IBM 990 - Manual (Page 115)

Chapter 3. I/O system structure 103
settings will override the OSA-Express feature port ability to auto-negotiate with its attached
Ethernet switch.
The OSA-Express FENET feature supports auto-negotiation with its attached Ethernet hub,
router, or switch. If you allow the LAN speed to default to auto-negotiation, the FENET
OSA-Express and the attached hub, router, or switch auto-negotiates the LAN speed setting
between them. If the attached Ethernet hub, router, or switch does not support
auto-negotiation, the OSA enters the LAN at the default speed of 100 Mbps in half-duplex
mode.
If you are not using auto-negotiate, the OSA will attempt to join the LAN at the specified
speed/mode; however, the speed/mode settings are only used when the OSA is first in the
LAN. If this fails, the OSA will attempt to join the LAN as if auto-negotiate were specified.
The OSA-Express FENET feature can be defined with CHPID type OSD or OSE. The HPDT
MPC mode is no longer available on the FENET for z990.
OSA-Express Token Ring (feature code 2367)
The z990 OSA-Express Token Ring feature (feature code 2367) occupies one I/O slot in the
z990 I/O cage. The feature has two independent ports, with one PCHID associated with each
port. The OSA-Express Token Ring feature can be defined with CHPID type OSD or OSE.
The OSA-Express Token Ring feature supports auto-sensing as well as any of the following
settings: 4 Mbps half- or full-duplex, 16 Mbps half- or full-duplex, and 100 Mbps full-duplex.
Regardless of the choice made, the network switch settings must agree with those of the
OSA-Express Token Ring feature. If the LAN speed defaults to auto-sense, the OSA-Express
Token Ring feature will sense the speed of the attached switch and insert into the LAN at the
appropriate speed. If the Token Ring feature is the first station on the LAN and the user
specifies auto-sense, it will default to a speed of 16 Mbps and will attempt to open in
full-duplex mode. If unsuccessful, it will default to half-duplex mode. The OSA-Express Token
Ring feature conforms to the IEEE 802.5 (ISO/IEC 8802.5) standard.
Each port has an RJ-45 receptacle and a DB-9 D shell receptacle for cabling to a Token Ring
MAU or Token Ring switch that is appropriate for the LAN speed. Only one of the port’s two
receptacles can be used at any time.
The RJ-45 receptacle is required to be attached using an EIA/TIA category 5 unshielded
twisted pair (UTP) cable that does not exceed 100 m (328 ft), or a shielded twisted pair (STP)
cable with a DB-9 D Shell connector.
Checksum offload for IPv4 packets when in QDIO mode
A function, called Checksum Offload, offered for the OSA-Express GbE and 1000BASE-T
Ethernet features, was introduced for the Linux for zSeries and z/OS environments.
Checksum Offload provides the capability of calculating the Transmission Control Protocol
(TCP), User Datagram Protocol (UDP), and Internet Protocol (IP) header checksums.
Checksum verifies the correctness of files. By moving the checksum calculations to a Gigabit
or 1000BASE-T Ethernet feature, host CPU cycles are reduced and performance is
improved.
When checksum is offloaded, the OSA-Express feature performs the checksum calculations
for Internet Protocol Version 4 (IPv4) packets. This function applies to packets that actually go
onto the Local Area Network (LAN) or come in from the LAN. When multiple IP stacks share
an OSA-Express, and an IP stack sends a packet to a next hop address owned by another IP
stack sharing the OSA-Express, OSA-Express sends the IP packet directly to the other IP
stack without placing it out on the LAN. Checksum Offload does not apply to such IP packets.