Cisco Ccie Fundamental Network Design An

[ Pobierz całość w formacie PDF ]
.Next, the segmentation and reassembly segments the CS PDU into 48-byte blocks.Then the ATM layer places each block intothe payload field of an ATM cell.For all cells except the last cell, a bit in the PT field is set to zero to indicate that the cell isnot the last cell in a series that represents a single frame.For the last cell, the bit in the PT field is set to one.When the cellarrives at its destination, the ATM layer extracts the payload field from the cell; the SAR sublayer reassembles the CS PDU;and the CS uses the CRC and the length field to verify that the frame has been transmitted and reassembled correctly.AAL5 is the adaptation layer used to transfer most non-SMDS data, such as classical IP over ATM and local-area network(LAN) emulation.ATM AddressingThe ATM Forum has adapted the subnetwork model of addressing in which the ATM layer is responsible for mappingnetwork-layer addresses to ATM addresses.Several ATM address formats have been developed.Public ATM networkstypically use E.164 numbers, which are also used by Narrowband ISDN (N-ISDN) networks.Figure 8-8 shows the format of private network ATM addresses.The three formats are Data Country Code (DCC),International Code Designator (ICD), and Network Service Access Point (NSAP) encapsulated E.164 addresses.Figure 8-8: ATM address formats.Fields of an ATM Addresshttp://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2008.htm (11 of 38) [9/16/2000 5:09:33 PM]Designing ATM InternetworksThe fields of an ATM address are as follows:AFI---One byte of authority and format identifier.The AFI field identifies the type of address.The defined values are45, 47, and 39 for E.164, ICD, and DCC addresses, respectively.DCC---Two bytes of data country code.DFI---One byte of domain specific part (DSP) format identifier.AA---Three bytes of administrative authority.RD---Two bytes of routing domain.Area---Two bytes of area identifier.ESI---Six bytes of end system identifier, which is an IEEE 802 Media Access Control (MAC) address.Sel---One byte of Network Service Access Point (NSAP) selector.ICD---Two bytes of international code designator.E.164---Eight bytes of Integrated Services Digital Network (ISDN) telephone number.The ATM address formats are modeled on ISO NSAP addresses, but they identify subnetwork point of attachment (SNPA)addresses.Incorporating the MAC address into the ATM address makes it easy to map ATM addresses into existing LANs.ATM MediaThe ATM Forum has defined multiple standards for encoding ATM over various types of media.Table 8-2 lists the framingtype and data rates for the various media, including unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cable.Table 8-2: ATM Physical RatesSingleData Rate Multimode Fiber Mode CoaxialFraming (Mbps) Fiber Cable UTP-3 UTP-5 STPDS-1 1.544 �E1 2.048 �DS-3 45 �E3 34 �STS-1 51 �SONET STS3c 155 � � � �SDH STM1SONET STS12c 622 � �SDH STM4TAXI 4B/5B 100 �http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2008.htm (12 of 38) [9/16/2000 5:09:33 PM]Designing ATM Internetworks8B/10B 155 � �(Fiber Channel)Because the FDDI chipset standard, TAXI 4B/5B, was readily available, the ATM Forum encouraged initial ATMdevelopment efforts by endorsing TAXI 4B/5B as one of the first ATM media encoding standards.Today, however, the mostcommon fiber interface is STS3c/STM.There are two standards for running ATM over copper cable: UTP-3 and UTP-5.The UTP-5 specification supports 155 Mbpswith NRZI encoding, while the UTP-3 specification supports 51 Mbps with CAP-16 encoding.CAP-16 is more difficult toimplement, so, while it may be cheaper to wire with UTP-3 cable, workstation cards designed for CAP-16-based UTP-3 maybe more expensive and will offer less bandwidth.Because ATM is designed to run over fiber and copper cable, investments in these media today will maintain their value whennetworks migrate to full ATM implementations as ATM technology matures.ATM Data Exchange InterfaceTo make ATM functionality available as soon as possible, the ATM Forum developed a standard known as the ATM DataExchange Interface (DXI).Network designers can use DXI to provide UNI support between Cisco routers and ATM networks,as shown in Figure 8-9.Figure 8-9: ATM DXI topology.The ATM data service unit (ADSU) receives data from the router in ATM DXI format over a High-Speed Serial Interface(HSSI).The DSU converts the data into ATM cells and transfers them to the ATM network over a DS-3/E3 line.ATM DXI is available in several modes:Mode 1a---Supports AAL5 only, a 9232 octet maximum, and a 16-bit FCS, and provides 1023 virtual circuits.Mode 1b---Supports AAL3/4 and AAL5, a 9224 octet maximum, and a 16-bit FCS.AAL5 support is the same as Mode1a.AAL3/4 is supported on one virtual circuit.Mode 2---Supports AAL3/4 and AAL5 with 16,777,215 virtual circuits, a 65535 octet maximum, and 32-bit FCS.On the router, data from upper-layer protocols is encapsulated into ATM DXI frame format.Figure 8-10 shows the format of aMode 1a ATM DXI frame.Figure 8-10: ATM DXI frame format.In Figure 8-11, a router configured as a data terminal equipment (DTE) device is connected to an ADSU.The ADSU isconfigured as a data communications equipment (DCE) device.The router sends ATM DXI frames to the ADSU, whichconverts the frames to ATM cells by processing them through the AAL5 CS and the SAR sublayer.The ATM layer attachesthe header, and the cells are sent out the ATM UNI interface.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2008.htm (13 of 38) [9/16/2000 5:09:33 PM]Designing ATM InternetworksFigure 8-11: ATM DXI Mode 1a and Mode 1b protocol architecture for AAL5.ATM DXI addressing consists of a DFA, which is equivalent to a Frame Relay data link connection identifier (DLCI).TheDSU maps the DFA into appropriate VPI and VCI values in the ATM cell [ Pobierz całość w formacie PDF ]

Archiwum