Switched Multimegabit Data
Service (SMDS) is a high-speed, packet-switched, datagram-based WAN
networking technology used for communication over public data networks (PDNs).
SMDS can use fiber- or copper-based media and supports speeds of
1.544 Mbps over Digital Signal level 1 (DS-1)
transmission facilities, or 44.736 Mbps over Digital
Signal level 3 (DS-3) transmission facilities. In addition, SMDS data
units are large enough to encapsulate entire IEEE 802.3, IEEE 802.5, and
Fiber-Distributed Data Interface (FDDI) frames. This chapter summarizes
the operational elements of the SMDS environment and outlines the
underlying protocol. A discussion of related technologies, such as
Distributed Queue Dual Bus (DQDB) is also provided. The chapter closes
with discussions of SMDS access classes and cell formats.
SMDS networks feature several underlying
entities to provide high-speed data service. These include customer
premises equipment (CPE), carrier
equipment, and the subscriber network interface (SNI). CPE
is terminal equipment typically owned and maintained by the customer.
CPE includes end devices, such as terminals and personal computers, and
intermediate nodes, such as routers, modems, and multiplexers.
Intermediate nodes, however, sometimes are provided by the SMDS carrier.
Carrier equipment
generally consists of high-speed WAN switches that must conform to
certain network equipment specifications, such as those outlined by Bell
Communications Research (Bellcore). These specifications define network
operations, the interface between a local carrier network and a
long-distance carrier network, and the interface between two switches
inside a single carrier network.
The SNI is the interface between CPE and
carrier equipment. This interface is the point at which the customer
network ends and the carrier network begins. The function of the SNI is
to render the technology and operation of the carrier SMDS network
transparent to the customer. Figure
14-1 illustrates the relationship between these three components of an SMDS
network.
Figure 14-1: The
SNI provides an interface between the CPE and the carrier
equipment in SMDS.
The SMDS Interface Protocol (SIP)
is used for communications between CPE and SMDS carrier equipment. SIP
provides connectionless service across the subscriber-network
interface (SNI), allowing the CPE to access the SMDS network. SIP
is based on the IEEE 802.6 Distributed Queue Dual Bus (DQDB)
standard for cell relay across metropolitan-area networks (MANs). The
DQDB was chosen as the basis for SIP because it is an open standard that
supports all the SMDS service features. In addition, DQDB was designed
for compatibility with current carrier transmission standards, and it is
aligned with emerging standards for Broadband ISDN (BISDN), which will
allow it to interoperate with broadband video and voice services. Figure
14-2 illustrates where SIP is used in an SMDS network.
Figure 14-2: SIP provides
connectionless service between the CPE and carrier equipment.
SIP consists of three levels. SIP
Level 3 operates at the Media Access Control (MAC) sublayer of the
data link layer of the OSI reference model. SIP Level 2
operates at the MAC sublayer of the data link layer. SIP Level 1
operates at the physical layer of the OSI reference model.
Figure 14-3 illustrates how SIP maps to the OSI reference model,
including the IEEE data link sublayers.
Figure 14-3: SIP provides
services associated with the physical and data link layers of the OSI
model.
SIP Level 3 begins operation when user
information is passed to SIP Level 3 in the form of SMDS
service data units (SDUs). SMDS
SDUs then are encapsulated in a SIP Level 3 header and trailer. The
resulting frame is called a Level 3 protocol
data unit (PDU). SIP Level 3 PDUs then are subsequently passed to
SIP Level 2.
SIP Level 2, which operates at the Media
Access Control (MAC) sublayer of the data Level layer, begins operating
when it receives SIP Level 3 PDUs. The PDUs then are segmented into
uniformly sized (53-octet) Level 2 PDUs, called cells. The cells are
passed to SIP Level 1 for placement on the physical medium.
SIP Level 1 operates at the physical
layer and provides the physical-link protocol that operates at DS-1 or
DS-3 rates between CPE devices and the network. SIP Level 1 consists of
the transmission system and Physical
Layer Convergency Protocol (PLCP) sublayers. The transmission
system sublayer defines the characteristics and method of
attachment to a DS-1 or DS-3 transmission link. The PLCP specifies how
SIP Level 2 cells are to be arranged relative to the DS-1 or DS-3 frame.
PLCP also defines
other management information.
Distributed
Queue Dual Bus (DQDB)
The Distributed
Queue Dual Bus (DQDB) is a data link layer communication protocol
designed for use in metropolitan-area networks (MANs). DQDB specifies a
network topology composed of two unidirectional logical buses that
interconnect multiple systems. It is defined in the IEEE 802.6 DQDB
standard.
An access DQDB describes just the
operation of the DQDB protocol (in SMDS, SIP) across a user-network
interface (in SMDS, across the SNI). Such operation is distinguished
from the operation of a DQDB protocol in any other environment (for
example, between carrier equipment within the SMDS PDN).
The access DQDB is composed of the basic
SMDS network components:
Figure 14-4 depicts a basic access DQDB,
with two CPE devices and one switch (carrier equipment) attached to the
dual bus.
Figure 14-4: A basic access
DQDB may consist of an end node, router, and a switch.
An SMDS access DQDB typically is arranged
in a single-CPE configuration or a multi-CPE configuration.
A single-CPE access DQDB configuration
consists of one switch in the carrier SMDS network and one CPE station
at the subscriber site. Single-CPE DQDB configurations create a two-node
DQDB subnetwork. Communication occurs only between the switch and the
one CPE device across the SNI. No contention is on the bus because no
other CPE devices attempt to access it.
A multi-CPE configuration consists of one
switch in the carrier SMDS network and a number of interconnected CPE
devices at the subscriber site (all belonging to the same subscriber).
In multi-CPE configurations, local communication between CPE devices is
possible. Some local communication will be visible to the switch serving
the SNI, and some will not.
Contention for the bus by multiple
devices requires the use of the DQDB distributed queuing algorithm,
which makes implementing a multi-CPE configuration more complicated than
implementing a
single-CPE configuration.
SMDS Access
Classes
SMDS access
classes enable SMDS networks to accommodate a broad range of traffic
requirements and equipment capabilities. Access classes constrain CPE
devices to a sustained or average rate of data transfer by establishing
a maximum sustained information transfer rate and a maximum allowed
degree of traffic burstiness. (Burstiness in this context is
the propensity of a network to experience sudden increases in bandwidth
demand.) SMDS access classes sometimes are implemented using a
credit-management scheme. In this case, a credit-management algorithm
creates and tracks a credit balance for each customer interface. As
packets are sent into the network, the credit balance is decremented.
New credits are allocated periodically, up to an established maximum.
Credit management is used only on DS-3 rate SMDS interfaces, not on DS-1
rate interfaces.
Five access classes are supported for
DS-3-rate access (corresponding to sustained information rates). Data
rates supported are 4, 10, 16, 25, and 34 Mbps.
SMDS protocol data units (PDUs) carry both
a source and a
destination address. SMDS addresses are 10-digit values resembling
conventional telephone numbers.
The SMDS addressing implementation offers
group addressing and security features.
SMDS group addresses allow a single
address to refer to multiple CPE stations, which specify the group
address in the Destination Address field of the PDU. The network makes
multiple copies of the PDU, which are delivered to all members of the
group. Group addresses reduce the amount of network resources required
for distributing routing information, resolving addresses, and
dynamically discovering network resources. SMDS group addressing is
analogous to multicasting on LANs.
SMDS implements two security features:
source address validation and address screening. Source address
validation ensures that the PDU source address is legitimately
assigned to the SNI from which it originated. Source address validation
prevents address spoofing, in which illegal traffic assumes the source
address of a legitimate device. Address screening allows a
subscriber to establish a private virtual network that excludes unwanted
traffic. If an address is disallowed, the data unit
is not delivered.
Figure 14-5 illustrates the format of the
SMDS Interface Protocol (SIP) Level 3 protocol data unit (PDU).
The following descriptions briefly
summarize the function of the SIP Level 3 PDU fields illustrated in Figure
14-5:
Figure 14-5: SIP Level 3
protocol data unit consists of 15 fields.
The following descriptions briefly
summarize the function of the SIP Level 3 PDU fields illustrated in Figure
14-5:
- The four most significant bits of
the Destination Address subfield contain the value 0001 (the
internationally defined country code for North America). The next
40 bits contain the binary-encoded value of the 10-digit SMDS
address. The final 16 (least-significant) bits are populated with
ones for padding
Figure 14-6 illustrates the format of the
SMDS Interface Protocol (SIP) Level 2 cell
format.
Figure 14-6: Seven
fields comprise the SMDS SIP Level 2 cell.
The following descriptions briefly
summarize the
function of the SIP Level 2 PDU fields illustrated in Figure 14-6:
Message
ID---Associates Level 2 cells with a Level 3 PDU.
The Message ID is
the same for all of the segments of a given Level 3 PDU. In a multi-CPE
configuration, Level 3 PDUs originating from different CPE devices must
have a different Message ID. This allows the SMDS network receiving
interleaved cells from different Level 3 PDUs to associate each Level 2
cell with the correct Level 3 PDU.
Segmentation
Unit---Contains
the data portion of the cell. If the Level 2 cell is empty, this field
is populated with zeros.
Payload
Length---Indicates
how many bytes of a Level 3 PDU actually are contained in the
Segmentation Unit field. If the Level 2 cell is empty, this field is
populated with zeros.
The Payload CRC value does not cover the
Access Control or
the Network Control Information fields.
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