QoS At Layer 2

There are methods of implementing quality of service at different layers. I would like to begin by talking about the methods of implementing QoS at Layer 2.

The Ethernet Standard specifies a frame as having a maximum size of 1,518 bytes. The IEEE defined an additional 4-byte tag for Ethernet Headers, increasing the maximum size to 1,522 bytes. This tag contains two valuable pieces of information that pertain to QoS, the 802.1w VLAN ID Field, and the 802.1p Priority Field.

I will begin by discussing the 802.1p Priority field. Three bits in the IEEE Ethernet Protocol Specification can be used to prioritize packets at the user level. The three bits allow for up to eight levels of prioritization (0-7). Any 802.1p switch is capable of reading this field and can then retransmit the packet across the network with the higher priority. The higher the number in the priority field, the higher priority the switch will give to the packet. The IEEE has made some recommendations for the numbering system to be used here, but it is at the discretion of the network administrator. IEEE recommendations are to use 0 for Best Effort, 1 for Background, 2 as a spare, 3 as Excellent Effort, 4 as Controlled Load, 5 as Video, 6 as Voice, and 7 as Network Control.

The 802.1q specification, also known as VLAN Tagging, refers to the process of identifying which VLAN a packet is coming from. A VLAN, or Virtual LAN, makes a physical switch into smaller logical groupings. For example, a 24-port switch may be divided into three smaller 8-port virtual switches to eliminate leakage of data from one section to the next. This is very useful in a VoIP deployment, as data and voice VLANs can be created to minimize the effect of data transmission on the voice network. VLANs work to reduce the number of collisions and lost packets on a network. There are two types of VLAN – Port-based and policy-based. A Port-based VLAN will assign certain ports on a switch to be members of a VLAN. This is the most common type of VLAN. A policy-based VLAN can be based on a variety of information, such as IP Address, MAC Address, or Protocol (i.e. IP vs. IPX). Most new switches support Port-based VLANs. It is possible to improve QoS by putting IP phones on a separate VLAN from data and other devices, thereby avoiding collisions with these devices.

To ensure QoS we can also select queuing methods for the packets that come into our router. There are six methods of queuing for QoS:
1. First-in First-Out is a straight-forward method of queuing. The first packet received by a switch is the first on sent out by the switch
2. Priority Queuing tells the switch to process high priority traffic first
3. Custom Queuing lets the administrator assign traffic to a certain queue to control how much data is sent each time to a particular queue
4. Fair Queuing uses a round-robin approach – each queue receives the same amount of bandwidth
5. Weighted Round Robin lets the administrator allocate different amounts of bandwidth to different priority queues while ensuring that lower priority queues still receive some bandwidth
6. Weighted Fair Queuing identifies logical conversations (high volume traffic between 2 peers) and makes sure that those conversations do not monopolize the link.

The Resource Reservation Protocol (RSVP) is a method used to reserve resources (or bandwidth) on an RSVP-Compliant router. Resources are allocated on a first-come-first-serve basis. A device requests a certain amount of bandwidth, and the router replies that the bandwidth is now reserved. It will not release the bandwidth until the device is done. This protocol does not carry data; it only acts as a control mechanism for RDP packets. It is not very widely used today.

Traffic Shaping is used to help alleviate network congestion. Network congestion can occur when multiple users contend for access to the same resources. Traffic Shaping lets you determine which packets are dropped due to congestion and which packets receive priority. It can take action based on various characteristics of traffic. It also relies on queuing mechanisms.
IP Address prioritization is as simple as it sounds – packets from certain IP addresses are given priority over packets from other IP addresses. This is ideal for devices with static IP Addresses, such as Call Servers, Signaling Servers, VoIP Gateways, etc. A Network Administrator can configure routers to filter and prioritize all packets originating from these IP Addresses and know they are from VoIP devices.

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