Cells and SAR
All types of cartage are transported over ATM as baby cells. Appliance beef of an optimal anchored size
allows the afterward benefits:
• Low latency, aerial throughput—Small beef can be confused actual bound from about-face to
switch with a low advancement adjournment for the abbreviate after abstracts from anniversary cell. Fixedsize
cells again can be relayed at a anticipated rate. ATM switches additionally use hardware-based
switching and bargain acclamation and accommodation spaces to acceleration corpuscle relay.
• Multiservice traffic—Traffic from abounding sources can be adapted into the aforementioned fixedlength
cells activity into and out of the ATM network. ATM beef can additionally be relayed at a
predictable rate, accouterment Quality of Account (QoS) or affirmed adapted commitment of realtime
data streams for casework like articulation and video.
ATM beef accept been connected to a 53-byte length: a 5-byte attack and a 48-byte payload.
On this basis, appliance some adjustment of fragmenting beyond pieces of abstracts (for example, an IP
packet) into 53-byte beef should be obvious. Part of the ATM corpuscle bearing action is
concerned with analysis and reassembly (SAR). On one ATM bend device, beyond packets
of higher-layer protocols are anecdotal or burst into 48-byte units. Anniversary burden assemblage is
attached to a 5-byte attack to accomplish a corpuscle and again relayed to an ATM switch.
Video
PBX
Router
Cells
ATM about-face ATM switch
UNI
LAN switch
ATM network
NNI
210 Chapter 6: Trunking with ATM LANE
On the far end of a affiliation at accession ATM bend device, beef are accustomed from an ATM
switch. ATM headers are removed, and the 48-byte payloads are reassembled into the original
larger packets. Appliance this process, any blazon of abstracts can be segmented, transported, and
reassembled appliance the aforementioned 53-byte ATM cells.
ATM Model
The ATM accepted uses a advertence archetypal to call the bureaucracy of its assorted operations,
similar to the 7-layer OSI model. Table 6-2 shows a allegory amid the layers of the OSI
and ATM advertence models. Notice that ATM operations are alone performed in the abstracts articulation and
physical layers. The college layers are still anxious with casual upper-layer protocols
downward to the abstracts articulation band to be candy into ATM cells.
The action of converting higher-layer abstracts blocks into ATM beef is performed in the ATM
Adaptation Band (AAL). This band contains several altered processes, anniversary tailored for the
segmentation and re-assembly of a altered higher-layer abstracts type. The connected AAL
processes are declared as follows:
• AAL1—Supports connection-oriented casework that can challenge busy band circuits.
AAL1 requires timing synchronization amid antecedent and destination, acceptance the
transport of real-time streams like articulation and video. Abstracts is sampled and put into cell
payloads, forth with adjustment advice afore transmission.
• AAL2—Supports connection-oriented casework for capricious bit amount (VBR) applications.
• AAL3/4—Supports both connection-oriented and connectionless services. AAL3/4 was
designed for use with Switched Multimegabit Abstracts Account (SMDS). Abstracts is segmented
into corpuscle payloads, while circadian back-up analysis (CRC) absurdity advice is added.
Table 6-2 ATM Advertence Model
OSI Advertence Archetypal ATM Advertence Model
Layer 7 (application) College Layers
Layer 6 (presentation)
Layer 5 (session)
Layer 4 (transport)
Layer 3 (network)
Layer 2 (data link) ATM Adaptation Layers (AAL)
ATM Layer
Layer 1 (physical) ATM Concrete Layer
ATM Review 211
• AAL5—Supports both connection-oriented and connectionless services. Non-SMDS
data, like TCP/IP over ATM and LANE, is transported in adjustment with simple
segmentation. A CRC amount is added to the end of the pre-segmented frame.
The ATM Band is acclimated to authorize admission to added ATM devices. Again beef are sent
across the ATM adjustment appliance the attack capacity of anniversary ATM cell.
Although ATM is advised to be media independent, it does accept a concrete band that converts
cells into bitstreams for the adapted media. As well, all timing advice and absurdity control
are handled in this layer. Accepted types of concrete media accommodate DS-3/E3, 155 Mbps over
multimode and single-mode cilia (OC3), 155 Mbps over cloistral twisted-pair cabling, 622
Mbps over cilia (OC12), and Synchronous Optical Network/Synchronous Digital Hierarchy
(SONET/SDH).
Virtual Circuits
ATM relies on admission to be congenital beyond the ATM adjustment in adjustment to broadcast beef end-toend.
Before beef are sent, admission charge be adjourned and set up. Abiding basic circuits
(PVCs) are manually congenital to abutment a agreed aisle through an ATM cloud. As the name
implies, these permanent, changeless admission abide in abode until you manually abolish them.
Switched basic circuits (SVCs) are aloof the opposite—they are dynamically congenital and torn
down by ATM switches as they are needed. Back one ATM bend accessory requires a new circuit,
it informs an ATM about-face that an SVC needs to be congenital to the destination.
Virtual circuits (VCs) can additionally be congenital in two ways: Point-to-point, area one accessory talks to
one device; and Point-to-multipoint, area one accessory communicates in one administering to many
end devices.
ATM uses specific analogue to analyze the hierarchical adjustment of its VCs. A virtual
channel Affiliation (VCC) is the affiliation that is set up beyond the ATM adjustment between
endpoints. A basic aisle (VP) is a array of basic channels that allotment a accepted aisle through
the switched network. Back a VP is used, you do not charge to about-face the alone VCs
contained in it—just about-face the all-embracing VP bundle. Lastly, a manual aisle is a array of
virtual paths.
ATM Addressing
Two types of addresses abide aural ATM: basic aisle and basic approach identifiers (VPI/
VCI), and adjustment account admission point (NSAP) addresses. Anniversary blazon of abode is acclimated for a
specific purpose and is declared in the sections that follow.
212 Chapter 6: Trunking with ATM LANE
VPI/VCI Addresses
Recall that an ATM corpuscle contains a 5-byte attack and a 48-byte payload. A 5-byte amplitude is not
sufficient to accommodate a ample abode amplitude for the antecedent or destination endpoints. In packet- or
frame-based networks, agreement the antecedent and destination addresses in anniversary anatomy is necessary
so that routers and switches can advanced frames correctly. Back ATM transports a corpuscle from
source to destination, however, alone the specific VC needs to be known.
ATM uses the VPI/VCI identifier aggregate to bear beef through a network. An ATM edge
device places the VPI/VCI ethics in a corpuscle afore presenting the corpuscle to an ATM switch. The
switch again references a forwarding table, apropos the VPI and VCI ethics to outbound switch
ports.
For allegory purposes, a UNI corpuscle contains an 8-bit VPI and a 16-bit VCI address. An NNI
cell expands the VPI to 12 $.25 and has a 16-bit VCI value. These ethics are alone locally
significant to an ATM about-face and do not accept to be globally unique.
NSAP Addresses
To analyze alone ATM accessories and endpoints, 20-byte adjustment account admission point
(NSAP) addresses are used. Anniversary accessory is appropriate to accept a different NSAP address. Typically
these addresses are accounting out as groups of 4 hex digits, afar by dots (for example,
47.0091.8100.0000.00c0.1004.d94e.0000.3750.a011.00). The rightmost two hex digits are
usually aggregate by themselves, with the aboriginal dot separator to the left. (This alignment will be
important back appliance LANE.) NSAP ATM addresses are composed of the following:
• Prefix—A 13-byte acreage that abnormally identifies every ATM about-face in the network. Cisco
ATM switches accept a predefined 7-byte amount of 47.0091.8100.0000, followed by a 6-byte
unique MAC abode preconfigured on anniversary switch.
• End-System Identifier (ESI)—A 6-byte acreage that abnormally identifies every device
attached to an ATM switch. A 6-byte MAC abode is usually acclimated in this field.
• Selector—A 1-byte acreage that identifies a action active on an ATM device. Cisco
devices usually use the selector amount to analyze an ATM subinterface number.
NOTE NSAP addresses are alone acclimated to body SVCs aural an ATM network. For PVCs, alone VPI/
VCI ethics are needed. Once an SVC or PVC has been built, the VPI/VCI brace is acclimated to relay
cells from about-face to switch.
Obviously, the admeasurement of the NSAP abode prohibits it from actuality agitated aural ATM cells. ATM
devices alone acquaint NSAP addresses aural signaling protocols, back SVCs are being
built or broken down.
LAN Emulation (LANE) 213
Inherent ATM Protocols
ATM uses two protocols to automate several functions, simplifying adjustment agreement and
operation. Integrated Local Management Interface (ILMI) is a agreement that provides an
automatic way for an ATM accessory to apprentice about its neighbors. For example, an ATM edge
device can apprentice the NSAP prefix abode from an ATM switch, rather than acute an
administrator to manually configure the address. As well, an ATM about-face can apprentice the ESI
portion of the NSAP abode of an ATM bend accessory through ILMI. This abstraction will be
important back activated to LANE.
Private Network-to-Network Interface (PNNI) is an ATM agreement that is acclimated to body and
dynamically breach bottomward SVCs. PNNI is alone acclimated amid ATM switches because it involves
SVC administration. PNNI is sometimes termed a Band 2 acquisition agreement because of its use
in free paths through an ATM network. In addition, PNNI enables ATM switches to
load antithesis cartage beyond assorted paths and alongside links, and provides back-up in case of
path failure.