet processing time is longer than in a hub - Monitoring the network is more complicated

General Benefits of Switching

Switches replace hubs in networking designs, and they are more expensive. So why is the desktop switching market doubling ever year with huge numbers sold? The price of switches is declining precipitously, while hubs are a mature technology with small price declines. This means that there is far less difference between switch costs and hub costs than there used to be, and the gap is narrowing.

Since switches are self learning, they are as easy to install as a hub. Just plug them in and go. And they operate on the same hardware layer as a hub, so there are no protocol issues.

There are two reasons for switches being included in network designs. First, a switch breaks one network into many small networks so the distance and repeater limitations are restarted. Second, this same segmentation isolates traffic and reduces collisions relieving network congestion. It is very easy to identify the need for distance and repeater extension, and to understand this benefit of switching. But the second benefit, relieving network congestion, is hard to identify and harder to understand the degree by which switches will help performance. Since all switches add small latency delays to packet processing, deploying switches unnecessarily can actually slow down network performance. So the next section pertains to the factors affecting the impact of switching to congested networks.

Switching in Your Network

The benefits of switching vary from network to network. Adding a switch for the first time has different implications than increasing the number of switched ports already installed. Understanding traffic patterns is very important to switching - the goal being to eliminate (or filter) as much traffic as possible. A switch installed in a location where it forwards almost all the traffic it receives will help much less than one that filters most of the traffic.

Networks that are not congested can actually be negatively impacted by adding switches. Packet processing delays, switch buffer limitations, and the retransmissions that can result sometimes slows performance compared with the hub based alternative. If your network is not congested, don't replace hubs with switches. How can you tell if performance problems are the result of network congestion? Measure utilization factors and collision rates.

Good Candidates for Performance Boosts from Switching

- Utilization more than 35% - Collision rates more than 10% Utilization load is the amount of total traffic as a percent of the theoretical maximum for the network type, 10 Mbps in Ethernet, 100 Mbps in Fast Ethernet. The collision rate is the number of packets with collisions as a percentage of total packages

Network response times (the user-visible part of network performance) suffers as the load on the network increases, and under heavy loads small increases in user traffic often results in significant decreases in performance. This is similar to automobile freeway dynamics, in that increasing loads results in increasing throughput up to a point, then further increases in demand results in rapid deterioration of true throughput. In Ethernet, collisions increase as the network is loaded, and this causes retransmissions and increases in load which cause even more collisions. The resulting network overload slows traffic considerably.