Datasheet

814
Encapsulation Types
ATM is a popular Layer 2 WAN protocol that establishes data-link layer com-
munications over physical Layer 1 circuits. The link between endpoints must
be established before data transmission may begin. Data bit rates of 155
Mbps over CAT5 cable and 622 Mbps using fiber-optic cable are possible,
with ATM network speeds approaching 10 Gbps. ATM is more expensive
compared to Frame Relay technology.
An ATM network is comprised of multiple ATM switches interconnected by
point-to-point links, which transmits data cells to destination ATM network
interface adapters (known as the ATM endpoints). Examples of ATM end-
points include CSUs, routers, switches, computers, and video coder-decod-
ers (codecs). Two types of ATM switch interfaces are known as either UNI
or NNI. UNI interfaces interconnect ATM end systems to ATM switches. NNI
interfaces connect ATM switches. UNI and NNI interfaces are also classified
by the owner of the interface. The telephone company is assigned publicly
owned interfaces, with private equipment being assigned to the end user
and is known as customer premises equipment (CPE).
The ATM cell headers are defined in either UNI or NNI format, depending on
the type of interface used. Some of the ATM cell header fields are as follows:
✦ Generic Flow Control (GFC): Field used to identify two or more devices
on an ATM network that are using the same ATM interface (multiplexing).
✦ Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI):
Fields used in determining the ATM cell destination while in transit
through an ATM switched network. Cells are tagged with VPI and VCI
values. A local translation table determines the port address of a
particular destination based on the VPI and VCI values.
✦ Payload Type (PT): Field used to show the type of data carried. Data
may be either user data or control data. If the cell is transporting user
data, the first bit is always set to 0. The bit value changes to 1 when
control data is used. The second bit in the payload type is assigned for
congestion. A 0-bit value represents no congestion, and a 1 bit is used
to register network congestion. If the third bit is on (a 1 bit), the cell is
marked as the last cell in a frame.
✦ Cell Loss Priority (CLP): Field responsible for management of buffering.
If the CLP bit equals 1, the cell will be dropped when congestion on the
network is discovered.
✦ Header Error Control (HEC): Field used in checksum calculations to
determine whether problems exist in the header.
WAN circuits are categorized into two kinds of connection types:
✦ Permanent virtual circuits (PVCs): PVCs establish direct connections
between sites and function similarly to a dedicated leased line. The
48_489628-bk07ch01.indd 81448_489628-bk07ch01.indd 814 3/1/10 10:15 PM3/1/10 10:15 PM