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14–2 MULTILINK ML2400 ETHERNET COMMUNICATIONS SWITCH – INSTRUCTION MANUAL
QUALITY OF SERVICE CHAPTER 14: QUALITY OF SERVICE
To make the preemptive queuing possible, most switches implement at least two queue
buffers. The MultiLink family of switches has two priority queues, 1 (low) and 0 (high).When
tagged packets enter a switch port, the switch responds by placing the packet into one of
the two queues, and depending on the precedence levels the queue could be rearranged
to meet the QoS requirements.
14.1.3 DiffServ and QoS
QoS refers to the level of preferential treatment a packet receives when it is being sent
through a network. QoS allows time sensitive packets such as voice and video, to be given
priority over time insensitive packets such as data. Differentiated Services (DiffServ or DS)
are a set of technologies defined by the IETF (Internet Engineering Task Force) to provide
quality of service for traffic on IP networks.
DiffServ is designed for use at the edge of an Enterprise where corporate traffic enters the
service provider environment. DiffServ is a layer-3 protocol and requires no specific layer-2
capability, allowing it to be used in the LAN, MAN, and WAN. DiffServ works by tagging
each packet (at the originating device or an intermediate switch) for the requested level of
service it requires across the network.
FIGURE 14–1: ToS and DSCP
DiffServ inserts a 6-bit DiffServ code point (DSCP) in the Type of Service (ToS) field of the IP
header, as shown in the picture above. Information in the DSCP allows nodes to determine
the Per Hop Behavior (PHB), which is an observable forwarding behavior for each packet.
Per hop behaviors are defined according to:
• Resources required (e.g., bandwidth, buffer size)
• Priority (based on application or business requirements)
• Traffic characteristics (e.g., delay, jitter, packet loss)
Nodes implement PHBs through buffer management and packet scheduling mechanisms.
This hop-by-hop allocation of resources is the basis by which DiffServ provides quality of
service for different types of communications traffic.
14.1.4 IP Precedence
IP Precedence utilizes the three precedence bits in the IPv4 header's Type of Service (ToS)
field to specify class of service for each packet. You can partition traffic in up to eight
classes of service using IP precedence. The queuing technologies throughout the network
can then use this signal to provide the appropriate expedited handling.
ToS Data FCS