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.Aggregateperformance numbers are media-independent, but the type of media used plays an importantrole in defining what the theoretical packet-per-second limitation is.Table 5-4 showscharacteristics of some of the more common media in use today.Table 5-4 Media CharacteristicsInterfram Minimum Maximum Valid Bandwide Gap Valid Frame Frame thEthernet 96 bits 64 bytes 1518 bytes 10 MbpsToken Ring 4 bits 32 bytes 16K bytes 16 MbpsFiber Distributed Data 0 34 bytes 4500 bytes 100 MbpsInterface (FDDI)Asynchronous Transfer 0 30 bytes (AAL5) 16K bytes (AAL5) 155 MbpsMode (ATM)Basic Rate Interface (BRI) 0 24 bytes (PPP) 1500 bytes (PPP) 128 KbpsPrimary Rate Interface 0 24 bytes (PPP) 1500 bytes (PPP) 1.472 Mbps(PRI)T1 0 14 bytes (HDLC) None (Theoretical) 1.5 Mbps4500 (Real)Fast Ethernet 96 bits 64 bytes 1518 bytes 100 MbpsCalculating the theoretical maximum packets per second involves all the variables listed inTable 5-4: interframe gap, bandwidth, and frame size.The formula to compute this number is:Bandwidth/Packet Size = Theoretical Maximum Packets per Second (where packet sizemay incorporate interframe gap in bits)Table 5-5 lists the theoretical packet-per-second limitations for three common media 10 MbpsEthernet, 16 Mbps Token Ring, and FDDI each for eight different Ethernet frame sizes.These CH01.book Page 174 Friday, January 7, 2000 5:35 PM174 Chapter 5: WAN Designeight frame sizes, widely used in the industry, are derived from the performance testingmethodology as outlined in the Internet standard for device benchmarking in RFC 1944.Thenumbers are derived by using the above formula.NOTE RFC 1944 has recently been made obsolete by RFC 2544.Table 5-5 Packet-per-Second LimitationEthernet Size 10-Mbps Ethernet 16-Mbps Token Ring FDDI(bytes) (pps) (pps) (pps)64 14,880 24,691 152,439128 8,445 13,793 85,616256 4,528 7,326 45,620512 2,349 3,780 23,585768 1,586 2,547 15,9031024 1,197 1,921 11,9961280 961 1,542 9,6301518 812 1,302 8,138More specific detail in how the numbers in Table 5-5 were derived for the three media (10 MbpsEthernet, 16 Mbps Token Ring, and FDDI) follow.10 Mbps Ethernet The frame size needs to incorporate the data and header bytes as well asthe bits used for the preamble and interframe gap, as shown in Figure 5-5. CH01.book Page 175 Friday, January 7, 2000 5:35 PMSelecting the WAN Hardware 175Figure 5-5 10 Mbps Ethernet FramesDirection of Data FlowInterframePreamble Ethernet Frame #1 Preamble Ethernet Frame #2Gap64 bits (8 x N Bytes, Where 96 bits 64 bitsN is 18 BytesHeader + User Data)In Figure 5-5 the fields have the following lengths:" Preamble 64 bits" Frame (8×N) bits (where N is Ethernet packet size in bytes, this includes 18 bytes ofheader)" Gap 96 bits16 Mbps Token Ring Neither token nor idles between packets are accounted for becausethe theoretical minima are hard to pin down, but by using only the frame format itself themaximum theoretical packets per second can be estimated, as shown in Figure 5-6.Because weare basing our initial frame on an Ethernet frame, note that we need to subtract the Ethernetheader bits for the correct calculation of the data portion.So, for a 64-byte Ethernet frame, weget 64 18 = 46 bytes of data for the Data portion of the Token Ring frame shown in Figure 5-6.Figure 5-6 16-Mbps Token Ring FramesDirection of Data FlowFrame #1 Frame #2D S CS S nSD AC FC DA SA RI Vendor Type DATA FCS ED FS SD AC FCA A tP P l8 8 8 48 48 48 8 8 8 24 16 8x 32 8 8 8 8 8(N-18)In Figure 5-6 the fields have the following lengths:" SD 8 bits" AC 8 bits" FC 8 bits" DA 48 bits" SA 48 bits" RI 48 bitsDE53XE01DE53XE02 CH01.book Page 176 Friday, January 7, 2000 5:35 PM176 Chapter 5: WAN Design" DSAP 8 bits" SSAP 8 bits" Control 8 bits" Vendor 24 bits" Type 16 bits" Data 8×(N 18) bits (where N is original Ethernet frame size)" FCS 32 bits" ED 8 bits" FS 8 bitsFDDI Neither token nor idles between packets are accounted for because the theoreticalminima are hard to pin down, but by using only the frame format itself the maximum theoreticalpackets per second can be estimated, as shown in Figure 5-7.Note that, because we are basingour initial frame on an Ethernet frame, we need to subtract the Ethernet header bits for thecorrect calculation of the data portion.So, for a 64 byte Ethernet frame, we get 64 18 = 46bytes of data for the Data portion of the FDDI frame shown in Figure 5-7.Figure 5-7 FDDI FramesDirection of Data FlowFrame #1 Frame #2D S CS S nPreamble SD FC DA SA Vendor Type DATA FCS ED FS PreambleA A tP P l64 8 8 48 48 8 8 8 24 16 8x 32 4 12(N-18)In Figure 5-7 the fields have the following lengths:" Preamble 64 bits" SD 8 bits" FC 8 bits" DA 48 bits" SA 48 bitsDE53XE03 CH01.book Page 177 Friday, January 7, 2000 5:35 PMSelecting the WAN Hardware 177" DSAP 8 bits" SSAP 8 bits" Control 8 bits" Vendor 24 bits" Type 16 bits" Data 8×(N 18) bits (where N is original Ethernet frame size)" FCS 32 bits" ED 4 bits" FS 12 bitsFrame and Packet Size The packet size is a major factor in determining the maximumpackets per second, and, in the theoretical test world, one packet size at a time is tested.Eightstandard packet sizes are tested: 64-, 128-, 256-, 512-, 768-, 1024-, 1280-, and 1518-bytepackets.Figure 5-8 shows a graph of the theoretical maximum packets per second for 10 MbpsEthernet.It is important to note that as the frame size increases, the maximum theoretical packets persecond decrease. 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.Aggregateperformance numbers are media-independent, but the type of media used plays an importantrole in defining what the theoretical packet-per-second limitation is.Table 5-4 showscharacteristics of some of the more common media in use today.Table 5-4 Media CharacteristicsInterfram Minimum Maximum Valid Bandwide Gap Valid Frame Frame thEthernet 96 bits 64 bytes 1518 bytes 10 MbpsToken Ring 4 bits 32 bytes 16K bytes 16 MbpsFiber Distributed Data 0 34 bytes 4500 bytes 100 MbpsInterface (FDDI)Asynchronous Transfer 0 30 bytes (AAL5) 16K bytes (AAL5) 155 MbpsMode (ATM)Basic Rate Interface (BRI) 0 24 bytes (PPP) 1500 bytes (PPP) 128 KbpsPrimary Rate Interface 0 24 bytes (PPP) 1500 bytes (PPP) 1.472 Mbps(PRI)T1 0 14 bytes (HDLC) None (Theoretical) 1.5 Mbps4500 (Real)Fast Ethernet 96 bits 64 bytes 1518 bytes 100 MbpsCalculating the theoretical maximum packets per second involves all the variables listed inTable 5-4: interframe gap, bandwidth, and frame size.The formula to compute this number is:Bandwidth/Packet Size = Theoretical Maximum Packets per Second (where packet sizemay incorporate interframe gap in bits)Table 5-5 lists the theoretical packet-per-second limitations for three common media 10 MbpsEthernet, 16 Mbps Token Ring, and FDDI each for eight different Ethernet frame sizes.These CH01.book Page 174 Friday, January 7, 2000 5:35 PM174 Chapter 5: WAN Designeight frame sizes, widely used in the industry, are derived from the performance testingmethodology as outlined in the Internet standard for device benchmarking in RFC 1944.Thenumbers are derived by using the above formula.NOTE RFC 1944 has recently been made obsolete by RFC 2544.Table 5-5 Packet-per-Second LimitationEthernet Size 10-Mbps Ethernet 16-Mbps Token Ring FDDI(bytes) (pps) (pps) (pps)64 14,880 24,691 152,439128 8,445 13,793 85,616256 4,528 7,326 45,620512 2,349 3,780 23,585768 1,586 2,547 15,9031024 1,197 1,921 11,9961280 961 1,542 9,6301518 812 1,302 8,138More specific detail in how the numbers in Table 5-5 were derived for the three media (10 MbpsEthernet, 16 Mbps Token Ring, and FDDI) follow.10 Mbps Ethernet The frame size needs to incorporate the data and header bytes as well asthe bits used for the preamble and interframe gap, as shown in Figure 5-5. CH01.book Page 175 Friday, January 7, 2000 5:35 PMSelecting the WAN Hardware 175Figure 5-5 10 Mbps Ethernet FramesDirection of Data FlowInterframePreamble Ethernet Frame #1 Preamble Ethernet Frame #2Gap64 bits (8 x N Bytes, Where 96 bits 64 bitsN is 18 BytesHeader + User Data)In Figure 5-5 the fields have the following lengths:" Preamble 64 bits" Frame (8×N) bits (where N is Ethernet packet size in bytes, this includes 18 bytes ofheader)" Gap 96 bits16 Mbps Token Ring Neither token nor idles between packets are accounted for becausethe theoretical minima are hard to pin down, but by using only the frame format itself themaximum theoretical packets per second can be estimated, as shown in Figure 5-6.Because weare basing our initial frame on an Ethernet frame, note that we need to subtract the Ethernetheader bits for the correct calculation of the data portion.So, for a 64-byte Ethernet frame, weget 64 18 = 46 bytes of data for the Data portion of the Token Ring frame shown in Figure 5-6.Figure 5-6 16-Mbps Token Ring FramesDirection of Data FlowFrame #1 Frame #2D S CS S nSD AC FC DA SA RI Vendor Type DATA FCS ED FS SD AC FCA A tP P l8 8 8 48 48 48 8 8 8 24 16 8x 32 8 8 8 8 8(N-18)In Figure 5-6 the fields have the following lengths:" SD 8 bits" AC 8 bits" FC 8 bits" DA 48 bits" SA 48 bits" RI 48 bitsDE53XE01DE53XE02 CH01.book Page 176 Friday, January 7, 2000 5:35 PM176 Chapter 5: WAN Design" DSAP 8 bits" SSAP 8 bits" Control 8 bits" Vendor 24 bits" Type 16 bits" Data 8×(N 18) bits (where N is original Ethernet frame size)" FCS 32 bits" ED 8 bits" FS 8 bitsFDDI Neither token nor idles between packets are accounted for because the theoreticalminima are hard to pin down, but by using only the frame format itself the maximum theoreticalpackets per second can be estimated, as shown in Figure 5-7.Note that, because we are basingour initial frame on an Ethernet frame, we need to subtract the Ethernet header bits for thecorrect calculation of the data portion.So, for a 64 byte Ethernet frame, we get 64 18 = 46bytes of data for the Data portion of the FDDI frame shown in Figure 5-7.Figure 5-7 FDDI FramesDirection of Data FlowFrame #1 Frame #2D S CS S nPreamble SD FC DA SA Vendor Type DATA FCS ED FS PreambleA A tP P l64 8 8 48 48 8 8 8 24 16 8x 32 4 12(N-18)In Figure 5-7 the fields have the following lengths:" Preamble 64 bits" SD 8 bits" FC 8 bits" DA 48 bits" SA 48 bitsDE53XE03 CH01.book Page 177 Friday, January 7, 2000 5:35 PMSelecting the WAN Hardware 177" DSAP 8 bits" SSAP 8 bits" Control 8 bits" Vendor 24 bits" Type 16 bits" Data 8×(N 18) bits (where N is original Ethernet frame size)" FCS 32 bits" ED 4 bits" FS 12 bitsFrame and Packet Size The packet size is a major factor in determining the maximumpackets per second, and, in the theoretical test world, one packet size at a time is tested.Eightstandard packet sizes are tested: 64-, 128-, 256-, 512-, 768-, 1024-, 1280-, and 1518-bytepackets.Figure 5-8 shows a graph of the theoretical maximum packets per second for 10 MbpsEthernet.It is important to note that as the frame size increases, the maximum theoretical packets persecond decrease. 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