DiffServ (Differentiated Services)

DiffServ is a QoS (Quality of Service) protocol for managing bandwidth allocation for Internet media connections (e.g. a VOIP voice connection). Overview here.

With low bandwidth lines, such as ADSL, users may feel jittering. Try to allow full upload bandwidth to eMule and do a phone call in G.711 at the same time, and you will surely understand what I mean.

Regarding Cisco docs, and others as well, the correct DiffServ value is EF (Expedited Forwarding). The EF DiffServ also contains the ToS Low Delay bit, which is treated by many routers without any configuration. This is called PFIFO (Priority Fist In First Out, or FIFO with Priority).

The values you must know:

• EF DiffServ decimal: 184
• EF DiffServ hexadecimal: 0xb8
• EF DiffServ binary: 10111000 (in red: DSCP field)
• EF DSCP decimal: 46
• EF DSCP hexadecimal: 0x2e
• EF DSCP binary: 101110

Remark: The DSCP field is contained within the 6 first bits of the DiffServ byte.

Here is the part where we getting crazy. Each constructor requires a different DiffServ or DSCP value.

• • Alcatel OmniPCX Office – EF DiffServ binary: 10111000 DIFFSERV_PHB_EF
  • Asterisk 1.0.9 (ie sip.conf) – EF DiffServ hexadecimal: tos = 0xb8
  • Grandstream (requires FW 1.0.6.3 and older) – EF DSCP decimal: 46
  • ZyXEL – EF DiffServ decimal: 184
  • TELES – EF DiffServ decimal: 184

• • snom comes with a TOS default of 160 and advises to use 184 (for RTP). For SIP a value of CS3 = TOS 96 would be advisable.

If any doubt, use a sniffer such as Wireshark (formerly Ethereal). If settings are correct, it will show the Differentiated Services Field in any of the RTP packets, with a value of 0xb8 (DSCP 0x2e: Expedited Forwarding: ECN: 0x00).

See also:

• http://www.cisco.com/en/US/products/ps6610/products_white_paper09186a00800a3e2f.shtml
• http://www.protocols.com/pbook/tcpip2.htm
• QoS